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Chemistry, Evolution, and Christian Worldview

In this assignment, students will analyze the current literature regarding the role of biochemistry in the creation of life on earth and explore the views of various scientists, experts on creation, theologians, and their own beliefs on how the earth was created while comparing them from the perspective of the Christian worldview.

In the book, A World from Dust: How the Periodic Table Shaped Life, biochemist Ben McFarland states, “Life is carried by a river made of chemistry that is pulled along, not by gravity, but by the predictable rules of chemical stability.”

After reading Chapter 1: Arsenic Life, explain in at least 250 words why McFarland believes that phosphorus is needed to make nucleic acids and why arsenic could not be a substitute for phosphorus in nucleic acids.

In Chapter 9: Cracked Open and Knit Together by Oxygen, McFarland details his thoughts on why there was an explosion in the number of fossil species found during the Cambrian period. In at least 250 words, explain what changes may have led to the “explosion” of species in the Cambrian period. Include your opinion as to whether these changes may have led to a dramatic increase in life during this time.

The Cambrian period in time is believed to have begun approximately 542 million years ago based on the abundance of the carbon-13 isotope. This period is characterized by a dramatic “explosion” in the number of different body plans that were found in the fossil record. The article “The Fossil Record of the Cambrian “Explosion”: Resolving the Tree of Life,” by Miller, discusses some of the controversies surrounding the Cambrian period. Read this article, and in at least 250 words, compare your viewpoints and beliefs on the creation of the earth with the viewpoints expressed by both Miller and McFarland about the Cambrian “explosion” events. Use supporting evidence from at least two other scientists, experts on creation, and/or theologians as you discuss why you agree or disagree with the viewpoints of the authors and compare these various viewpoints with biblical teachings from the perspective of the Christian worldview.

Attachments are attached for reading.

Prepare this assignment according to the guidelines found in the APA Style Guide, located in the Student Success Center. An abstract is not required.

This assignment uses a rubric. Review the rubric prior to beginning the assignment to become familiar with the expectations for successful completion.

Arsenic Life?

a puzzling biochemical press conference // the natural laboratory at Mono Lake // a global

scientific test // phosphorus builds, arsenic kills // how to mop up arsenic // magnesium’s
fitting relationship // a less wonderful life is more predictable

GE OL OGY SH A PE S CH E M IST RY AT MONO L A K E

In December 2, 2010, at 11:16 a.m., I received the first of three emails from students
in my biochemistry class, all asking if I had heard the news. A press conference at
11 a.m. had announced that scientists had discovered a bacterium that uses arsenic
instead of phosphorus in its DNA. Soon there was a hashtag for this: #arseniclife.
We were excited and a little puzzled. I had just lectured about how phosphorus was
uniquely useful to DNA. I shrugged and mumbled something about how textbooks
can be rewritten.

Today, the dust has settled— and the textbook reads the same as ever. DNA is made
of phosphorus, never arsenic. That December press conference was followed by two
full years of multiple experiments in labs around the world. It confirmed what the
textbook said all along, yet the story was well worth it. The “arsenic life” story was
never just about microbiology. It’s about science itself, how we know things, and the
nature of natural history.

Everyone should know this story. It will temper expectations when the next press-
conference- induced hashtag makes its way halfway around the world while science is
still lacing up its boots. More than that, it shows something deep about what kind of
world we live in, something underreported because it is so intricate and comes from
so many different places. There is a hidden order that makes some sense of biology
and even sociology, and that hidden order is chemistry.

All life, from a lakewater bacterium to the neurons firing in your brain as you read
this, is hemmed in. It is free to randomly adapt to its surroundings with nearly infi-
nite creativity, but its overall path is as constrained as if it were walking on the deck
of a ship crossing the ocean. The ultimate movement, on the scale of billions of years,
is shaped by chemical rules.

One of these rules is that phosphorus makes good DNA, while arsenic does not. To
reach this conclusion, we have to start where the arsenic life story started. The home
of the purported arsenic- using bacteria that caused all this trouble is in a remote spot
of California called Mono Lake.

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2 A Wor l d f rom Du s t

Mono Lake sits at the bottom of a basin just east of Yosemite, at the foot of Mount
Conness. It is one of the oldest lakes in North America, peaceful and uniquely beauti-
ful, with towers of rugged tufa that look like they belong on an alien moon. It is also
poisonous. Its arsenic- laced waters put the surrounding ecosystem under constant
chemical threat.

Mono Lake is an unusual place, hosting unusual life, which even uses unusual
atoms. Its unusual chemistry comes from its unusual geology. From above, Mono
Lake is an asymmetric shape, its west shore squarish and angular, its east a round
arc. This sharp western edge is cracked by a geological fault, traced by Highway 395.
West of this fault, the Sierra Nevada mountains are pushed up by deep geological
forces, while to the east, the lake is pulled down. The mountains to the west catch
the moisture so that Mono Lake itself is arid and rain- shadowed. This is where the
Nevada desert begins.

Highway 395’s fault still bubbles with potential volcanoes. The pressure from
underneath raises the entire Sierra Nevada range by about a millimeter a year, tilting
it up and away from the Mono Lake basin. Sometimes the pressure is released explo-
sively. A volcano 250 years ago suddenly formed a new island in the middle of lake.

The water in Mono Lake is a concentrated, liquid form of the Sierra Nevadas. Like
the Dead Sea, Mono Lake sits at the bottom of a bowl of rock, with many streams
running in but no streams running out. Once an atom arrives at Mono Lake, it can-
not leave unless it is light enough to evaporate up, liquid enough to seep down, or
lucky enough to be eaten by an animal that can walk or fly away. Water carries heavy
rock atoms down from the surrounding hills and they are trapped.

At Mono Lake, the water is as much mountain as it is lake. The dissolved rock,
especially the calcium, makes Mono Lake’s water very “hard.” It is so hard that when
it evaporates, towers of tufa rock are left behind (Figure 1.1). The classic album cover
to Pink Floyd’s Wish You Were Here was taken at Mono Lake. It shows a diver’s legs
projecting out of blue water with wrinkled towers of sand- colored rock all around,
as if his splash was turned to stone. Sometimes the water in lakes like this can get
so “hard” that a bird sitting on the water too long will calcify. The rock creeps up
and coats the bird’s feathers, eventually overtaking and ossifying the entire bird, like
something from the works of Edgar Allan Poe.

Tufa towers are built from chemistry, and the work is done by time. You can make
your own by mixing baking soda, table salt, Epsom salts, and Borax in a gallon of
water, then adding calcium chloride. Your bucket will include six elements important
to rocks (and to life): sodium, chlorine, magnesium, sulfur, calcium, and carbon.
Only the boron (added as Borax) does not play a major role in this book. After mix-
ing, all you need is the patience to wait for the water to evaporate. Over months,
inexorably, the calcium will link together with carbonate from the baking soda to
make limestone tufa.

As the calcium turns into tufa, the other chemicals stay dissolved in the lake
water. The first two, sodium and chlorine, come from table salt and dissolve well
in water. Too heavy to evaporate, sodium and chlorine remain trapped in Mono
Lake and make it twice as salty as the ocean. Most of the remaining rocky atoms are

McFarland, Ben. A World from Dust : How the Periodic Table Shaped Life, Oxford University Press, Incorporated, 2016.
ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/gcu/detail.action?docID=4413924.
Created from gcu on 2021-03-26 08:27:58.
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3 Arsenic Life?

surrounded by shells of oxygen atoms in water— carbon as carbonate (that was the
baking soda), sulfur as sulfate (Epsom salts), boron as borate (Borax), even arsenic as
arsenate (not included here because I presume you don’t want poison in your bucket).

Much of this book is spent thinking about what these chemicals are doing at the
atomic level, and there are two major differences when you shrink to this scale. The
first is that everything is constantly moving. A river on a still day may look serene, but
at the biological level it is filled with animals moving, plants waving, and currents
flowing. This motion is magnified at the tiniest levels, where molecules wiggle in
place or zip about like bees in a bottle.

Second, when molecules fit together, they only care about two things: shape and
charge. Shape is familiar— all atoms are spheres that can stack together like a super-
market display of oranges— but charge is unusual. Unless you work with wires or
rub your feet across shag carpeting, you don’t normally sense charge imbalances at
our macro level. But at the nanometer level, charge moves things around. Each atom
is made of heavy protons with a positive charge and light electrons with negative
charge. When these charges are symmetric and balanced, the overall charge is neu-
tral, but when they fall askew, a chain of domino effects can start, and chemistry can
happen.

Fig. 1.1 At Mono Lake, did the unusual chemistry that formed the Tufa towers also reshape the
elements in microbial DNA? Also, note how magnesium interacts with DNA’s phosphates.

4 A Wor l d f rom Du s t

Here it’s truer than ever that opposites attract, because positive and negative
charges pull together. If the pulling forces put two negative electrons between two
positive protons, the negatives tie the positives together with a chemical bond.
A group of likewise bonded atoms is a molecule. If opposite charges are on two dif-
ferent molecules, the two will pull together. The chemistry of Mono Lake— all chem-
istry, in fact— is formed from patterns of positive– negative interactions.

Each “- ate” molecule noted earlier is a different element coated with three or four
negative oxygen atoms. These are strong negative charges and can pull on neighbor-
ing molecules. Sometimes a water molecule comes too close, so that the “- ate” mol-
ecule’s negative oxygen attracts the water’s positive hydrogen, and yanks it right off.
Take away an H+ from H2O water, and OH

– hydroxide molecule is left behind.
So when rock dissolves in water, the rocky “- ate” molecules make enough OH–

hydroxide to change the water chemistry. This change is measured by the pH scale. If
almost all of the water is intact, the pH is 7 and the solution is balanced and neutral.
If water is pulled apart to make OH– hydroxide, the pH is above 7 and the solution is
basic. If instead it’s pulled to make H+ hydrogens, the pH is below 7 and the solution
is acidic.

One rule of thumb is that dark granite is acidic, while light limestone is basic.
The light- colored spires in Mono Lake indicate basic conditions, and the lake’s pH is
around 10. It has as much hydroxide “base” as milk of magnesia and is only a pH step
or two away from ammonia. These basic solutions feel slightly slippery, almost thick,
as if you could dive in and leave behind a splash of stone.

So Mono Lake is no garden spot, but neither is it dead. Local geology made the
lakewater hard and basic, which shapes the local biology. Fish cannot survive in
Mono Lake’s high- pH water, but smaller, more nimble organisms can adapt and get
everything they need.

In spring, the lake turns green with life as lake algae gorge on the swollen streams’
runoff. The rivulets that feed the lake bring rock molecules such as phosphate for
food. Sunlight gives algae power to grow, so they will piece small molecules together
into larger molecules that life can use: sugars, fats, and proteins. Brine shrimp eat
the algae, and birds eat the brine shrimp— not to mention the black flies that swarm,
breed, and thrive on the lakeshore, which from a human perspective may be a little
too productive.

But not all rocks are good for life. The high levels of dissolved phosphate in the
spring runoff are accompanied by high levels of dissolved arsenic in the oxygenated
form of arsenate, which is also concentrated in the lake. Life has learned to live with
the arsenic. It was this extreme ecosystem that attracted scientists curious about the
biochemistry formed from this poisoned geology.

BAC T E R I A T H AT DE F Y A R SE N IC

This brings us to the December 2010 press conference about arsenic life. Ever since
the 1980s, the era of the cold fusion debacle, scientists have learned to be suspicious
of science by press conference. Still, this had something that cold fusion didn’t: peer

5 Arsenic Life?

review and publication in a prominent journal. A group of scientists, including mem-
bers of the NASA Astrobiology Institute, published evidence in the journal Science
that bacteria from Mono Lake could use arsenic instead of phosphorus.

The question asked in the 2010 Science paper was provocative: Did the extreme
environment of Mono Lake force life to build with poisonous elements? In particular,
could a bacterium in Mono Lake have performed the biochemical alchemy of living
on arsenic, turning a poisonous sword into a productive plowshare?

Felisa Wolfe- Simon is listed in the first, best spot on the 2010 Science paper that
claimed that bacteria can pull this off. She grew bacteria in the lab from Mono Lake
mud. Day after day, she would reduce the phosphate levels while keeping the arsenic
around, in order to prove that the bacteria were not only tolerating the arsenic but
actually using it. Day after day, she came into the lab and the bacteria were still defi-
antly growing.

Wolfe- Simon removed the phosphate because of a pattern in the periodic table.
There, the symbol for arsenic (As) is directly beneath the symbol for phosphorus (P).
Each column in the periodic table contains a family of elements that have a simi-
lar electron arrangement and therefore often do the same chemistry. Phosphorus is
used by all life to build DNA and cell membranes. Once the phosphate was removed,
the bacteria would be forced to borrow the next- best thing from the environment
instead: arsenate.

The fact that elements in a column of the periodic table are chemically similar is
incredibly useful in the lab. If you want to tweak a molecule, try switching out some
of its elements for others in the same column. They often will bond the same but will
have different shapes that change the chemistry just a little. In 2006, chemists built a
new superconductor from lanthanum, oxygen, iron, and phosphorus. If phosphorus
worked, the researchers reasoned, why not try its chemical cousin one box down,
arsenic? They did, and it worked, if anything, even better.

In superconductors the two can substitute, so why not in DNA? Before finding
the Mono Lake bacteria, Wolfe- Simon asked the same question in a 2009 paper
titled “Did nature also choose arsenic?” She speculated that a “shadow biosphere” of
arsenate- using organisms may have played a part in life’s origins— in other words, an
alien biochemistry.

The problem is that for all known organisms, phosphorus is essential for life, while
arsenic is only essential for death. In this case, their chemical family resemblance
explains why arsenic is such a dangerous poison (and useful operatic plot device).
Today both molecules are found surrounded with oxygens, as the “- ate” molecules
phosphate and arsenate. As the periodic table would allow us to predict, their shapes
are the same, and their sizes are also practically the same: if phosphate is a handball,
then arsenate is a racquetball. If you can mistake one for the other when rummaging
around inside a gym bag, then the cell likewise can grab an arsenate when it intends
to use a phosphate.

