The Cambrian Radiation

Halkieria evangelista: from the Lower Cambrian. Image from WikiCommonsHalkieria evangelista: from the Lower Cambrian. Image from WikiCommons For the vast majority of Earth's history, there existed only single-celled organisms. During the Cryogenian Period, 850-630 Ma (millions of years ago), the Earth experienced a series of deep glaciations, sometimes referred to as "Snowball Earth." As the ice waned, Ediacaran organisms made their first appearance. Ediacarans were large, strange, and probably sessile creatures of unknown affinity who lived between 630-542 Ma.

At 542.0 ± 1 Ma, life on Earth radically changed, with the appearance of Trichophycus pedum, a mud burrower with complex movement. The first appearance of Trichophycus pedum marks the beginning of the Cambrian Period (542-488 Ma). From 542 Ma, and especially during the middle Cambrian, animal diversity rapidly increased. This is referred to as the Cambrian Explosion or the Cambrian Radiation.

Sudden Appearance?

Did animal phyla suddenly appear in the Cambrian Explosion?

Paleontologists have discovered that new animal forms almost always appear abruptly--not gradually--in the fossil record, without any obvious connections to the animals that came before.
Explore Evolution, p. 22

About 530 million years ago, more than half of the major animal groups (called phyla) appear suddenly in the fossil record.
Explore Evolution, p. 22

Anomalacaris: a fearsome predator of the Cambrian. Image from WikiCommonsAnomalacaris: a fearsome predator of the Cambrian. Image from WikiCommons

Summary of problems with claim: The events of the Cambrian explosion are subject of ongoing debate and research. Some scientists argue that the fossils we see in the Cambrian represent the ancestors of modern phyla before those different groups had fully separated, and that the phyla truly emerged over a longer period of time. There are also questions about the preservation of fossils before the Cambrian, and it is possible that the explosion of fossils during the Cambrian represents a shift toward predator-resistant (and readily fossilized) exo-skeletons, rather than a shift in the actual diversity of life.

Full discussion: To suggest that, during the Cambrian explosion, "more than half of the major animal groups (called phyla) appear suddenly in the fossil record" (Explore Evolution, p. 22) stretches the true state of affairs. A number of fossils discovered from that period of time possess traits characteristic of modern phyla. Other species found at that time cannot be clearly classified in any modern phyla at all. Fossils from the period following the Cambrian, an era known as the Ordovician, more clearly show the distinct groups possessing the traits associated with many modern phyla. Fossil deposits before the Cambrian are rarer, making it difficult to be sure how sudden any appearances were.

Palaeontology 50(1):1–22. "Change through time in realized ecospace. Top line represents all recorded modes of life, middle line represents modes of life of skeletal fauna only; bottom line records mean number of modes of life for single assemblages. For the Recent, the open circle represents those recent taxa with readily preserved hard parts, and the open circle containing an asterisk represents those taxa with a diverse fossil record."" title="Modes of Life as Function of Time: Figure 8 from Richard K. Bambach, Andrew M. Bush, Douglas H. Erwin (2007) "Autecology and The Filling of Ecospace: Key Metazoan Radiations" Palaeontology 50(1):1–22. "Change through time in realized ecospace. Top line represents all recorded modes of life, middle line represents modes of life of skeletal fauna only; bottom line records mean number of modes of life for single assemblages. For the Recent, the open circle represents those recent taxa with readily preserved hard parts, and the open circle containing an asterisk represents those taxa with a diverse fossil record."" class="image image-img_assist_custom" width="360" height="271" />Modes of Life as Function of Time: Figure 8 from Richard K. Bambach, Andrew M. Bush, Douglas H. Erwin (2007) "Autecology and The Filling of Ecospace: Key Metazoan Radiations" Palaeontology 50(1):1–22. "Change through time in realized ecospace. Top line represents all recorded modes of life, middle line represents modes of life of skeletal fauna only; bottom line records mean number of modes of life for single assemblages. For the Recent, the open circle represents those recent taxa with readily preserved hard parts, and the open circle containing an asterisk represents those taxa with a diverse fossil record."