Phosphate and arsenate may look the same, but they do not act the same. If you
ingest too much arsenic, you will experience headache, confusion, and sleepiness.
Your body tries to expel it out of, ahem, one end or the other. Many different parts

6 A Wor l d f rom Du s t

of your body break down at once. Your stomach and muscles ache and convulse, and
your kidneys malfunction. Everything that grows will misfire, whether it’s your hair
falling out in clumps or your fingernails turning white. Enough arsenic causes coma
and death.

The difference between life- giving phosphate and life- stealing arsenate is all in
the timing. Phosphate latches onto other molecules and stays bound for days, but
arsenate binds and then drops off in a matter of seconds. Phosphate is the studious
engineer that builds bonds to last, while arsenate is much more easily distracted.
Arsenate’s bigger size makes for longer bonds allowing just enough room for water.
Water, pressing in from all directions, squeezes into arsenate and breaks its bonds.
In a test tube, arsenate chemicals break down in seconds. Arsenic simply does not
cohere with the body’s proteins and metabolites— it sticks but does not stick around.

Arsenic is toxic when it is used by the body in the place of phosphate. It is like
an old post- it that falls off too fast. Phosphate sends essential messages for muscle
contraction and growth throughout the body, which arsenate disconnects. When the
phosphate messages die, so does the cell. Rapidly living cells (hair and kidneys) are
the first to feel the pinch, but all cells use phosphorus, so all cells are in danger. Heart
cells without energy shut down and die.

Arsenate also carries a second kind of danger. Because it is bigger than phos-
phorus, arsenic has more room to carry extra electrons. In a cell, arsenate absorbs
electrons like an electronic sponge, and this sabotages the cell’s electron balance.
Arsenate sheds its electrons randomly, and these extra electrons react powerfully
with molecules, especially oxygen. Random reactions shred the cell like interior
shrapnel. In an experiment in which yeast cells were fed arsenic, when the electronic
balance was disrupted, the DNA fragmented like a dropped egg. Arsenic also pushes
proteins out of shape by interfering with their sulfur atoms.

From this perspective, it seems like folly to grow cells in arsenic- rich, phosphate-
poor broth, but the proof is in the experiment. Wolfe- Simon’s logic was also based
on the periodic table, and in the fact that microbes thrive in Mono Lake, tolerating
chemical conditions that destroy plants and animals in hours. A single- celled organ-
ism without complex systems may be able to heave a microbial sigh and turn in the
absence of phosphate to the next- best thing, even if the next- best thing is normally
poisonous. Could a microbe turn an element of death into one of life?

Wolfe- Simon’s experiments showed that the microbes could live in high levels of
arsenic and very low levels of phosphate. This was an exciting result, but still, just the
beginning. Survival in arsenate does not mean the cell is made of arsenate. Atoms are
too small to see with a microscope, so Wolfe- Simon and company looked at the cells
themselves, and observed bacteria that looked inflated and shot through with huge
holes filled with arsenate. Bacteria usually do this when faced with a toxin— they
shove the toxin inside a bubble like they’re cleaning house by shoving all the extra
stuff in a closet. Just don’t open the closet.

Wolfe- Simon and colleagues still had to show that the arsenate replaced
phosphate in essential molecules. Inside the cell, phosphate sticks to proteins,
coats cell membranes, and f loats around as small energy- bearing molecules like

7 Arsenic Life?

adenosine triphosphate (“ATP,” in which “TP” means tri- phosphate). Perhaps
most important, phosphate forms the backbone of DNA, the molecule that tells
the cell how to make all the other molecules. If Wolfe- Simon could find arsenate
not just lightly stuck to, but permanently fixed in the backbone of DNA, that
would close the case.

But the smoking gun wasn’t found. The rest of Wolfe- Simon’s paper contains
several surprisingly weak experiments. Isolated DNA showed a faint shimmer of
arsenate in a large DNA molecule, but the shimmer was so faint it could have been
background noise. X- rays shot through the cells produced a complex shape of data.
This did not look like plain old arsenate, so it might have been bound to something,
but then again, it didn’t clearly look like anything else either. The data were faint and
the error bars shown bracketing the data were too large for many scientists’ comfort,
including my own. Instead of a firm conclusion, we had a mystery.

T H E 2 011– 2 012 MONO L A K E BAC T E R I A TOU R

Six months later, Science magazine took a step I had never seen before. It published
eight separate critiques, each short and argumentative, all written by scientists ques-
tioning Wolfe- Simon’s paper. Chemists cited arsenate’s fragile bonding chemistry;
biologists questioned the techniques; everyone asked why more detailed experi-
ments weren’t done. (Now, this is the kind of comment that can always be leveled at a
paper— it is much easier to propose an experiment than to carry one out.)

The real hope of changing minds was not in print but in the lab. Wolfe- Simon
and others sent bacteria around the world, and other scientists set to work. One
researcher in Canada even blogged her experiments daily. In summer 2012, a year
and a half after Science first published Wolfe- Simon’s results, two papers appeared in
Science, followed by two more in other journals, constituting a parade of evidence:

1. A lab from Switzerland found the Mono Lake bacteria grew at “very low” con-
centrations of phosphate but not at “very, very low” concentrations. Some trace phos-
phorus could have contaminated Wolfe- Simon’s original media. This Swiss lab also
used a technique called mass spectrometry that could essentially weigh individual
molecules to look for heavy arsenic atoms. Arsenic levels in DNA were too low to be
measured, meaning that the DNA was at least 99.99% phosphate.

2. Rosie Redfield and her lab were the live- blogging Canadian scientists. They
also found that the Mono Lake bacteria would not grow in “very, very low” concen-
trations of phosphate, and their mass spectrometry results also came up empty. The
bacteria acted the same in Vancouver as they did in Switzerland. Finally, Redfield
stored the DNA in water for two months, and, anticlimactically, nothing happened.
Every arsenate- linked molecule known to chemists reacts with water, but this
DNA was as sturdy as normal phosphate- linked DNA, mostly likely because it was
phosphate- linked DNA.

This is enough evidence to provide the verdict that the Mono Lake bacteria are
not arsenate utilizing but are merely arsenate resistant. This showed that they are not

8 A Wor l d f rom Du s t

arsenic eaters, but it didn’t show what they are. How are they so good at discrimi-
nating phosphate handballs from arsenate racquetballs? Two more papers answered
these questions:

3. Researchers from Israel used a complex and slow technique with X- rays that
can see individual atoms. They examined phosphate- binding proteins outside the
cell that find phosphate and let it into the cell. In an arsenate- rich environment,
these phosphate- binding proteins would be challenged with thousands of arsenate
imposters for every phosphate. This lab looked closely at the Mono Lake bacteria’s
phosphate- binding protein, to find out why it was especially good at accepting phos-
phate and binding arsenic.

Their pictures showed that the Mono Lake protein has a phosphate- binding hole.
This hole is like a lock, and phosphate fits inside like a key. Phosphate’s four oxy-
gens are spread out from each other and stick out evenly from the central phospho-
rus. This tetrahedral structure is like a camera tripod with the top camera- holding
arms extended fully up: the phosphorus is at the middle where the four arms come
together and at the tip of each arm is an oxygen atom. The phosphate- binding site on
the protein is a “lock” that mirrors phosphate’s four negative oxygens with a slightly
larger tetrahedral hole of four positively charged hydrogen atoms (Figure 1.2). The
unbalanced charges in both the protein and the phosphate become balanced as oppo-
site charges attract and tiny magnets snap the phosphate into place.

Fig. 1.2 Phosphate and arsenate molecules both adopt similar geometries, but a protein can
distinguish between them by the O- H angle shown at the top of each molecule.

9 Arsenic Life?

The Mono Lake protein has one twist, but it’s a good one. On each tetrahedron,
one of the oxygens has pulled an extra hydrogen from water. The angle for this
hydrogen is 95 degrees for phosphate and it is 109 degrees for arsenate. This struc-
tural difference can discriminate between toxin and nutrient. The Mono Lake pro-
tein puts a negative oxygen atom that fits with a 95- degree angle for hydrogen but
not a 109- degree angle. This one feature holds phosphate in place while arsenate
slips out.

4. The last piece of evidence was making the numbers add up. Wolfe- Simon’s very
low phosphate level was not enough for a cell to live on, so how did they make up
the difference? A lab from Miami found out a somewhat grisly secret: these bacteria
scraped up phosphate by cannibalizing the ribosome, the most important molecule
in the cell.

Ribosomes are protein- making molecules made from RNA (a molecule that looks
just like DNA except for one oxygen- hydrogen pair). Every protein in a cell is assem-
bled by a ribosome. A full quarter of the dry weight of a bacterium is in the ribosomes
alone. About one- quarter of the mass of RNA is phosphate, making it a tempting tar-
get for a phosphate- starved bacterium. The Miami lab saw the Mono Lake bacterium
breaking down its ribosomes for their phosphate backbones. Needless to say, this is
not a sustainable strategy in the long term. The cell is chopping up its furniture to
feed its furnace.

All four studies converged on a single conclusion: the Mono Lake bacterium resists
using arsenate as much as possible. Life as we know it will accept no substitutes,
and does not incorporate arsenate but finds new ways to reject it. This was not what
Wolfe- Simon was looking for, which makes it even more convincing. The biology
changes constantly, but it is molded around a solid, incontrovertible fact of chem-
istry: arsenate is not suitable for building DNA. Even in Mono Lake, arsenate DNA
would be a house of cards thousands high, falling apart within seconds.

PHOSPHORUS: T H E L A ST E L E M E N T STA N DI NG

All this re- emphasizes something biologists have known for a long time, that phos-
phorus has an unavoidable association with life. In Mono Lake it makes algae bloom
in the spring. This extends back in time. Old rocks, after glaciers pass through, have
suspiciously high amounts of phosphate. It looks like advancing glaciers scraped
phosphate out from rocks and fed it to the sea, seeding a spring- like burst of plant
and microbe activity. More phosphorus made more life, as geology led to biology
through chemistry. This has implications for all of biochemistry, because other ele-
ments that go well with phosphorus also go well with life.

In terms of the periodic table, phosphorus has biological advantages over arsenate
below it, over nitrate above it, over sulfate to its right, and over silicate to its left. In
fact, it has advantages over every other element on the periodic table in forming a
medium- strength bond to negative charge. This makes it the best choice on the table
for energy and information chemistry.

10 A Wor l d f rom Du s t

To understand the chemical advantage of phosphorus, let’s shrink to the scale
where we can watch atoms move, bond, and fly apart. Instead of an atmosphere, a
microbe is surrounded by a blizzard of flowing water molecules. Everything skitters
around like a butterfly unless it is tied down. Movement is mediated by the constant
random jostling and flow of molecules. It’s like trying to move through the crowd
at a big outdoor festival. In this turmoil, the best shapes will fit together like three-
dimensional puzzle pieces, propelled by random motion and guided by electric fields
of charge. Stable arrangements of atoms that fit together with shape and charge will
hold together better (like phosphate in the Mono Lake phosphate- binding protein).

In this context, a molecule needs three characteristics to be useful for building
life’s structures:

1. It must be available in water.
2. It must have a fitting shape and charge for its purpose.
3. Its bonds should last as long as they need to (and no longer).

Both arsenate and phosphate are available in Mono Lake, and have similar use-
ful shapes and charges, but the third point distinguishes the two: phosphate bonds
for a long time, while arsenate does not. Even in a lakeful of arsenate, life chooses
phosphate.

To understand phosphate’s unique qualities, start where a chemist starts: its tet-
rahedral (that is, four- sided) shape, as shown in Figure 1.2. When dissolved in water,
the molecule tumbles freely, and the four oxygens whirl around the phosphorus cen-
ter. From a distance, these atoms blur together into a tiny ball of negative charge,
joining with the opposite charges on water molecules in an intricate dance.

No other element is used for this purpose by life because no other element has
chemical properties like phosphorus. All of the possible chemical options are shown
on the periodic table. Take a moment to find the periodic table (Figure 0.1) at the
beginning of this book and consider all those boxes, each representing a different
element. (As we go through this book, refer back to that table like a map. It works like
the map of Middle Earth in Tolkien’s Lord of the Rings.) At first, it seems like the 90
naturally occurring elements give lots of options to make something else like phos-
phate. But the options quickly narrow.

First we have to cross off the elements in the first two rows of the periodic table
(from #1 hydrogen through #10 neon). These elements are all too small to fit the four
bonds to oxygen needed for our tetrahedral arrangement, so they don’t fit rule #2.
Then we have to cross off the elements in the third row and below (after #37 rubid-
ium). These are big enough, but too rare or too insoluble in their oxygen- bound “- ate”
form, so they don’t fit rule #1.

Then consider the columns. The number of electrons exposed in the outermost
level of each atom increases from left to right in the table. The farthest left column
has one electron and the next column has two (as does the block of elements run-
ning from #21 scandium to #30 zinc in general); #5 boron’s column has three avail-
able electrons, #6 carbon’s has four, on out to the rightmost row, which has eight

11 Arsenic Life?

electrons. About one electron in the outer shell is needed for each bond to oxygen. So
we have to cross off everything left of #5 boron, which has too few electrons to bind
four oxygens, and also cross off the rightmost row, which has too many electrons.
This rightmost row, in fact, is entirely composed of gases that don’t react with any-
thing else, snobbishly refusing to form bonds (and somewhat spitefully named the
“noble gases”).

When you sit back and look at our once- promising collection of candidates, every-
thing is crossed off except for aluminum, silicon, phosphorus, sulfur, and chloride.
(We skipped over a few exceptions, but even those are crossed off with a little more
investigation.) Just by asking how life can form a tetrahedron in water, we have
reduced the possibilities from 90 naturally occurring elements to five.

But four of these five don’t work as well as phosphorus. The oxygen- covered forms
of aluminum and silicon are rocks, not metaphorically, but literally. They form strong
bonds to each other and like to share their oxygens in long, amorphous linked chains.
Sand and glass are sharp- edged silicate materials. These are too solid to allow for the
flow a living organism needs to change and respond to its environment. They are as
frozen in place as a victim of Medusa and would slice through the flowing structures
of life.

On the right end is chlorine, which surrounded by four oxygens is perchlorate.
Perchlorate reacts so readily that it is used in rocket fuel. Perchlorate is related to the
molecules in chlorine bleach. Like arsenate, it is poison, not food. Some fascinating
microbes eat chlorate molecules, but they do not appear to build with them.