Ecologically, the Cambrian fossils represent a smooth extension of the rate of diversification before and after. An analysis of the lifestyles of the Cambrian fossils, Ediacaran (pre-Cambrian) fossils, and fossils from eras after the Cambrian shows a steady increase in ecological complexity, not an explosion of diversity. The nature of that expansion is informative, though.

Cambrian fossils include the first predators capable of hunting and capturing prey (rather than passive filter-feeders). This behavioral development had adaptive consequences for concurrent species, and some evidence supports the contention that the Cambrian fossils seem more diverse simply because hard bodyparts — evolved as protection against predators — preserved better than the soft bodies that preceded the Cambrian. For these and other reasons, the record of pre-Cambrian fossils is not necessarily adequate for a full evaluation of the predecessors of Cambrian fauna.

New fossil finds, and improved understanding of the biological basis for the changing body forms we find in the Cambrian, have led to revisions of the claim that so many phyla emerged in the Cambrian explosion. Fossilized remains found in Australia appear to represent a group of pre-Cambrian chordates, representatives of the phylum to which humans belong. More detailed analysis of Cambrian fossils has also shown some species to be significant branches off of the tree which led to modern species within the same phylum (Derek E. G. Briggs and Richard A. Fortey, 2005, "Wonderful strife: systematics, stem groups, and the phylogenetic signal of the Cambrian radiation," Paleobiology 31:94-112), a finding which indicates that the origin of the phylum itself lies earlier.

Study of the Cambrian fossils has also revealed that some of the examples of divergent Cambrian phyla may have been premature. Some fossils possess features indicating that they evolved from a time before certain existing phyla emerged. Because they possess certain traits in common with existing phyla, certain authors assign them to one phylum or the other, but such assignments are debatable, and new knowledge about the relationships between the modern groups has caused some such assignments to be reevaluated. Thus, the range of time over which modern phyla are seen to emerge tends to get wider as we improve our understanding of both modern and fossilized life.

Finally, our improving understanding of developmental biology is allowing scientists to better understand why the Ediacaran, Cambrian and Ordovician saw so many new body forms enter the fossil record, and why so many features of modern living things emerged during those geologic periods. A major theme of emerging from the field of developmental biology is the discovery that the process of forming the body from a fertilized egg cell is controlled by a group of genes which regulate the way that body segments form (whether the segments of an insect's exoskeleton, or the segments visible in the muscles of a fish fillet and the human spinal column). These genes, including the Hox genes mentioned elsewhere in Explore Evolution are shared by nearly all multicellular organisms, from sea sponges to humans. Those genes appear to have duplicated, diverged and played an expanded role in structuring development during the same period when the major body forms begin appearing in the fossil record (Jordi Garcia-Fernàndez, 2005, "The genesis and evolution of homeobox gene clusters" Nature Reviews Genetics 6, 881-892). Paleontologists and developmental biologists have begun working together on the hypothesis that the diversification of these genes drove the changes in body forms during that period (see, for instance, Robert Carroll, 2000, "Towards a new evolutionary synthesis," Trends in Ecology and Evolutionary Biology 15(1):27-32, or Sean Carroll's popular treatment Endless Forms Most Beautiful, ch. 6). Once critical developmental pathways became established, it may have become harder to produce major new body forms, and that may explain why those basic forms seem to appear and stabilize relatively rapidly during that period.

Evolutionary developmental biology is an active area of research, a field of study only opened in the last 10-20 years, which has the potential to radically reshape how we think about the processes driving diversity, and the framework within which we interpret fossils from the Cambrian and from earlier eras. Students should not be taught simply that fossil forms suddenly appear, they need to be taught the developmental biology, and provided with a conceptual framework so that they can appreciate the ways that life 500 million years ago differed from life as they know it. That would provide students with a map which would guide their exploration of evolution. The approach taken by Explore Evolution simply discourages students from pursuing ideas in this cutting edge field.

Advocates of the artifact hypothesis say that the Cambrian explosion is not real; it is only the result--or an "artifact"--of having too small a sample of fossils to work with.
Explore Evolution, p. 30

No paleontologists say this about the Cambrian explosion. Explore Evolution does not cite references for this claim, but any casual examination of the peer-reviewed literature about the Cambrian explosion will fail to turn up a single instance of a paleontologist claiming the Cambrian explosion was "not real."