That leaves phosphate and sulfate. Like phosphate, sulfate can bond biological
molecules and is found doing so outside the cell for chemical reasons described later.
But phosphate can perform the chemical trick of linking itself when sulfate cannot.
If a cell chains two sulfates together in water, half of them will be gone in minutes,
while two chained phosphates will last a thousand days. As a chaining molecule,
phosphate is eminently transferable. One oxygen can be its left hand and another its
right hand. Phosphate can be passed between all sorts of different groups by switch-
ing what it’s holding with its left hand, and then its right.

Some cell signals are turned “on” by attaching phosphate to the turned- on mol-
ecule through one of its oxygen hands. I imagine the phosphate glowing faint green,
both because pure phosphorus can glow green, and because it signals that a pathway
is “on,” a “green light” to the rest of the cell. As the signal propagates, more proteins
are attached to phosphate, and the cell begins to fill with glowing green beacons.
Eventually a protein is turned on that tells the cell to move something, to make some-
thing from DNA, or to do whatever the cell may require. Phosphate is transferring
information from one part of the cell to another. Because phosphate is bound to the
protein with a medium- strength bond, that bond can be broken with a flick of an
atom, allowing the signal to be turned off as quickly as it was turned on.

Phosphate may be even more useful for energy transfer than it is for signaling.
Two or three phosphates linked together are held in tension. The medium- strength
bonds that hold the phosphates together also hold 7 to 10 negative oxygens next to
each other. These oxygens repel the negative oxygens in water, protecting the bond

12 A Wor l d f rom Du s t

from being broken by water’s reactivity, but they also repel each other (if opposite
charges attract, like charges repel). When the bond is broken, the oxygens repel apart
and accelerate the breaking, releasing energy as they go. Even more important, the
released phosphate interacts better with water. Overall, because ATP is more stable
broken than joined together, breaking its three phosphates apart releases energy.

Four slightly different triple- phosphate (abbreviated “TP”) groups are found in the
cell, called ATP, GTP, CTP, or TTP, depending on what molecular “handle” they have
attached to them (abbreviated A, G, C, or T). Your energy- transferring processes turn
the food you eat into linked phosphate groups, the most important of which is ATP.
Your muscles get energy to move by splitting phosphates off ATP. Also, the first thing
you do to break down the sugar glucose is put a phosphate on it. Phosphate both
sends signals and holds chemical power.

Finally, phosphate’s energy helps build the most important of all biomolecules, the
long- term information storage molecule DNA and its sister molecule, the short- term
information transfer molecule mRNA. There are four types of “TP” molecules, each
of which matches one of the four “letters” in DNA.

The four oxygens that surround phosphorus give it a negative charge in water, even
when it is linked on its left and right. This is why both DNA and RNA end with “A” for
“acid,” because a negatively charged phosphate is an acid. An acid is something that
has shed a positively charged hydrogen in water, leaving a negative charge behind.

Phosphate may be the only way for nature to make abundant charged chains in
water. The negative charges repel and spread the long DNA chain out like a ticker-
tape, which is more easily read than a tangled mess. The whole point of DNA is to
hold information that is read like an open book, and phosphate keeps the book open.

Steven Benner is a chemist who redesigns DNA’s “bases,” which is another name
for what I have called the “handle” end of the nucleotide, the A, C, T, and G parts of
DNA. Benner has chemically welded together new alternative bases with remarkable
success, and he has gone on the books as saying that the naturally occurring base
structure “is a stupid design” that he can improve. I take his word for it on that.

But the reason Benner redesigns bases is that he couldn’t redesign the phosphate
at the other end of the molecule. No chemist can redesign the periodic table to find
another element that links DNA as well as phosphorus. Years ago Benner started
his design program by trying molecules other than phosphate in the DNA that
had none of phosphate’s negative charge. They would tangle and fold without phos-
phate’s charged self- repulsion. Benner’s research bore fruit only when he turned to
the other end of the molecule. (Other scientists have introduced positive nitrogen-
based charges to the backbone, but nitrogen doesn’t work for phosphate’s other pur-
poses, because three nitrates together are no more stable than three sulfates together.)

In a cell, phosphate is so important and abundant that it causes the problems of
abundance, particularly when it comes to charge. If all of these phosphates are trans-
ferring information and energy inside the cell, wouldn’t that mean that the inside of
the cell should have a negative charge overall? Such a huge charge imbalance would
crumple the cell, or promote frequent electrical discharges that would undermine the
stability needed for life.

13 Arsenic Life?

The cell needs something else to balance phosphate’s charge. It needs something
positive that can stick around well, but not too well. It needs something discreet that
gets out of the way when it’s not needed. So another element must stand beside phos-
phate, if phosphate’s negative charges are to work inside the cell.

M AGN E SI U M : PHOSPH AT E ’ S I N V ISI BL E PA RT N E R

When we teach about ATP in high school biology, we teach it wrong, because we have
to. By editing it down from “adenosine triphosphate” to “ATP” we make it possible
to fit into a high schooler’s brain along with the rest of adolescence. Come to think of
it, it’s a miracle that we get ATP to stick in there, considering. But our terminology,
shrunken to three letters, edits out ATP’s constant chemical partner.

Negatively charged ATP must be balanced with positive charge. In the cell, posi-
tively charged magnesium provides this balance. The chemist in me says we should
write it as Mg- ATP, but the contingencies of history make that collection of five let-
ters pretty much unpronounceable in English (“Em- gat- puh”?). We are stuck with
“ATP” just like we are stuck with QWERTY keyboards. Whether our labels recognize
it or not, magnesium is essential wherever phosphate functions, from ATP to RNA.

Many elements have two positive charges like magnesium, but magnesium’s size
sets it apart. If we need a positive charge, we need a metal on the left side of the peri-
odic table, so cross off the whole right side of the table. On the left side, water reacts
with each element and takes away its outer electrons, so the leftmost column loses its
one outer electron and has one positive charge as a result. One positive charge is too
weak to effectively balance the negative phosphates, so cross that column off. Three
positive charges are too strong— they’d stick but we’d never we able to pry them off
to move phosphate around. This excludes the column under aluminum on the right.
Between these two extremes are elements that are mostly +2.

Next we consider abundance (rule #1). Since phosphate is abundant in the cell, its
balancing agent must also be abundant. Like before, we cross off the bottom half of
the table because the bigger elements down there are not abundant enough. Even if
the first long row, from scandium to zinc, was abundant enough (which I’m not sure
about), all of those elements stick a little too tightly to phosphate as well. They form
high- strength phosphate bonds and solid phosphate rocks, not medium- strength
phosphate bonds.

We are left with one column: beryllium, magnesium, calcium (the three atoms
below them went home during the previous steps). Since atomic size increases as you
move down on the periodic table, beryllium is smallest and calcium is largest. All
three of these have two positive charges to effectively counterbalance phosphate’s
negative charge.

But like Goldilocks with her three porridge bowls, we can reduce this list from
three options to one. First we have to account for the fact that we need something that
sticks to three phosphates in ATP, not just one. Magnesium fits perfectly between
the oxygens on two adjacent phosphates in ATP, but calcium is too big. This makes
magnesium the optimal elemental partner for phosphate.

14 A Wor l d f rom Du s t

What about beryllium? Even though it’s small and appears as easy to make as the
other small elements, because of a quirk of nuclear physics, not much beryllium is
actually made in stars (see Chapter 3). This is actually very good for us, because beryl-
lium is stickier than magnesium, so sticky that it is toxic to life. Beryllium allergy is
caused when it sticks to an oxygen- rich pocket on the surface of a cell and provokes
an immune response. Because the beryllium never unsticks, it sets off alarms and
immune inflammation that wreck the body.

Magnesium’s stickiness to phosphate is used by the bacterium that causes tuber-
culosis. It puts magnesium in a protein toxin that uses the magnesium’s positive
charge as something like bait. The magnesium attracts the phosphates from your
RNA strands and then reacts with them, snapping them into little pieces. Magnesium
forms the edge of a knife that tuberculosis uses against RNA.

But the tuberculosis bacterium must be careful with this magnesium knife,
because it can cut up the bacteria’s RNA, too. The microbe makes another protein
that acts as a sheath for the knife and covers the magnesium. Even though magne-
sium is just one atom among thousands in the protein chain, it is at the very center of
how this protein knife works. Chemists are already trying to design a small molecule
that can stick to magnesium and blunt tuberculosis’s magnesium knife.

Because of the phosphate- magnesium balance, RNA in the cell carried around a
positively charged cloud of magnesium ions, a sort of magnesium aura. Magnesium
is especially important when RNA must fold up into a compact shape to do some
work, like when the ribosome folds up to build new proteins. The ribosome’s negative
phosphates are knit together with magnesium, stabilizing it with a web of positive and
negative charges. Ribosomes in plants without magnesium fall apart, and the plants
turn old before their time because they are missing this one element.

Many smaller chains of RNA form compact structures with specific shapes that help
specific reactions along. For a century we’ve known that proteins do this, which we call
enzymes. When RNA does an enzyme’s job, it’s called a ribozyme. Scientists who study
ribozymes know the value of magnesium. If they forget to add magnesium to their exper-
iments, their ribozymes will fall apart. That’s the kind of thing you don’t forget twice.

Experiments have systematically stepped through the periodic table and tested
how different metals can tie together the phosphate chain of an RNA ribozyme. This
reveals that magnesium is indeed the best element for this job. Magnesium, with its
two positive charges, worked so well that we can say, given the choice between one
magnesium and a hundred singly charged potassium atoms, this RNA would still
choose magnesium. Magnesium also binds on a timescale faster than milliseconds.
Magnesium fits quickly as well as tightly into the RNA chain of the ribozyme.

Magnesium helps with DNA, too. For example, it is an essential ingredient when
enzymes proofread DNA. On Figure 1.1, where DNA rises from Mono Lake, mag-
nesium is found in its favorite place, nestled between the DNA phosphates. Most
pictures of DNA don’t include this aspect of its structure because the magnesium
moves around so much, but if the magnesium wasn’t there, the negative phosphates
would crumple the cell within seconds. Magnesium and phosphate pair as effectively
as steak and red wine.

15 Arsenic Life?

CH E M IC A L RU L E S A N D A WON DE R F U L DISPU T E

The magnesium- phosphate pairing is only the first of many. In a sense, chemistry is
the study of pairings. Chemistry studies the bonds between atoms, and every bond is
a pairing with two and only two sides. Even the immature form of chemistry known
as alchemy had a place for this idea.

On the biochemical timescale, some pairs are permanent, some are fleeting, and
some are repulsive and anti- bonding. Some are very picky about direction, and some
don’t care which way is up. But all of these bonds come from atoms attracting and
repelling through electron imbalances. All are the consequences of the contrast
between two physical pieces: miniscule, flighty, negatively charged electrons danc-
ing about; and heavy, solid, positively charged atomic nuclei anchoring the electrons
through opposites attracting.

The same physical laws that say that positive and negative charges attract add up
to say that, in water, magnesium and phosphate pair especially well. One member
may be less famous, like magnesium, but general chemical rules tell us that it is still
important. When that pairing chemistry is changed even slightly, like if arsenate
pairs more fleetingly with its partners, the consequences can be life- threatening.

Life has a vested interest in collecting phosphate and magnesium, and in rejecting
arsenate. A set of protein sensors monitors levels of each and keeps them in balance.
The phosphate- binding (arsenate- rejecting) protein from Mono Lake bacterium lets
phosphate in. At least two proteins open doors into the cell specifically for magne-
sium. One particular magnesium transporter opens in response to an ATP- magne-
sium “key,” again showing the complementarity of those two molecules.

Not all elements are welcome. Another protein patrols the interior of the bac-
terium E. coli, looking for arsenate that sneaked inside. When arsenate fits into its
arsenate- shaped binding site, the protein changes shape and binds DNA, which acti-
vates arsenate- cleaning proteins that block arsenate’s oxygens with carbon- hydrogen
groups. The intruder is neutralized.

Similar arsenate- cleaning proteins are found in plants and animals, especially
those chronically exposed to arsenic. In some cases, the periodic table can explain
mysterious chemical effects. For example, Leishmania parasites are susceptible to
treatments with the element antimony (#51 Sb on the table). But in certain areas
of India, such treatments don’t work. Some scientists think they know why, with
some experiments in mice to back them up: antimony is right below arsenic on the
periodic table and the two are therefore chemically similar. In those areas of India,
arsenic frequently contaminates drinking water, and the people who live there have
upgraded their internal arsenate- cleaning processes. Because antimony is so chemi-
cally similar to arsenic, the two are swept up by the same processes, throwing out the
antimony that would otherwise destroy Leishmania. The law of unintended conse-
quences extends to chemistry and can be understood with a glance at the periodic
table’s columns.

A flotilla of proteins in the cell respond to specific elements, whether as sensors
deep inside the cell or transporters opening doors in the cell membrane for needed

16 A Wor l d f rom Du s t

elements. In the E. coli bacterium, at least five different transporters bring iron inside
the cell, which is a huge energetic investment and shows that iron must be pretty
important to life. There are also five transporter proteins for zinc, although only
two let zinc in and the other three push it out. Copper has two doors opening out;
nickel, manganese, and molybdenum each have one door opening in; some magne-
sium proteins also work for calcium; and one protein ejects both nickel and cobalt.
Sodium and potassium also have proteins responsible for them, pushing sodium out
and potassium in.

In this way, the proteins in the cell tell us which elements are crucial for life. Why
in and not out, or out and not in? Each has its own chemical reason and rule. Many,
like phosphate, play irreplaceable and unique roles. Each element has a distinct his-
tory, too. Some have been around since the original quickening of life, and some
came late to widespread biochemical use. This history was ordered and sequenced by
the rules of chemistry. It even has a chemical direction, moving from left to right on
certain segments of the periodic table.

All of this says that, whatever life is, it is not entirely random. Neither is it entirely
determined. In chemistry, gases are random, solids are set, and liquids are the in-
between stage that flows with motion that is random at the atomic level but predict-
able at the human level. Life itself is more like a liquid than a solid or a gas. Life is
carried by a river made of chemistry that is pulled along, not by gravity, but by the
predictable rules of chemical stability.

The great science communicator Stephen Jay Gould disagreed with this. Gould’s
book Wonderful Life (1990) describes the evolution of life as a “lottery” with “thou-
sands of improbable stages” (pp. 47, 238). Most strikingly, Gould refers to the “tape of
life”: “Wind back the tape of life … let it play again from an identical starting point,
and the chance becomes vanishingly small that anything like human intelligence
would grace the replay” (p. 14). Gould was speaking of an event that we will return
to in Chapter 9, but his argument has been so successful that it needs to be addressed
from the beginning.

Gould documented well the damage done by wrong scientific stories. This made
him skeptical of grand narratives. So this book’s grand narrative must start from
the evidence before it is knit together in a narrative. This story is built from three
areas of evidence: rocks (geology), genes (biology), and the chemical rules that tie
the two together. To represent these other disciplines, I’d like to introduce Gould to
the chemist R. J. P. Williams.

R. J. P. Williams cowrote a book titled Evolution’s Destiny: Co- evolving Chemistry
of the Environment and Life (2012) about how chemistry guided evolution. I owe
Williams for much of this story. In fact, Williams has been writing books like this
for decades now, based on chemical laws. Only in the past few years has the biology
caught up with Williams’s chemical predictions, and by and large, it tells the story
he expected.

Overall, I think Gould is right when it comes to individual species, but I think he
is wrong at the broader levels of ecosystems and planetary evolution. Williams gives
reasoning, evidence, and tools that tell a grand narrative tied together by chemistry.

17 Arsenic Life?

From the proper perspective within this narrative, natural history can indeed be pre-
dicted. In the past decade, microbiologists have also challenged Gould by actually
running the tape of life repeatedly in the lab, and they too have found predictable
patterns.

From this, I conclude that the tape of life is likely more predictable than Gould
thought— something less like a tape and more like a river, its liquid flow channeled by
the solid banks of chemical laws. We’ve already seen why this river would flow with
phosphorus and magnesium, and not with arsenic. The remaining chapters will show
why for the rest of the elements.

In each case we begin with the living, thriving specimens at hand. The chem-
istry they nimbly perform tells of the underlying rules and pairings of chemistry.
Chemists can mimic (as through a glass darkly) some aspects of life in the lab. These
experiments tell us the chemical rules, and the best summary of these is in the grid of
the periodic table. The periodic table is a map that will guide us through the history
of chemistry on this planet.

By paying close attention to biochemistry, a story begins to emerge of order in dis-
order, of ingenious organisms persevering through ages of time, of poison and food,
of sunlight and water, death and life. It is a story written in elements and channeled
by chemical rules of energy, flow, and pairing. We enter the story midway along the
road of life, and we begin by looking behind us and counting the footprints.

Article

The Fossil Record of the
Cambrian “Explosion”:
Resolving the Tree of Life^

ß. Miller

The Cambrian “explosion” has been the focus of extensive scientific study, discussion,
and debate for decades. It has also received considerable attention by evolution critics
as posing challenges to evolution.

In the last number of years, fossil discoveries from around the world, and particularly
in China, have enabled the reconstruction of many of the deep branches within
the invertebrate animal tree of life. Fossils representing “sister groups” and “stem
groups” for living phyla have been recognized within the latest Precambrian
(Neoproterozoic) and Cambrian. Important transitional steps between living phyla
and their common ancestors are preserved. These include the rise of mollusks from
their common ancestor with the annelids, the evolution of arthropods from lobopods
and priapulid worms, the likely evolution of brachiopods from tommotiids, and the
rise of chordates and echinoderms from early deuterostomes.

With continued new discoveries, the early evolutionary record of the animal phyla
is becoming ever better resolved. The tree of life as a model for the diversification
of life over time remains robust, and strongly supported by the Neoproterozoic and
Cambrian fossil record.

T
he most fundamental claim of bio-
logical evolution is that all living
organisms represent the outer tips

of a diversifying, upward-branching tree
of life. The “Tree of Life” is an extreme-
ly powerful metaphor that captures the
essence of evolution. Like the branches
of a tree, as we trace individual lines
of descent (lineages) back into the past
(down the tree), they converge with other
lineages toward their common ancestors.
Similarly, these ancient lineages them-
selves converge with others back in time.
Thus, all organisms, both living and ex-
tinct, are ultimately cormected by an
unbroken chain of descent with modifica-
tion to a common ancestral trunk among
single-celled organisms in the distant past.

This tree metaphor applies as much to the
emergence of the first representatives of
the major groups of living invertebrates

(such as snails, crabs, or sea urchins) as it
does to the first appearance and diversi-
fication of dinosaurs, birds, or mammals.
This early diversification of invertebrates
apparently occurred around the time of
the Precambrian/Cambrian boundary over
a time interval of a few tens of millions of
years. This period of rapid evolutionary
diversification has been called the “Cam-
brian Explosion.”

The Cambrian explosion has been the
focus of extensive scientific study, dis-
cussion, and debate for decades, and is
increasingly receiving attention in the
popular media. It has also received con-
siderable recent attention by evolution

Keith B. Miller

Keith Miller is a fellow of the ASA and current member of the Executive
Council. He received his PhD in geology from the University of Rochester
and has been teaching at Kansas State University since 1990. His research
interests are in paleoecology and paleoclimatology. He has ivritten and spoken
widely on the public understanding of the nature and limits of science.

Volume 66, Number 2, June 2014 67

irtlele

The Fossil Record of the Cambrian “Explosion”: Resolving the Tree of Life

critics as posing challenges to evolution. These crit-
ics argue that the expected transitions between major
invertebrate groups (phyla) are absent, and that the
suddenness of their appearance in the fossil record
demonstrates that evolutionary explanations are not
viable.

What are some of the arguments of the evolution
critics? John Morris of the Institute for Creation
Research writes.

If evolution is correct, the first life was quite
simple, evolving more complexity over time. Yet
the Cambrian Explosion of Life has revealed life’s
complexity from the start, giving evolution a black
eye. The vast array of complex life that appears
in the lowest (or oldest) stratigraphie layer of rock,
with no apparent ancestors, goes hard against
evolutionary dogma. Evolution’s desperate
attempt to fill this gap with more simple ancestral
fossils has added more injury … Think of the
magnitude of this problem from an evolutionary
perspective. Many and varied forms of complex
multi-celled life suddenly sprang into existence
without any trace of less complex predecessors.
There are numerous single-celled forms at lower
stratigraphie levels, but these offer scant help in
solving the mystery. Not one basic type or phyla
[sic] of marine invertebrate is supported by an
ancestral line between single-celled life and the
participants in the Cambrian Explosion, nor are
the basic phyla related to one another. How did
evolution ever get started?^

Intelligent design advocate Stephen Meyer and others
have written:

To say that the fauna of the Cambrian period
appeared in a geologically sudden manner
also implies the absence of clear transitional
intermediates connecting the complex Cambrian
animals with those simpler living forms foimd in
lower strata. Indeed, in almost all cases, the body
plans and structures present in Cambrian period
animals have no clear morphological antecedents
in earlier strata.’

And

A third feature of the Cambrian explosion (as well
as the subsequent fossil record) bears mentioning.
The major body plans that arise in the Cambrian
period exhibit considerable morphological
isolation from one another (or “disparity”) and
then subsequent “stasis.” Though all Cambrian

and subsequent animals fall clearly within one of
a limited number of basic body plans, each of these
body plans exhibits clear morphological differences
(and thus disparity) from the others. The animal
body plans (as represented in the fossil record) do
not grade imperceptibly one into another, either
at a specific time in geological history or over the
course of geological history. Instead, the body
plans of the animals characterizing the separate
phyla maintain their distinctive morphological
and organizational features and thus their isolation
from one another, over time.*

Are these critiques warranted? To what extent is
the Cambrian explosion really problematic for the
evolutionary picture of an unbroken tree of life
extending back to the earliest life on Earth?

Defining the Cambrian
‘^Explosion”
The relative rapidity of the diversification of inverte-
brates during the Cambrian “explosion” is set against
the backdrop of the earth’s geologic and biologic
history. Geologic time is unfamiliar to most people,
and its shear vastness is difficult to grasp.

Two lines of evidence impact our understanding
of the durafion of the animal diversification that
led to the appearance of the major groups of living
invertebrates. The first is the dating of critical lev-
els within the geological timeline such as the Pre-
cambrian-Cambrian boundary and various important
fossil-bearing horizons. The second is the time of
appearance of the first widely recognized fossil repre-
sentatives of the major living groups (phyla) of inver-
tebrate animals. The latter is in considerable fiux as
new fossil discoveries are made.

Originally, the base of the Cambrian had been set at
the earliest appearance of organisms with mineralized
skeletons — particularly trilobites. However, a diverse
collection of tiny mineralized plates, tubes, and scales
was discovered to lie below the earliest trilobites.^
This interval of “small shelly fossils” was designat-
ed the Tommotian. Because of the presence of even
earlier tiny mineralized tubes and simple burrows,
there was no internationally accepted definition for
the boundary until 1994. At that time, the base of the
Cambrian was placed at the first appearance of a par-
ticular collection of small fossil burrows characterized
by Treptichnus pedum.^

68 Perspectives on Science and Ctiristian Faith

Keith B. iVIiller

Until the early 1990s, the age of the Precambrian-
Cambrian boundary was not tightly constrained,
and was estimated to be about 575 million years ago.
However, in 1993, new radiometric dates from close
to the accepted Precambrian-Cambrian boundary re-
vealed that it was significantly younger —about 544
million years.^ A more precise date of 542 ± 0.3 mil-
lion years has recently been formally accepted by the
International Commission on Stratigraphy. The basis
for this date was the discovery that a sharp world-
wide fall (or negative spike) in the abundance of the
isotope carbon-13 was coincident with the Cambrian
boundary as previously defined. In Oman, this iso-
topic marker also coincides with a volcanic ash layer
that yielded the 542-million-year date using uranium/
lead radiometric methods.^ This horizon also marks
the last occurrence of several fossils characteristic of
the underlying late Precambrian Ediacaran Period.’
Such extinction events are commonly used to sub-
divide the geologic time scale.

The earliest diverse fossil invertebrate communities
of the Cambrian are represented by the Chengjiang,
in China. These deposits are dated at 525-520 mil-
lion years. The famous Burgess Shale is consider-
ably younger, dating at about 505 million years, and
the end of the Cambrian Period is set at 490 million
years. The Cambrian Period thus lasted for 52 million
years, and the Early Cambrian alone was an extended
period of time lasting 32 million years.” To put this
in perspective, the time elapsed from the extinction
of the dinosaurs at the end of the Cretaceous to the
present has been 65 million years. The Cambrian was
a very long period of time (see fig. 1).

If the Cambrian “explosion” is understood to com-
prise the time from the base of the Cambrian to the
Chengjiang fossil beds, then this period of diversifi-
cation in animal body plans appears to have lasted
about 20 million years. However, not all living animal
phyla with a fossil record first appear within this time
window. The colonial skeleton-bearing bryozoans,
for example, are not known from the fossil record
until near the end of the Cambrian around 491 million
years ago.” In addition, most of the Early Cambrian
fossils recognized as related to modern phyla are
actually intermediates or stem groups (see discussion
below). Furthermore, recent refined dating of first
appearances of the Early Cambrian stem groups has
indicated that even the “explosive” start of the Cam-
brian diversification was mLore gradual and episodic
than previous thought.^^

Defining the Cambrian “explosion” is not as straight-
forward as it might seem. Although there was clearly
a major burst of evolutionary innovation and diversi-
fication in the first 20 million years or so of the Cam-
brian, this was preceded by an extended period of
about 40 million years during which metazoans arose
and attained critical levels of anatomical complexity.
Significantly, several living invertebrate phyla have
a fossil record that extends into the late Neoprotero-
zoic before the Cambrian. Sponges have been recog-
nized as early as 580 million years, cnidarians (the
group including jellyfish and anemones) are present
among the Ediacaran animals at around 555 million
years, and the stem groups for some other phyla
were also likely part of the Ediacaran communities.
The Ediacaran saw the appearance of organisms with
the fundamental features that would characterize the
later Cambrian organisms (such as three tissue layers,
and bilaterally symmetric bodies with a mouth and
anus), as well as the first representatives of modern
phyla. The base of the Cambrian is not marked by a
sharp dramatic appearance of living phyla without
Precambrian roots. It is a subjectively defined point
in a continuum.

– 4 9 0 – =

– 5 0 0 –

-510

-520

-530

^ £
& Ö

Earliest known
bryozoans

— Burgess Shale

Toyonian

Botomian

Aidabanian

Chengjiang and
-.Sirius Passet biotas

First Trilobites

Ediacaran

Tommotian .s^.^,, shelly fossils”
diversification

Nemakit- * – F’fst halkleriids, moliusks
Daidynian *—Treptichnus pedum trace

Cloudina & Namacaiathus

•— skeletal fossils, Stem
mollusk Kimberelta

«-Claimed bilaterian
trace fossils

First Ediacara biota
t—members

»-First sponges
«-Doushantuo embryos

Figure 1. Timeline showing the interval from the late Neoproterozoic
(Ediacaran) through the Cambrian. Marked on the timeline are the
positions in time of some of the more important fossil localities, and
the time of first appearances of selected metazoan groups.

-550 –

– 5 6 0 –

-570

– 5 8 0 –

Volume 66, Number 2, June 2014 69

The Fossil Record of the Cambrian “Explosion”: Resolving the Tree of Life

Drawing Trees and
Assigning Names
The procedure of classifying organisms is called tax-
onomy, and the general name for individual groups is
“taxa.” The first question that needs to be addressed
is “What is a phylum?” A phylum is often identified
as a group of organisms sharing a basic “body plan”
or a group united by a common organization of the
body. However, phyla can be understood fundamen-
tally, like all other taxonomic categories, as groupings
of taxa that are more closely related to each other than
to any other group.

The most widely accepted method for grouping
organisms today is called cladistics.” In cladistics,
all taxonomic groups are monophyletic, that is, all
of the members of the group are descended from
a common ancestor that is the founding member of
that taxon. A branch of the tree of life whose mem-
bers all share the same ancestor is called a “clade” —
thus the term “cladistics.” A taxon or taxonomic
group that is the closest relative of another group,
and that shares the same common ancestor, is called
a “sister taxon” or “sister group.” The early represen-
tatives of two sister groups commonly resemble each
other more than the descendant relatives resemble
the ancestors of their clade. As a result, placing these
organisms into their correct monophyletic groups
can be very difficult. Thus, primitive organisms with-
in a given phylum may bear close similarities to those
from another closely related sister phylum. In fact,
the assignment of a given organism or fossil specimen
to a phylum can be just as problematic as assignments
to lower-ranked taxa such as classes, orders, families,
and so forth.” This fact alone indicates that biologi-
cal diversity is more a continuum than a collection of
discrete groups.