Many paleontologists now estimate the Cambrian explosion took place over a period of 10 million years or less... If Earth's whole history were a timeline the length of an American football field, the Cambrian explosion time would take up just 4 inches of the football field's total length.
Explore Evolution, p. 22

We need better math in creationist textbooks:
American football field = 100 yards
1 yard = 3 feet
football field = 300 feet
1 foot = 12 inches
football field = 3600 inches
4 inches/3600 inches = 0.0011 = 0.11%

age of the Earth = 4.54 billion years = 4,540 million years
length of Cambrian Explosion according to
Explore Evolution = ~10 million years
10 million / 4540 million = 0.0022 = 0.22%

0.11% does not equal 0.22%. Q.E.D.

The Cambrian Radiation in the Geologic Record

The Precambrian/Cambrian boundary appears relatively abruptly when examined from the perspective of large, shelled fossils. However, because a geologic process taking many millions of years may leave behind only a few inches of rock, when geologists say "suddenly" it has a different meaning than the common usage. Trilobite Fragments: from the Cambrian Andrews Mountain Member of the Campito Formation, White-Inyo Mtns., California. Photo by Steven Newton.Trilobite Fragments: from the Cambrian Andrews Mountain Formation, White-Inyo Mtns., California. Photo by Steven Newton.

Precambrian rocks are usually free of bioturbation, which is the destruction of fine layering by animals burrowing into soft sediment. In rock forming today, sediments are usually bioturbated. The absence of bioturbation indicates something is unusual—for example, anoxic conditions, where there is not enough oxygen dissolved in water to sustain life. Today such conditions are rare.

Just prior to the Cambrian/Precambrian boundary, worldwide massive carbonate deposits (limestone, dolomite) heralded the end of "Snowball Earth," a 220 million year period of deep glaciation now called the Cryogenic Period (850-630 Ma). Above these carbonate deposits geologists start to find the signs of complex life: burrowing, bioturbation, shells. Fragments of shelled animals are much more common than whole fossils of the animals themselves.

Peter Ward of the University of Washington beautifully describes the boundary this way:

I kicked an empty pop can with my thick field boots as I walked along the country road near Addy, surrounded by the roadside sandstones, vestiges of that long-ago world. I was walking stratigraphically upward in these sediments; they lie at an angle, tilted about 30 degrees from their original horizontal. As I walked northward along the road I was thus going up through time, into ever higher and thus younger beds of these sandstones. With each step I passed upward through thousands of years of time; with my quarter-mile hike along the road I had traversed several millions of years among these buff-colored sandstones. I was somewhat disappointed. I was training to become a paleontologist and disdained geological phenomena not associated with fossils…

"I had been lulled by the walk and the endless slabs of sandstone showing nothing but featureless bedding planes. The small slab now in my hands thus elicited no immediate response. I stared with unseeing eyes at the small oblong shell and tossed the rock before the message from my eyes finally burned through into my brain. The small rock followed a beautiful ballistic arc down the talus slope as I realized that I had just seen an unmistakable announcement of life…

"I picked up another piece and saw more shells, amid even more wondrous fossils. I saw the heads of large trilobites, looking something like large crabs yet very different, fossils with segments and strange crescent-shaped eyes unlike anything now living. I was surrounded by fossils, sitting atop a teaming graveyard, a joyous assemblage announcing that after 3 billion years skeletonized life had arrived. I was sitting on the base of the Cambrian System, the start of the Paleozoic era, the beginning of the Phanerozoic, the time of life.

Peter Ward, 1991. On Methuselah's Trail: Living Fossils and the Great Extinctions, p. 27

An important caveat here is that the base the Cambrian, at 542 Ma, is not the beginning of life at all. Nor will the Cambrian Radiation occur for another ~7 Ma, at 535 Ma. So what the geologist sees in outcrops—the appearance of large, shelled organisms—is not the complete story.

Creationist Statistics

Statistical Sampling 101 …Suppose you find a big box of marbles. You reach in and grab six marbles at random. When you remove the marbles, you discover that each marble is either red, green, or blue…This sample is so small that is may not be representative of all the colors in the box…However, you keep going until you've pulled about 1,000 marbles out of the box. You look at them all, and still find only red, green, and blue ones. There are still some marbles left in the box. What colors would you guess they are?
Explore Evolution, p. 31

What Explore Evolution is really saying here is this: We do not see abundant trace fossils because there were no animals to make them. Therefore Cambrian animals had no ancestors, but were suddenly "created." This is wrong in that it fails to consider the type of animal that existed at that time. Marbles: from  Wikicommons Marbles: from Wikicommons

But in an additional sense, this example shows why every scientist must have a working knowledge of statistics. Yes, the likelihood is that the next marble produced in this scenario would be red, green, or blue. But if among the thousands of marbles, you had planted a white marble, then even though this white marble was present, it would be very unlikely to be found. That's not the same thing as saying it could not be there. In fact, not until every single marble was removed could you say with any assurance that this hypothetical box contained only red, green, or blue. There might have been 1 white, 2 black, 3 purple, and so on. But the very small numbers of these odd colors make it very unlikely that would they be found in a random draw.

There's an old joke that goes like this:

A businessman, a philosopher, and a scientist are on a train traveling through the countryside. They see a solitary black sheep grazing in a field.

The businessman says, "Sheep are black."

The philosopher says, "At least one sheep is black."

The scientist says, "At least one side of one sheep is black."

Soft-bodied fossils

Explore Evolution claims that the ancestors of the Cambrian phyla could not have been soft-bodied

Summary of problems with claim: This claim is based solely on an quotation from a Discovery Institute Fellow, a toxicologist whose credentials are misrepresented to claim he is a "marine paleobiologist." This claim is a variant of the "gaps in the fossil record" argument, a gap that is being steadily filled by scientists.

Full discussion: This is yet another argument based on a creationist antecedent, "Complex life forms appear suddenly in the Cambrian explosion, with no ancestral fossils." This argument first appeared in Henry Morris's book Scientific Creationism in 1985 (pp 80-81). The authors of Explore Evolution then recast the argument, and cite an interesting source.

This point has been further emphasized by a recent Precambrian fossil find near Chengjiang, China. Scientists there recently discovered incredibly preserved microscopic fossils of sponge embryos. (Sponges are obviously soft-bodied. Their embryos are small and soft-bodied, too—other than their tiny spicules.) Paul Chien, a marine paleobiologist at the University of San Francisco argues that this discovery poses a grave difficulty for the artifact hypothesis. If the Precambrian rocks can preserve microscopic soft-bodied organisms, why don't they contain the ancestors to the Cambrian animals? (footnote 28)
Explore Evolution, p. 31

Who is Paul Chien? What are his credentials? What peer-reviewed evidence is cited in footnote 28?

The USF webpage lists Chien's research interests thusly

Prof. Chien is interested in the physiology and ecology of inter-tidal organisms. His research has involved the transport of amino acids and metal ions across cell membranes and the detoxification mechanisms of metal ions.

He is also a "senior fellow" of the Center for Science and Culture, a part of the Discovery Institute, where his credentials are listed somewhat differently.

Paul Chien is a Professor in the Department of Biology at the University of San Francisco and he was elected Chairman of his department twice. He received his Ph.D. in Biology from the University of California at Irvine's Department of Developmental & Cell Biology. He has held such positions as Postdoctoral Fellow in the Department of Environmental Engineering at the California Institute of Technology, Pasadena (CIT); Instructor of Biology at The Chinese University of Hong Kong; and a consultant to both the Kerckhoff Marine Laboratory of the CIT, and the Scanning Electron Microscopy & Micro X-ray Analyst in the Biology Department of Santa Clara University, California. Dr. Chien's work has been published in over fifty technical journals and he has spoken internationally, and on numerous occasions, from Brazil to mainland China-where he has also been involved in cooperative research programs. Dr. Chien edited and translated Phillip Johnson's book Darwin on Trial into Chinese as well as Jonathan Wells' Icons of Evolution.

A search of Web of Science (July 2007) reveals that he is the author of 15 peer-reviewed articles, but none in the area of "marine paleobiology". The most recent article is dated 1998, and is not in any relevant field of biology at all; the title is "Relocation of civilization centers in ancient China: Environmental factors". The most recent biology-related article is from 1995; all of the articles seem to be focused on heavy metal toxicity and antidotes in marine animals, particularly worms (e.g. Uptake, Binding and Clearance of Divalent Cadmium in Gycera dibranchiata (Annelida-Polychaeta); MA Rice and PK Chien; Marine Biology 53 (1): 33-39 1979). He is also apparently a creationist, judging from his statements in a 1997 interview in The Real Issue, which is a publication of the Christian Leadership Ministries, whose Statement of Faith includes the belief in the inerrancy of the Bible.