Further complicating the assignment of fossil organ-
isms to phyla is that the anatomical characteristics that
are used to define living phyla did not appear simulta-
neously, but were added over time. This has resulted
in the distinction between “crown groups” and “stem
groups” in the scientific literature^^ (fig. 2). This termi-
nology can be applied to any level of the taxonomic
hierarchy. A crown group phylum is composed of all
the living organisms assigned to that phylum, plus
all the extinct organisms that were descended from
the common ancestor of those living organisms. The
stem group is composed of extinct organisms more
closely related to one particular living phylum than

to any other, but that were not descended from the
common ancestor of the living representatives of that
phylum. Stem groups typically do not possess all of
the defining characters of the crown group of that
phylum. It turns out that the organisms appearing
in the Early Cambrian are, with few exceptions, not
crown groups but stem groups. That is, the complete
suite of characters defining the living phyla had not
yet appeared. Many crown groups do not appear in
the fossil record until well after the Cambrian.̂ ^

The existence of stem groups provides a way to under-
stand how the basic body plan of a living invertebrate
could have been built up in steps. The major inverte-
brate groups are often portrayed by evolution critics
as possessing anatomies that are both irreducible in
organization and separated from other groups by
unbridgeable gaps. However, the identification of
stem and sister groups explicitly recognizes the exis-
tence of fossil taxa that possess transitional morpholo-
gies between recognized modern taxonomic groups
(including phyla).

Some critics of evolution make much of the “top-
down” versus the “bottom-up” pattern of appearance
of higher taxa. That is, phylum-level diversity reaches
its peak in the fossil record before class-level diver-
sity, and the class-level diversity before that of orders,
and so forth. These critics interpret this apparent
“top-down” pattern as contrary to expectations from
evolutionary theory. For example, Stephen Meyer
and others have argued:

Instead of showing a gradual bottom-up origin
of the basic body plans, where smaller-scale
diversification or speciation precedes the advent
of large-scale morphological disparity, disparity
precedes diversity. Indeed, the fossil record shows
a “top-down” pattern in which morphological
disparity between many separate body plans
emerges suddenly and prior to the occurrence of
species-level (or higher) diversification on those
basic themes. ‘̂

However, this pattern is an artifact, being gener-
ated by the way in which species are assigned to
higher taxa. The classification system is hierarchi-
cal with species being grouped into ever larger and
more inclusive categories. When this classification
hierarchy is applied to a diversifying evolutionary
tree, a “top-down” pattern will automatically result.
Consider species belonging to a single evolving
line of descent given genus-level status. This genus
is then grouped with other closely related lines of

70 Perspectives on Science and Christian Faith

Keith B. Miiler

descent into a family. The common ancestors of these
genera are by definition included within that family.
Those ancestors must logically be older than any of
the other species within the family. Thus the family-
level taxon would appear in the fossil record before
most of the genera included within it. Another way
of looking at this is the fact that the first appearance
of any higher taxon will be the same as the first appear-
ance of the oldest lower taxon within the group. For
example, a phylum must be as old as the oldest class it
contains. Most phyla contain multiple classes, which
in turn include multiple orders, and so forth. Thus,

each higher taxon will appear as early as the first of
the included lower taxa. The “top-down” pattern of
taxa appearance is therefore entirely consistent with
a branching tree of life.

There is one last bias in our reconstruction of the
past that is generated by the process of assigning
organisms to a particular phylum. Because phyla
are defined by particular anatomical character traits,
they cannot be recognized in the fossil record until
after those specific characters evolve. However, the
splitting of the branch of the tree of life to which

Taxon D

Taxon A Taxon

crown group stem group stem crown group

Taxon C

stem lineage
of taxon D

Figure 2. Diagram illustrating the difference between stem and crown groups. The crown group includes the living organisms that possess
the characters used to define a modern taxonomic group, and all of the extinct fossil organisms that were descended from the last
common ancestor of all members of the crown group. The extinct fossil organisms of the stem group possess some, but not all, of the
characters diagnostic of the crown group, and are more closely related to the crown group than any other organisms. A sister group
includes those organisms that are more closely related to the total group (crown and stem group) than to any other group of organisms.
In this diagram, taxon A and taxon B are sister groups, and taxon C is a sister group to the more inclusive taxon D. (This diagram was
modified from the Palaeos website, http://www.palaeos.org/Crown_group.)

Volume 66, Number 2, June 2014 71

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The Fossil Record of the Cambrian “Explosion”: Resolving the Tree of Life

a phylum belongs may have occurred many millions
of years previous to the evolution of those charac-
ters. The characters that we use to define a phylum
very likely do not correspond to the characters that
actually marked the initial separation of that evolu-
tionary lineage from its closest relative. The actual
first appearance of a fossil assignable to a particular
phylum thus would likely occur after significant ana-
tomical evolution has occurred along that particular
branch of the tree. Branching points in the tree of life
will always be older than the named

The Completeness of the
Fossil Record
There are two opposite errors which need to be coun-
tered about the fossil record: (1) that it is so incomplete
as to be of no value in interpreting patterns and trends
in the history of life, and (2) that it is so good that we
should expect a relatively complete record of the
details of evolutionary transitions within all or most
lineages.

What then is the quality of the fossil record? It can
be confidently stated that only a very small fracfion
of the species that once lived on Earth have been
preserved in the rock record and subsequently dis-
covered and described by science.^’

There is an entire field of scientific research referred
to as “taphonomy”—literally, “the study of death.”
Taphonomic research includes investigating those
processes active from the time of death of an organ-
ism until its final burial by sediment. These process-
es include decomposition, scavenging, mechanical
destruction, transportation, and chemical dissolu-
tion and alteration. The ways in which the remains
of organisms are subsequently mechanically and
chemically altered after burial are also examined —
including the various processes of fossilization. Buri-
al and “fossilization” of an organism’s remains in no
way guarantees its ultimate preservation as a fossil.
Processes such as dissolution and recrystallization
can remove all record of fossils from the rock. What
we collect as fossils are thus the “lucky” organisms
that have avoided the wide spectrum of destructive
pre- and post-depositional processes arrayed against
them.

Soft-bodied organisms and organisms with non-
mineralized skeletons have very little chance of

preservation under most environmental conditions.
Until the Cambrian, nearly all organisms were soft
bodied, and even today the majority of species in
marine communities are soft bodied. The discov-
ery of new soft-bodied fossil localifies is always met
with great enthusiasm. These localities typically turn
up new species with unusual morphologies, and
new higher taxa can be erected on the basis of a few
specimens! Such localities are also erratically and
widely spaced geographically and in geologic time.

Even those organisms with preservable hard parts are
unlikely to be preserved under “normal” conditions.
Studies of the fate of clam shells in shallow coastal
waters reveal that shells are rapidly destroyed by
scavenging, boring, chemical dissolution, and break-
age. Environments with high sedimentation rates,
or those with occasional rapid sedimentation during
major storm events, tend to favor the incorporation of
shells into the sedimentary record, and their ultimate
preservation as fossils.̂ ”

The potential for fossil preservafion varies dramati-
cally from environment to environment. Preservation
is enhanced under conditions that limit destructive
physical and biological processes. Thus marine and
fresh water environments with low oxygen levels,
high salinities, or relatively high rates of sediment
deposition favor preservation. Similarly, in some en-
vironments biochemical conditions can favor the early
mineralization of skeletons and even soft tissues by
a variety of compounds (e.g., carbonate, silica, pyrite,
phosphate). The likelihood of preservation is thus
highly variable. As a result, the fossil record is biased
toward sampling the biota of certain types of environ-
ments, and against sampling the biota of others.

In addition to these preservational biases, the erosion,
deformation, and metamorphism of originally fos-
siliferous sedimentary rock have eliminated signifi-
cant portions of the fossil record over geologic time.
Furthermore, much of the fossil-bearing sedimen-
tary record is hidden in the subsurface, or located in
poorly accessible or little studied geographic areas.
For these reasons, of those once-living species actu-
ally preserved in the fossil record, only a small por-
tion have been discovered and described by science.
However, there is also the promise, and reality, of
continued new and important discovery as new sedi-
mentary units are examined, and new techniques are
applied. The rapidity with which new fossil discover-

72 Perspectives on Science and Christian Faitti

Keith B. Miller

Íes are being made within Neoproterozoic and Cam-
brian strata is actually quite remarkable.^^

The forces arrayed against fossil preservation also
guarantee that the earliest fossils known for a given
animal group will always date to some time after that
group first evolved. The fossil record always pro-
vides only minimum ages for the first appearance of
organisms.

Because of the biases of the fossil record, the most
abundant and geographically widespread species of
hardpart-bearing organisms would tend to be best
represented. Also, short-lived species that belonged
to rapidly evolving lines of descent are less likely to be
preserved than long-lived stable species. Because evo-
lutionary change is probably most rapid within small
isolated populations, a detailed species-by-species
record of such evolutionary transitions is unlikely to
be preserved. Furthermore, capturing such evolution-
ary events in the fossil record requires the fortuitous
sampling of the particular geographic locality where
the changes occurred.

Using the model of a branching tree of life, the expec-
tation is for the preservation of isolated branches on
an originally very bushy evolutionary tree. A few
of these branches (lines of descent) would be fairly
complete, while most are reconstructed with only
fragmentary evidence. As a result, the large-scale
patterns of evolutionary history can generally be
better discerned than species-by-species transitions.
Evolutionary trends over longer periods of time and
across greater anatomical transitions can be followed
by reconstructing the sequences in which anatomical
features were acquired within an evolving branch of
the tree of life.

The Precannbrian Fossil Record
A very important concern is what organisms existed
before the Cambrian “explosion.” Were there Precam-
brian precursors, or did the Cambrian “explosion”
really happen in a biological vacuum? Many critics
of evolution claim that the Precambrian is devoid of
fossils that could represent body plans ancestral to
those of the Cambrian invertebrates.

The words of Darwin are often cited as evidence of
the seriousness of the problem for evolution.

There is another and allied difficulty, which is
much more serious. I allude to the manner in

which species belonging to several of the main
divisions of the animal kingdom suddenly appear
in the lowest known fossiliferous rocks. Most
of the arguments which have convinced me that
all the existing species of the same group are
descended from a single progenitor, apply with
equal force to the earliest known species.^

When Darwin published his model of descent with
modification by means of natural selection, knowl-
edge of the fossil record was in its infancy. In par-
ticular, the Precambrian and Early Cambrian fossil
record was virtually unknown. Even the fossils of
the now famous Burgess Shale and similar units were
as yet undiscovered. After nearly a century and a half
of paleontological work, the situation has changed
dramatically. In keeping with evolutionary expecta-
tions, fossils are now known from the late Precambri-
an and early Cambrian that record several dramatic
transitions in the history of life.

The presence of Late Precambrian animals was recog-
nized in the 1950s and became widely publicized by
the early 1970s. These are the famous Ediacaran fos-
sils named for fossil-rich beds in the Ediacara Hills of
South Australia and now recognized at sites through-
out the world. These organisms are typically pre-
served as impressions in sandstones and siltstones.
Associated with these fossils are trails and simple
burrows of organisms that show a limited increase in
complexity and diversity toward the Cambrian.

The record of life actually extends far beyond the
Ediacaran fossils (-575-542 My) into the deep geo-
logic past. Fossils of algae, protists, and bacteria
are present throughout much of the Precambrian. The
earliest convincing fossils of bacteria are recognized
in rocks 3.5 billion years old, and chemical signatures
point to the presence of life even earlier. Finely lay-
ered mounds (called stromatolites) produced by the
activity of mat-building bacteria and algae appear
at about this time and become relatively abundant
by around 2.7 billion years ago. Evidence of eukary-
otic algae, possessing membrane-bounded nuclei
and internal organelles, dates to about 1500 million
years ago, or earlier if chemical evidence is accepted.
Multicellularity had appeared by 1000 million years
ago in the form of diverse and relatively advanced
seaweeds.^’ The earliest fossils of metazoans (multi-
celled animals) may be represented by simple disk-
shaped fossils found in rocks 610-600 million years

Volume 66, Number 2, June 2014 73

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The Fossil Record of the Cambrian “Explosion”: Resolving the Tree of Life

The earliest unambiguous indication of the rise
of metazoan life is preserved in the spectacular
phosphorite deposits of the Doushantuo Formafion
of China dating to at least 580 million years ago.
Phosphate can preserve organisms and tissues in such
great detail that individual cells can often be recog-
nized. Where environmental conditions are ideal for
this type of preservation, extraordinary fossil depos-
its may result. In the case of the Doushantuo, phos-
phatization has preserved not only a variety of algal
remains, but also the cellular tissues and spicules of
sponges.^^ These sponges appear to belong to the class
Demospongia. However, even more spectacular with-
in the Doushantuo phosphorites is the preservation
of metazoan eggs and early embryos. These embryos
are of uncertain affinities, but they may represent
stem cnidarians (the phylum including “jellyfish,”
anemones, and corals) or even bilaterians (animals
with bilateral symmetry).^” Recently described milli-
meter-sized phosphatic tubes with internal chambers
and apical budding also suggest a cnidarian affinity.̂ ^

The Ediacaran biota provide the next window into
the rise of metazoans. These fossil-bearing units
span from about 575 million years to the base of the
Cambrian (an interval of -33 million years), and are
found in south Australia, Namibia, the White Sea coast
of Russia, and Newfoundland. The enigmatic soft-
bodied organisms were preserved as impressions, or
molds, on the surfaces of sandstone and siltstone lay-
ers. These sediment layers accumulated in shallow-
marine environments where the seafioor was covered
by firm microbial algal mats. The microbial mats cov-
ering the seafioor appear to have been important in
determining the lifestyles of the Ediacaran organisms,
as well as their unique mode of preservation.^^

Most soft-bodied impressions of the Ediacaran
can roughly be placed into three general groups —
disks; fronds; and ñat-bodied, bilaterally-symmetric
forms. The biological affinity of these fossils is very
difficult to determine and highly debated.^’ Disks
are the earliest appearing, and most common, Edia-
caran fossils. A few disk-shaped fossils are fossil
impressions of sponges. One such form appears to
be a sponge that might be assignable to the modern
class of Hexactinellida.^”

Many disk-shaped impressions have often been iden-
tified as medusoids (“jellyfish”) but many appear to
have been attached to the bottom, and none bear clear
structures that would place them in a living group.

Some do clearly possess tentacles around their mar-
gins, suggesting a stem or sister group relationship
to the cnidarians. Furthermore, recent descriptions
of very small phosphatized fossils that predate the
Cambrian by 25 million years or more have demon-
strated the presence of cnidarians that might even be
stem anthozoans (the cnidarian class that includes
anemones and corals) .̂ ^

The frond-shaped forms include organisms that
were attached to the bottom by a stalk, and others
that appear to have been free lying. These fossils
have also been assigned by some workers to a group
of modern cnidarians (the “sea pens”) or to cteno-
phores. However, like the disks, the fronds are fairly
diverse and some may be unrelated to living phyla.”̂
Others, although likely not able to be placed into
a living cnidarian group, may be stem cnidarians, or
even stem anthozoans. The discovery of better pre-
served fronds in the Cambrian that closely resemble
some of the Ediacaran fossils would seem to support
this interpretation.^^

The bilaterally symmetric forms of the Ediacaran are
the most diverse and most enigmatic fossils of the
late Precambrian. Some of these fossils may represent
early experiments on the pathway to the living phy-
la.’* For example, Dickinsonia and the similar Yorgia
are fairly large flat highly segmented forms that some
workers have interpreted as annelids or stem anne-
lids, while others have seen resemblances to other
worm phyla or even chordates. These organisms do
appear to have been able to move about the bottom
as seen by associated crawling and resting traces.
Even if not members of a living phylum, these organ-
isms appear to at least be mobile bilateral metazoans
(or bilaterians).

Another bilateral form that has been the subject
of much recent attention is the 555-million-
year-old mollusk-like Kimberella (see fig. 3). This
organism appears to have lacked several features
characteristic of modern moliusks and thus has been
interpreted as a stem mollusk.’^ Scratch marks found
associated with Kimberella indicate that it had some
form of feeding structure (though probably not a true
mollusk radula) that enabled it to graze the abundant
algal mats.

An important, but less attenfion-getting, component
of the Ediacaran fossil record is the presence of trace
fossils such as trails, burrows, and feeding traces.

74 Perspectives on Science and Christian Faith

Keith B. Miller

Figure 3. Examples of stem mollusks and annelids, and of
halwaxiids, a possible sister group of the annelids, include (A) the
probable stem mollusk Kimberelia from the Ediaoaran; (B) the
Cambrian stem mollusk Odontogriphus; (C) the early Cambrian
halwaxiid i-laikieria with mineralized solerites covering the body, and
anterior and posterior mollusk-like shells; (D) the early Cambrian
halwaxiid Wiwaxia covered in unmineralized chitinous sclerites
similar to the setae of annelids, and possessing long ribbed spines;
(E) the recently described middle Cambrian halwaxiid Orthozanclus
with slender unmineralized chitinous spines and a single anterior
mollusk-like shell; and (F) the middle Cambrian stem annelid
Canadia with rigid setae extending from lateral outgrowths of the
body. (A is modified from reconstruction by M. A. Fedonkinand B. M.
Waggoner, “The Late Precambrian Fossil Kimbereiia Is a Mollusc-
like Bilaterian Organism,” Nature 388 [1997]: 868-71. B is redrawn
from reconstruction in J. B. Caron, A. Scheltema, C. Schänder, and
D. Rudkin, “A Soft-Bodied Mollusc with Radula from the Middle
Cambrian Burgess Shale,” Nature 442 [2006]; 159-63. C is based
on the illustration in Susannah Porter’s website http://www.geol
.ucsb.edu/faculty/porter/Early_Animals.html by Jennifer Osborne.
D is based on the illustration at the website of the Burgess Shale
Geoscience Foundation http://www.burgess-shale.bc.ca/discover
-burgess-shale/ancient-creatures/wiwaxia. E is redrawn from
S. Conway Morris and J. B. Caron, “Halwaxiids and the Early
Evolution of the Lophotrochozoans,” Science 315 [2007]: 1255-
8. F is drawn based on specimen shown at the Royal Ontario
Museum website, http://burgess-shale.rom.on.ca/en/fossil-gallery
/list-species.php.)

Except in the few cases mentioned above, there are
no body fossils preserved of the organisms that made
these traces. These traces tend to be small unbranched
sediment-filled burrows that run horizontally along
the sediment surface or under the microbial algal
mats. Somewhat more complex burrows appear
toward the base of the Cambrian, including irregu-
larly branching burrows and shallow vertical bur-
rows.’^ These traces are important because they point
to the existence of small worm-like organisms that
were probably feeding on and in the algal mats that
covered extensive areas of the seafioor. The biological
identity of these burrowing organisms is unknown,
although they were clearly bilaterian.

There is one more set of fossils that are known from
the late Ediacaran (550-543 million years) that reveal
yet another aspect of the metazoan diversity before
the Cambrian. These fossils include tiny calcified or
phosphatized tubes, cones, and goblet-shaped struc-
tures that record the presence of animals capable
of producing mineralized skeletons. They are com-
monly embedded within algal buildups that formed
reef-like structures, and are locally quite abundant.^”
These algal-metazoan reefs foreshadow the later
algal reefs of the Cambrian. The very peculiar cm-
sized goblet-shaped Namacalathus (found as calci-
fied fossils) lived attached to the algal mounds by
stalks. Although the preserved shape of these fossils
is consistent with that of cnidarians, their biology is
uncertain. The tiny partitioned and budded tubes
of Sinocydocyclicus bear a strong resemblance to the
skeletons of some primitive corals.̂ * The cone-in-cone
structures of Cloudina, and the more tubular Sinotu-
bulites could have been produced by various types of
worms such as serpulids. However, as with the trace
fossils, the identity of the actual tube formers remains
unknown. A significant observation of the Cloudina
fossils is that many of them are perforated by bor-
ings. These borings provide the first clear evidence of
prédation before the Cambrian.

It is clear from the above discussion of the latest Pre-
cambrian, that the Cambrian “explosion” did not
occur in a biological vacuum. Although many of the
fossil specimens are enigmatic and difficult to clas-
sify, they nonetheless show significant biological di-
versity. Furthermore, at least a few living phyla had
already appeared by the beginning of the Cambrian,
and other forms likely represented sister groups or
stem groups related to later-evolving phyla.

Volume 66, Number 2, June 2014 75

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The Fossil Record of the Cambrian “Explosion”: Resolving the Tree of Life

The Cambrian Record of
Evolutionary Transitions
One of the most important features of the Cambrian
“explosion” was the rapid diversification of organ-
isms with shells, plates, and various other types
of hard parts. A wide variety of soft-bodied organ-
isms are also known from the Cambrian. Although
some fossils can be assigned to living phyla, there
are also specimens that appear to represent stem
groups or intermediates between modern phyla, as
well as specimens of unknown relationship. Repre-
sentatives of several living classes and other lower
taxonomic categories also appear in the Cambrian.
A few deposits with exceptionally good preserva-
tion of fossils, such as the Burgess Shale in Canada,
contribute to the wide range of taxa known from the
Cambrian. Such deposits with exceptional preserva-
tion are known as Konservat-Lagerstätten (from the
German “conservation deposits”). Similar deposits
have since been found around the world in the Ear-
ly to Middle Cambrian, notably the Early Cambrian
Chengjiang fauna of China. Additionally, trace fossils
become much more varied, complex, and abundant in
the Cambrian, suggesting a newly widened range of
animal activity.

As stated earlier, the first appearance of the burrow
Treptichnus pedum defines the base of the Cambrian.
The organisms forming those burrows were likely
priapulid worms, a worm phylum that is well repre-
sented among the Chengjiang and Burgess fossils.^’
Significantly, the early Cambrian is marked by a sig-
nificant increase in the diversity of burrows associ-
ated with the onset of vertical mixing of the sediment
by organisms, and the destruction and loss of the
algal mat-grounds that characterized the Ediacaran.
This “substrate revolution” from stable firm ocean
floors to soft, muddy, turbid ones, had a major impact
on the bottom-dwelling organisms of the Cambrian.””
Organisms responded by becoming more mobile, and
by moving below the sediment surface and into the
overlying water column.

Some of the very first fossils to appear near the base
of the Cambrian are tiny skeletal plates, spines,
tubes, and cap-shaped shells that have been called
the “small shelly fossils.”*^ Among these are the spic-
ules of different groups of sponges and the shells
of the earliest known “crown group” moUusks and
brachiopods. However, the biological identities of
many of these tiny skeletal elements were completely

unknown until fairly recently. Well-preserved com-
plete fossils in the Chengjiang, and other fossil lager-
stätten around the world, have revealed that many
of these small shelly fossils were actually the spines
and “armoring” of larger metazoans. More detailed
analysis of other fossils has revealed that they may
represent the stem groups of living phyla rather than
evolutionary dead ends.

The discovery of complete specimens from later in the
early Cambrian has revealed that a variety of scales,
plates, and spines found among the “small shelly fos-
sils” actually fit together and overlapped to cover the
bodies of slug-like organisms.*^ These organisms are
the halkieriids and wiwaxiids (fig. 3). The halkieriids
bore conical mollusk-like shells as well as calcare-
ous structures similar to the chitinous bristles typical
of polychaete annelid worms. The slightly younger
Wiwaxia was covered in scale-like and spine-like
structures even closer to those of the polychaetes, cind
also possessed a radula diagnostic of mollusks. These
various unusual organisms bear resemblances to both
mollusks and polychaete annelid worms, which are
closely related phyla.*^ Thus these organisms would
appear to be positioned somewhere on the evolution-
ary tree near the branching point of the mollusks with
the annelids. Stem group polychaete annelid worms
also appear in the early Cambrian.**

The first likely “crown group” mollusks appear
in the earliest Cambrian as part of the small shellys.
While recognizable as mollusks, many of these fos-
sils belong either to sister groups or to stem groups
of living classes. Cap-shaped fossils called helcionel-
loids are interpreted as monoplacophoran-like crown
group mollusks. There is good fossil evidence of
the transition from^ these primitive cap-shaped hel-
cionelloids to the first bivalves by way of the extinct
group of rostroconchs. The hinged valves of clams
appear to have evolved by the lateral compression
of cap-shaped shells and then the thinning and loss
of shell material along the hinge line.*^ There are also
likely fossil transitions from coiled helcionelloids to
the first gastropods.

Another important group of orgarüsms represented
by small plates in the early Cambrian are the lobo-
pods. Lobopodians, until very recently an enigmatic
group of strange fossils, were “caterpillar-like” organ-
isms with fleshy lobed limbs and mineralized plates
or spines running along their backs. They are similar
to the living Onychophora, or velvet worms, but are

76 Perspectives on Science and Christian Faith

Keith B. Miller

considered a distinct group.””” The oldest known lobo-
podian bears certain similarities to a distinctive group
of worms called the palaeoscolecid priapulids that
also bore small plates or tubercles along their bod-
ies.*^ Lobopods may have been derived from these
worms that also have an early Cambrian fossil record.
Furthermore, the lobopods have become recognized
as the critical link in reconstructing the assembly of
the arthropod body plan. They have anatomical fea-
tures in common with the arthropods, particularly
with peculiar Cambrian stem arthropods such as
Opabinia and Anomalocaris that are preserved in the
younger Chengjiang and Burgess fossil beds. These
later organisms possessed lobopod limbs but also
had gill flaps along their bodies and jointed feeding
appendages. Intermediates between lobopodians and
the early stem group arthropods have also been dis-
covered that possessed gills.*^ Of even greater interest
is the evidence available from the extraordinary pres-
ervation of muscle tissue in a few of these transitional
organisms. These specimens suggest a progression of
steps in the transformation of internal anatomy from
lobopodians to true arthropods.*’

The tommotiids, a group of tiny roughly conical-
shaped shells composed of calcium phosphate, have
been, until recently, one of the most enigmatic of
the small shelly fossils. However, new discoveries
of articulated specimens have shown that pairs of
symmetrical skeletal elements fit together to form
an open cone that was attached to the seafioor at the
base. An opening at the base indicates the presence of
a muscular attachment structure likely similar to the
pedicle of brachiopods. The paired shells also have
features similar to the tiny paterinids, crown group
brachipods with calcium phosphate shells that also
appear in the early Cambrian.^” These fossils there-
fore appear to represent stem brachiopods that were
themselves derived from armored tubular filter feed-
ers attached to the seafloor (fig. 4).

The living phoronid worms are a phylum closely
related to the brachiopods. Like the brachiopods,
they are filter-feeders using a ring of ciliated tentacles
called a lophophore. However, unlike brachiopods,
they are not enclosed within paired shells but con-
struct chitinous tubes. The recent description of an
early Cambrian unmineralized, “soft-shelled” lin-
gulid brachiopod strongly suggests that phoronids
evolved from crown-group brachiopods by the loss of
a mineralized shell.̂ ^ This transitional form also pro-
vides evidence for the transformation of the muscu-

lature from that typical of shelled brachiopods to the
longitudinal arrangement of phoronids. These “soft-
shelled” brachiopods are likely stem-phoronids.

Following the appearance of the small shelly fossils,
the diverse metazoan fossil communities of the
Chengjiang in China are dated at around 525- 520
million years, 20 million years after the beginning of
the Cambrian. The exceptional preservation in these
fossil beds is similar to that of the Burgess Shale
deposits that are dated around 515-505 million years.

Figure 4. These fossils illustrate the transition from tommotiids
to braehiopods: (A) the conical phosphatic shell of the tommotiid
Eccentrottieca with an opening at the apex (seale bar 0.5mm);
(B) the tommotiid and stem brachiopod Paterimitra with a conical
shell of articulated phosphatic sclerites, a “pedicle tube” for
attachment, and an upper valve (scale bar 0.2mm); (C) the bivalved
Micrina, the most brachiopod-iike tommotid yet known (scaie
bar 0.5m); and (D) the early Cambrian crown group brachiopod
Psitoria (sheil about 1 cm across). (A is drawn from an image in
C. B. Skovsted, G. A. Brock, J. R. Paterson, L. E. Holmer, and G. E.
Budd, “The Scleritome of Eccentrotheca from the Lower Cambrian
of South Australia; Lophophorate Affinities and Implications for
Tommotiid Phyiogeny,” Geology 36 [2008]: 1 7 1 – ^ . B is drawn from
an illustration in C. B. Skovsted, L. E. Holmer, C. M. Larsson, A. E.
S. Högström, G. A. Brock, T P Topper, U. Balthasar, S. P Stolk, and
J. R. Paterson, “The Scleritome of Paterimitra: An Early Cambrian
Stem Group Brachiopod from South Austraiia,” Proceedings of the
Royal Society B 276 [2009]: 1651-6. C is drawn from L. E. Holmer,
C. B. Skovsted, G. A. Brock, J. L. Valentine, and J. R. Paterson,
“The Early Cambrian Tommotiid Micrina, A Sessile Bivalved Stem
Group Brachiopod,” Bioiogy Letters 4 [2008]: 724-8. D is drawn
from an iilustration at the website http://www.museumwales.ac.uk
/en/1625/.)

Volume 66, Number 2, June 2014 77

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The Fossil Record of the Cambrian “Explosion”: Resolving the Tree of Life

These extraordinary fossil sites give us our best views
into the composition of marine biological communi-
ties from these times, preserving both soft-bodied
organisms and those with mineralized skeletons.^^
These beds contain abundant and diverse sponges
and cnidarians, as well as priapulid worms, annelid
worms, lobopods, stem mollusks such as Wiwaxia,
and brachiopods. However, probably the most dra-
matic characteristic of the Chengjiang- and Burgess
Shale-type deposits is the abundance and diversity of
arthropods.

Arthropods comprise 50% or more of all of the fos-
sil specimens collected from these beds. These fossils
include stem arthropods such as the anomalocarids,
trilobites which came to dominate the Paleozoic, and
some species that appear to be crustaceans and chelic-
erates. However, most of the fossils belong to primi-
tive stem groups that likely represent evolutionary
dead ends after the appearance of true arthropods
but before the rise of most living arthropod groups.
In the Burgess Shale, one such primitive species {Mar-
rella) alone comprises a third of all fossil specimens.
These fossils show unusual arrangements, and types,
of appendages.

The chordates (that include vertebrates), hemi-
chordates (that include the living “acorn worms”),
and echinoderms (that include the living starfish and
echinoids) are all deuterostomes and have the same
pattern of early embryo development. Although
the modern representatives of these phyla appear
extremely different, they are actually closely related
branches on the tree of life, and are understood to
have evolved from a common ancestor. Some rare, but
very significant, specimens in the Chengjiang seem to
be stem chordates and stem echinoderms, as well as
specimens that have been interpreted as organisms
close to the common ancestors of chordates and echi-
noderms. These rather simple Cambrian organisms
possess the anatomical characteristics that would
be expected in organisms that had acquired some,
but not all, of the distinctive features of chordates
or echinoderms.

A newly described group of primitive soft-bodied
deuterostomes, called vetulocystids, bears similari-
ties to some of the bizarre early echinoderms. These
organisms rested on the bottom and possessed echino-
derm-like respiratory openings and two ribbed cones
that likely represented the mouth and anus. Unlike all

living echinoderms, however, they lacked any calcite
skeletal plates.̂ ^ They may represent organisms that
belonged to a sister group ancestral to the first stem-
group echinoderms. The most primitive echinoderms
were characterized by flattened, nearly bilaterally
symmetrical forms. The earliest stage of echinoderm
evolution is represented by Ctenoimbricata from the
early middle Cambrian. These flattened stem echino-
derms were completely covered on their lower side
by calcite plates, but were largely uncalcified on their
upper (dorsal) sides.’* The ctenocystoids and cinctans
were similar stem echinoderms that show increasing
coverage of their dorsal sides by interlocking calcite
plates (fig. 5).

Figure 5. Stem echinoderms and the early evolution of the
Echinodermata as iiiustrated by (A) the soft-bodied ventulocystid
Ventulocystis, a primitive deuterostome, possibly a sister group
to the stem echinoderms; (B) Ctenoimbricata is the most primitive
known stem echinoderm with only scattered calcified elements on
the dorsal side; (C) the ctenocystid Courtessolea, a siightiy more
derived stem echinoderm; (D) the cinctan Sotocinctus, a stem
echinoderm with a body compieteiy covered by caicite piates and
a “taii” appendage; and (E) the stem group soiute Syringocrinus
with a “taii” and feeding arm appendage. (A is drawn from an illus-
tration in D-G. Shu, S. Conway iVIorris, J. Han, Z-F. Zhang, and
J-N. Liu, “Ancestral Echinoderms from the Chengjiang Deposits
of China,” Nature 430 [2002]: 422-8. B, C, and D are redrawn
from S. Zamora, i. A. Rahman, and A. B. Smith, “Plated Cambrian
Bilaterians Reveal the Earliest Stages of Echinoderm Evolution,”
PLoS 0NE7, no. 6 [2012], e38296. doi:10.1371. E is redrawn from
the Paiaeos website http://palaeos.com/metazoa/deuterostomia
/homalozoa/soluta.html.)

78 Perspectives on Science and Christian Faith

Keith B. IVIiller

Another very primitive stem group of deuterostomes,
called ventulicolians, has also recently been described
that might represent the anatomy of organisms near
the base of the deuterostome evolutionary branch
that were ancestral to both the chordates and echi-
noderms. These soft-bodied organisms possessed
segmentation and oval structures interpreted as gill
slits, and a terminal mouth.̂ ^ The most primitive
group of chordates are the urochordates, or tunicates,
that have a sack-like adult body that filters seawater
through pharyngeal slits. In their tadpole-like larval
form, they possess stiff notochords (a structure diag-
nostic of chordates) that is lost in the adult form. A
likely tunicate has been described from the Chengji-
ang. ‘̂’ Another group of primitive chordates are the
cephalochordates (represented today by the lancelets)
that possess a notochord as adults, pharyngeal slits,
and muscles arranged in parallel bundles. Some fos-
sils have been interpreted as stem cephalochordates.^^
Lastly, and of particular interest, is a fossil that may
be a stem vertebrate.’^ Myllokunmingia, in addition
to a notochord, gill pouches and muscle bundles, also
appears to have had some structures characteristic
of vertebrates. These vertebrate features include a
cavity surrounding the heart, a dorsal fin, and carti-
lage around the head and as a series of elements along
the notochord. The Chengjiang thus includes fossil
specimens that occupy several significant transitional
stages from primitive deuterostomes to stem echino-
derms and stem chordates (fig. 6).

Conclusions
Given our current, and continually growing, knowl-
edge of the deep past, it is increasingly clear that
the rise of multicellular animals is not an impenetrable
mystery. While there is much that is not known, and
some that will never be known, there is also much that
has been discovered, and much excitement for what
will yet be learned. New discoveries and analyses are
continually adding to our knowledge of evolutionary
transitions in the latest Precambrian and Cambrian.

The Cambrian “explosion” was a time of great evolu-
tionary significance, as it established the anatomical
templates for much of the diversification to come. It
was also extraordinary in that it was a time of accel-
erated evolutionary change for marine organisms
across the animal kingdom. However, despite its rela-
tive rapidity, the time during which the rise of mod-
ern animal phyla occurred was still a lengthy interval.

with the Early Cambrian alone lasting 32 million
years. Furthermore, critical evolutionary innovations
were established in the 40 million years of the Edia-
caran preceding the Cambrian.

D
Figure 6. The evolution of chordates from primitive deuterostomes
as illustrated by (A) the vetulicolian Vetulicoia interpreted as
a stem deuterostome with some features suggestive of chordates;
(B) the lancelet-like stem chordate Haikoueiia (about 3cm long);
(C) Haikouichthys, another likely stem chordate (about 2.5cm
long); and (D) Myllokunmingia, is a possible stem vertebrate (about
3cm long). (A is drawn from an illustration in D-G. Shu, S. Conway
Morris, J. Han, L. Chen, X-L. Zhang, Z-F. Zhang, H-Q. Liu, Y. Li, and
J-N. Liu, “Primitive Deuterstomes from the Chengjiang Lagerstätte
(Lower Cambrian, China),” Nature 4^4 [2001 j : 419-24. B is redrawn
from J-Y. Chen, D-Y. Huang, and C-W. Li, “An Early Cambrian
Craniate-Like Chordate,” Nature 402 [1999J: 518-22. C is modified
from a reconstruction in X-G. Zhang and X-G. Hou, “Evidence for a
Single Median Fin-Fold and Tail in the Lower Cambrian Vertebrate,
Haikouichthys ercaicunensis,” Journai of Evolutionary Bioiogy 17,
no. 5 [2004J: 1162-6. D is drawn from an illustration in D-G. Shu,
H-L. Luo, S. Conway Morris, X-L. Zhang, S-X. Hu, L. Chen, J. Han,
M. Zhu, Y. Li, and L-Z. Chen, “Lower Cambrian Vertebrates from
South China,” Nature 402 [1999]: 42-6.)

Volume 66, Number 2, June 2014 79

irtíeli
The Fossil Record of the Cambrian “Explosion”: Resolving the Tree of Life

T h e a n i m a l s of t h e C a m b r i a n d i d n o t a p p e a r i n all
t h e i r m o d e r n c o m p l e x i t y o u t of a v o i d , b u t r a t h e r t h e y
p r o v i d e p o i n t e r s to t h e i r c o m m o n a n c e s t r y . D e s p i t e
t h e c l a i m s of e v o l u t i o n s k e p t i c s , t h e fossil r e c o r d p r o –
v i d e s m u l t i p l e e x a m p l e s of o r g a n i s m s d i s p l a y i n g
t r a n s i t i o n a l a n a t o m i e s . A s w e h a v e s e e n , t h e s e fossil
o r g a n i s m s w e r e l a r g e l y r e p r e s e n t a t i v e of s t e m g r o u p s
t h a t p o s s e s s e d s o m e , b u t n o t all, of t h e d i a g n o s t i c
f e a t u r e s t h a t define t h e m a j o r g r o u p s of l i v i n g o r g a n –
i s m s . T h e a n a t o m i c a l c h a r a c t e r s t h a t define t h e b o d y
p l a n s of t h e major l i v i n g a n i m a l p h y l a c a n b e s e e n t o
h a v e b e e n a c q u i r e d p i e c e m e a l d u r i n g t h e e a r l y e v o l u –
t i o n of t h e m e t a z o a n s . J u s t as w i t h all o t h e r t a x o n o m i c
g r o u p s (e.g., classes, o r d e r s , families, g e n e r a , species),
t h e d i v i s i o n s b e t w e e n p h y l a b r e a k d o w n as w e m o v e
closer to t h e i r t i m e s of o r i g i n f r o m c o m m o n a n c e s t o r s .
W h i l e t h e p i c t u r e is i n c o m p l e t e , r e c e n t s p e c t a c u l a r
fossil d i s c o v e r i e s s t r o n g l y s u p p o r t t h e c o n c l u s i o n t h a t
t h e major b r a n c h e s of t h e a n i m a l t r e e of life a r e j o i n e d
to a c o m m o n m e t a z o a n t r u n k . T h e t r e e of life c o n t i n –
u e s t o s t a n d tall. »és

Acknowledgments
I would like to thank the editors at BioLogos for
initially encouraging me to write a blog series on
the Cambrian explosion. Those essays formed the
foundation for this article. I also greatly appreci-
ate the very thorough and constructive comments
by an anonymous PSCF reviewer. Any errors of fact
or interpretation are mine alone.

Notes
^Some material in this article appeared earlier in the six-part
blog series “The Cambrian ‘Explosion,’ Transitional Forms,
and the Tree of Life” on The BioLogos Forum, December
2010 to March 2011, http://biologos.org/.
Ĵ. D. Morris, “The Burgess Shale and Complex Life,” Acts &
Facts 37, no.lO (2008): 13.

^ . C. Meyer, M. Ross, P. Nelson, and P. Chien, “The
Cambrian Explosion: Biology’s Big Bang/’ in Darwinism,
Design and Public Education, ed. J. A. Campbell and S. C.
Meyer (Lansing, MI: Michigan State University Press,
2003), 326.

^Ibid., 333.
^A. Y. Rozanov, “The Precambrian-Cambrian Boundary in
Siberia,” Episodes 7 (1984): 20-4; A. Y. Rozanov and A. Y.
Zhuravlev, “The Lower Cambrian Fossil Record of the
Soviet Union,” in Origin and Early Evolution of the Metazoa,
ed. J. H. Lipps and P. W. Signor (New York: Plenum Press,
1992), 205-82.

^E. Landing, “Precambrian-Cambrian Boundary Global
Stratotype Ratified and a New Perspective of Cambrian
Time,” Geology 22, no. 2 (1994): 179-82.

^S. A. Bowring, J. P. Grotzinger, C. E. Isachsen, A. H. Knoll,
S. M. Pelechaty, and P. Kolosov, “Calibrating Rates of
Early Cambrian Evolution,” Science 261 (1993): 1293-8.

T. M. Gradstein et al, A Geologic Time Scale 2004 (New
York: Cambridge University Press, 2004).

‘J. E. Amthor, J. P. Grotzinger, S. Schröder, S. A. Bowring,
J. Ramezani, M. W. Martin, and A. Matter, “Extinction of
Cloudina and Namacalathus at the Precambrian-Cambrian
Boundary in Oman,” Geology 31 (2003): 431-4.

î E. Landing, S. A. Bowring, K. L. Davidek, S. R. Westrop,
G. Geyer, and W. Heldmaier, “Duration of the Early
Cambrian: U-Pb Ages of Volcanic Ashes from Avalon
and Gondwana,” Canadian Journal of Earth Sciences 35
(1998): 329-38.

“E. Landing, A. English, and J. D. Keppie, “Cambrian Ori-
gin of All Skeletonized Metazoan Phyla — Discovery of
Earth’s Oldest Bryozoans (Upper Cambrian, Southern
Mexico),” Geology ?>?, (2010): 547-50.

î A. C. Maloof, S. M. Porter, J. L. Moore, F. Ö. Dudas, S. A.
Bowring, J. A. Higgins, D. A. Fike, and M. P. Eddy, “The
Earliest Cam^brian Record of Animals and Ocean Geo-
chemical Change,” Geological Society of America Bulletin
122, no. 11-12 (2010): 1731-74.

‘̂ An excellent introduction to the interpretation of clado-
grams and evolutionary trees is T. R. Gregory, “Under-
standing Evolutionary Trees,” Evolution: Education &
Outreach 1 (2008): 121-37. For a discussion of how clado-
grams help counter incorrect views of evolution, see also
K.B. Miller, “Countering Common Misconceptions of
Evolution in the Paleontology Classroom,” in Teaching
Paleontology in the 21st Century, ed. M. M. Yacobucci and
R. Lockwood, The Paleontological Society Special Publica-
tions 12 (2012): 109-22.
“̂•See the discussion in J. W. Valentine, “The Nature of
Phyla,” in On the Origin of Phyla (Chicago, IL: Univer-
sity of Chicago Press, 2004), 7-39. Also see K. B. Miller,
“Common Descent, Transitional Forms, and the Fossil
Record,” in Perspectives on an Evolving Creation, ed. K.B.
Miller (Grand Rapids, MI: Wm. B. Eerdmans, 2003),
152-81.

“G. Budd, “Climbing Life’s Tree,” Nature 412 (2001): 487.
•̂”G. E. Budd and S. Jensen, “A Critical Reappraisal of the
Fossil Record of the Bilaterian Phyla,” Biological Reviews
75 (2000): 253-95; S. Conway Morris, “The Cambrian
‘Explosion’: Slow-Fuse or Megatonnage?,” Proceedings of
the National Academy of Science 97, no. 9 (2000): 4426-9.

^^Meyer, Ross, Nelson, and Chien, “The Cambrian Explo-
sion: Biology’s Big Bang,” 346.

^̂ See the discussion in J. W. Valentine, “The Nature of
Phyla,” in On the Origin of Phyla, 7-39.

“A more expanded discussion of this topic can be found
in K. B. Miller, “Common Descent, Transitional Forms,
and the Fossil Record,” 152-81.

2°K. H. Meldahl, K. W. Flessa, and A. H. Cutler, “Time-
Averaging and Postmortem Skeletal Survival in Benthic
Fossil Assemblages: Quantitative Comparisons among
Holocene Environments,” Paleobiology 23 (1997): 207-29.

^^Illustrating this point are the numerous significant fossil
discoveries that have occurred since the publication of
the essay on the Cambrian explosion by David Campbell
and myself only ten years ago. D. Campbell and K. B.
Miller, “The ‘Cambrian Explosion’: A Challenge to

80 Perspectives on Science and Christian Faith

Keith B. iVIiiier

Evolutionary Theory?,” in Perspectives on an Evolving
Creation, ed. Miller, 182-204.

^C. Darwin, On the Origin of Species by Means of Natural
Selection, 6th ed. (1872), 234-5.

^Summaries of the early fossil record of life can be found
in A. H. Knoll, Life on a Young Planet: The First Three Bil-
lion Years of Evolution on Earth (Princeton, NJ: Princeton
University Press, 2003), 277. For descriptions of diverse
eukaryotic algae, see X. Yuan, Z. Chen, S. Xiao, C. Zhou,
and H. Hua, “An Early Ediacaran Assemblage of Macro-
scopic and Morphologically Differentiated Eukaryotes,”
Nature 470 (2011): 390-3.

^̂ A new fossil discovery from Australia has indicated the
presence of possible sponge-grade metazoans in rocks
640-650 million years ago. See A. C. Maloof, C. V. Rose,
R. Beach, B. M. Samuels, C. C. Calmet, D. H. Erwin, Gerald
R. Poirier, N. Yao, and F. J. Simons, “Possible Animal-
Body Fossils in Pre-marinoan Limestones from South Aus-
tralia,” Nature Geoscience 3 (2010): 653-59.

^C-W. Li, J-Y. Chen, and T-E. Hua, “Precambrian Sponges
with Cellular Structures,” Science 279 (1998): 879-82.

^yy. Chen, P. Oliveri, C-W. Li, G-Q. Zhou, F. Gao, J. W.
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4457-62; S. Xiao and A. H. Knoll, “Phosphatized Animal
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from the Precambrian of Southwest China,” Science 312,
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and X. Yuan, “Rare Helical Spheroidal Fossils from the
Doushantuo Lagerstätte: Ediacaran Animal Embryos
Come of Age?,” Geology 35, no. 2 (2007): 115-8.

‘̂S. Xiao, X. Yuan, and A. H. Knoll, “Eumetazoan Fossils
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*̂A. Seilacher, “Biomat-Related Lifestyles in the Precambri-
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°̂J. G. Gehling and K. Rigby, “Long Expected Sponges from
the Neoproterozoic Ediacara Fauna of South Australia,”
Journal of Paleontology 70, no. 2 (1996): 185-95.

3̂ J-Y. Chen, P. Oliveri, F. Gao, S. Q. Dornbos, C-W. Li, D.J.
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^K^. M. Narbonne, M. Laflamme, C. Greentree, and
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Rangeomorphs from Spaniard’s Bay, Newfoundland,”
Journal of Paleontology 83, no. 4 (2009): 503-23.

‘̂’S. Conway Morris, “Ediacaran-Like Fossils in Cambrian
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‘••J. Dzik, “Anatomical Information Content in the Ediacaran
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^̂ M. A. Fedonkin and B. M. Waggoner, “The Late Precam-
brian Fossil Kimberella Is a Mollusc-Like Bilaterian Organ-
ism,” Nature 388 (1997): 868-71.

‘•”T. P. Crimes, “The Record of Trace Fossils across the Pro-
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(2005): 323-56.

^’Z. Chen, S. Bengtson, C-M. Zhou, H. Hua, and Z. Yue,
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and E. W. Mountjoy, “Namacalathus-Cloudina Assemblage
in Neoproterozoic Miette Group (Byng Formation), Brit-
ish Columbia: Canada’s Oldest Shelly Fossils,” Geology 29
(2001): 1091-4; J. P. Grotzinger, W. A. Watters, and A. H.
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Reefs of the Terminal Proterozoic Nama Group, Namibia,”
Paleobiology 26, no. 3 (2000): 334-59.
*̂S. Xiao, X. Yuan, and A. H. Knoll, “Eumetazoan Fossils
in Terminal Proterozoic Phosphorites?,” PNAS 97, no. 25
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‘̂J. Vannier, I. Calandra, C. Gaillard, and A. Zylinska, “Pria-
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•”•D. J. Bottjer, J. W. Hagadorn, and S. Q. Dornbos, “The Cam-
brian Substrate Revolution,” GSA Today 10 (2000): 1-9.

”̂ For detailed descriptions of the variety of small shelly fos-
sils, see Rozanov and Zhuravlev, “The Lower Cambrian
Fossil Record of the Soviet Union,” 205-82; and Z-W. Jiang,
“The Lower Cambrian Fossil Record of China,” in Origin
and Early Evolution of the Metazoa, ed. J. H. Lipps and P. W.
Signor (New York: Plenum Press, 1992), 311-33.

”̂ J. Dzik, “Early Metazoan Evolution and the Meaning of
Its Fossil Record,” Evolutionary Biology 27 (1993): 339-86;
S. Conway Morris and J. S. Peel, “Articulated Halkieriids
from the Lower Cambrian of North Greenland and Their
Role in Early Protostome Evolution,” Philosophical Trans-
actions of the Royal Society London B 347 (1995): 305-58. See
also J. B. Caron, A. Scheltema, C. Schänder, and D. Rud-
kin, “A Soft-Bodied Mollusc with Radula from the Middle
Cambrian Burgess Shale,” Nature 442 (2006): 159-63.

•̂ S. Conway Morris and J. B. Caron, “Halwaxiids and the
Early Evolution of the Lophotrochozoans,” Science 315
(2007): 1255-8.

Volume 66, Number 2, June 2014 81

irtiili-
The Fossil Record of the Cambrian “Explosion”: Resolving the Tree of Life

^]. Vinther, D. Eibye-Jacobsen, and D. A. T. Harper, “An
Early Cambrian Stem Polychaete with Pygidial Cirri,”
Biology Letters 7 (2011): 929-32.

«A. P. Gubanov, A. V. Kouchinsky, and J. S. Peel, “The First
Evolutionary-Adaptive Lineage within Fossil Molluscs,”
Lethaia 32 (1999): 155-7; A. V. Kouchinsky, “Shell Micro-
structures of the Early Cambrian Anabarella and Watson-
ella as New Evidence on the Origin of the Rostroconchia,”
Lethaia 32 (1999): 173-80; A. P. Gubanov and J. S. Peel,
” Oelandiella, and the Earliest Cambrian Helcionelloid Mol-
lusc from Siberia,” Palaeontology 42, pt. 2 (1999): 211-22;
and P. Yu. Parkhaev, “Shell Chirality in Cambrian Gastro-
pods and Sinistral Members of the Genus Aldanella Vosto-
kova,” 1962, Paleontological journal 41, no. 3 (2007): 233-40.

•̂”L. Ramsköld, “Homologies in Cambrian Onychophora,”
Lethaia Ib (1992): 443-60; L. Ramsköld and H. Xianguang,
“New Early Cambrian Animal and Onychophoran Affini-
ties of Enigmatic Metazoans,” Nature 351 (1991): 225-8.

^̂ J. Liu, D. Shu, J. Han, Z. Zhang, and X. Zhang, “Origin,
Diversification, and Relationships of Cambrian Lobo-
pods,” Gondwana Research 14 (2008): 277-83.

^̂ Ĵ-Y. Chen, L. Ramsköld, and G-Q. Zhou, “Evidence for
Monophyly and Arthropod Affinity of Cambrian Giant
Predators,” Science 264 (1994): 1304-8; G. E. Budd, “The
Morphology of Opabinia regalis and the Reconstruction of
the Arthropod Stem Group,” Lethaia 29 (1996): 1-14. Also
see discussion of the transitions from lobopods to crown
group arthropods in J-Y. Chen, “The Origins and Key
Innovations of Vertebrates and Arthropods,” Palaeoworld
20 (2011): 257-78; and J. Dzik, “The Xenusian-to-Anomalo-
caridid Transition within the Lobopodians,” Bollettino delta
Societa Paleontológica Italiana 50, no. 1 (2011): 65-74.

*’G. E. Budd, “Arthropod Body-Plan Evolution in the Cam-
brian with an Example from Anomalocaridid Muscle,”
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=°C. B. Skovsted, G. A. Brock, J. R. Paterson, L. E. Holmer,
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Lower Cambrian of South Australia: Lophophorate Affini-
ties and Implications for Tommotiid Phyiogeny,” Geol-
ogy 36 (2008): 171-4; C. B. Skovsted, L. E. Holmer, C. M.
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of Paterimitra: An Early Cambrian Stem Group Brachiopod
from South Australia,” Proceedings of the Royal Society B
276 (2009): 1651-6; and L. E. Holmer, C. B. Skovsted, G. A.
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brian Tommotiid Micrina, a Sessile Bivalved Stem Group
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=’U. Balthasar and N. J. Butterfield, “Early Cambrian ‘Soft-
Shelled’ Brachiopods as Possible Stem-Group Phoronids,”
Acta Palaeontologica Polonica 54, no. 2 (2009): 307-14.

^^Excellent descriptions of these fossil communities can be
found in the following books: D. Briggs, D. Erwin, and
F. Collier, The Fossils of the Burgess Shale (Washington,
DC: Smithsonian Institution Press, 1994); S. Conway Mor-
ris, The Crucible of Greation: The Burgess Shale and the Rise
of Animals (New York: Oxford University Press, 1998);
J. Chen and G. Zhou, “Biology of the Chengjiang Fauna,”
in The Gambrian Explosion and the Fossil Record, Bulletin of
the National Museum of Natural Science No. 10, ed. Junyuan
Chen, Yen-nien Cheng, and H.V. Iten (Taichung, Taiwan,
China: 1997), 11-105.

”D-G. Shu, S. Conway Morris, I. Han, Z-F. Zhang, and J-N.
Liu, “Ancestral Echinoderms from the Chengjiang Depos-
its of China,” Nature 430 (2002): 422-8.

^For photographs and descriptions of early stem echino-
derms from the middle Cambrian, see S. Zammora, I. A.
Rahman, and A. B. Smith, “Plated Cambrian Bilaterians
Reveal the Earliest Stages of Echinoderm Evolution,”
PLoS ONE 7, no. 6 (2012): e38296. doi:10.1371; S. Zamora,
“Middle Cambrian Echinoderms from North Spain Show
Echinoderms Diversified Earlier in Gondwana,” Geology
38, no. 6 (2010): 507-10.

^̂ D-G. Shu, S. Conway Morris, J. Han, L. Chen, X-L. Zhang,
Z-F. Zhang, H-Q. Liu, Y. Li, and J-N. Liu, “Primitive Deu-
terostomes from the Chengjiang Lagerstätte (Lower Cam-
brian, China),” Nature 414 (2001): 419-24.

56D-G. Shu, L. Chen, J. Han, and X-L. Zhang, “An Early
Cambrian Tunicate from China,” Nature 411 (2001): 472-3.

=7-Y. Chen, J. Dzik, G. D. Edgecombe, L. Ramsköld, and
G-Q. Zhou, “A Possible Early Cambrian Chordate,” Nature
377 (1995): 720-2; J-Y. Chen, D-Y. Huang, and C-W. Li,
“An Early Cambrian Craniate-Like Chordate,” Nature 402
(1999): 518-22.

=̂ D-G. Shu, H-L. Luo, S. Conway Morris, X-L. Zhang, S-X.
Hu, L. Chen, J. Han, M. Zhu,Y. Li, and L-Z. Chen, “Low-
er Cambrian Vertebrates from South China,” Nature 402
(1999): 42-6; D-G. Shu, S. Conway Morris, J. Han, Z-F.
Zhang, K. Yasui, P. Janvier, L. Chen, X-L. Zhang, J-N. Liu,
Y. Li, and H-Q. Liu, “Head and Backbone of the Early
Cambrian Vertebrate Haikouichthys” Nature 421 (2003):
526-9. See discussion of the origin of vertebrates in J-Y.
Chen, “The Origins and Key Innovations of Vertebrates
and Arthropods,” Palaeoworld 20 (2011): 257-78.

ASA Members: Submit comments and questions on this com-
munication at www.asa3.org^FORUIVlS-»PSCF DISCUSSION.

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