But when I read Genesis chapter one, the fifth day seems to read very much like the fossil record we see now because it talks about all the creatures teeming in the oceans. Now, to me that sounds like the Cambrian explosion in a very short period of time, [the animals] are all there.

Why, then, is Paul K. Chien described by the authors as a "marine paleobiologist"? He appears to be a toxicologist, whose last peer-reviewed paper appeared in 1998. Those articles in "over fifty technical journals", cited by the Discovery Institute, somehow never made it into the Web of Science. Finally, in the interview cited above, when the interviewer asks him directly if he should be described as a paleontologist, he replies "Not really; that's not my purpose." We can only speculate as to the "purpose" of the authors who do describe him as a "paleobiologist".

What about the references cited in footnote 28? There are two of them. One is a paper (not peer-reviewed) by Chien et al., presented at the North American Paleontological Convention at Berkeley in 2001, entitled "SEM observation of Precambrian sponge embryos from southern China, revealing ultrastructures including yolk granules, secretion granules, cytoskeleton and nuclei". This paper, unsurprisingly, is not indexed in the Web of Science.

The other citation from footnote 28 is Hagadorn, et al., Science, 314:291-294, 2006, "Cellular and subscellular structure of neoproterozoic animal embryos". That paper has already been cited 10 times (Web of Science search performed in July 2007). None of the papers citing Hagadorn et al., cite Chien's 2001 contribution. The Hagadorn paper does not cite Chien either. All of these observations contribute to the perception that Chien's credentials in this area are nil, and that his non-peer-reviewed paper of 2001 has had no impact on this field. His concern about the lack of Precambrian fossils of ancestors to the Cambrian fauna should thus also be viewed with a more critical eye than was used by the authors of this textbook.

But is this question, despite its lack of academic credentials, a valid concern? Why don't we find these missing fossils? More importantly, is a gap in the fossil record a good reason to cast doubt on evolutionary theory and common descent? Probably not. This gap in the fossil record, like all gaps identified by the creationists, is being filled. For some of this more recent information, see the references cited below.


"Darwin's dilemma: the realities of the Cambrian 'explosion'", Morris SC, Philosophical Transactions of the Royal Society B-Biological Sciences 361(1470): 1069-1083 2006

"Fossilized embryos are widespread but the record is temporally and taxonomically biased.", Donoghue PCJ et al. Evolution and Development 8(2):232-238, 2006

Cal Academy of Sciences display

Did a 1990s Cal Academy of Sciences display on Cambrian phyla showed the phyla connected without common ancestors?

…A fossil exhibit on display at the California Academy of Sciences in the 1990s. It showed fossils arranged in the familiar branching-tree pattern… the phyla lines are parallel, illustrating that each phylum remains distinct--separate from the other phyla--during the entire time it appears in the fossil record.
Explore Evolution, p. 34

Summary of problems with claim: If description of exhibit is accurate, this display does not undermine evolution. Even if a hypothetical exhibit were inaccurate, one mistaken exhibit is not evidence against evolution any more than a misspelling on a picture caption changes the spelling of the word.

Full discussion:

Attempts by NCSE to verify this with the California Academy of Sciences have proven fruitless; this was so long ago that the information is unverifiable.

But if we accept their premise and assume that the diagram is a faithful representation of the CAS exhibit, then several things are wrong with Explore Evolution's claims:

1. Explore Evolution Misunderstands the Definition of Phyla

Phyla are ways of classifying body plans of animals. We are part of Phylum Chordata, for example, meaning that we have a spinal cord. So are birds, fish, snakes, and so on. Phylum Cnidaria is the home of animals without a spinal cord and with stinging cells; jellies and corals and anemones are all cnidarians.

2. Phyla Remain Distinct

Phyla are not expected to change within the fossil record; they are expected to evolve in parallel, separate branches. While Phylum Chordata plays many variations on the theme of spinal cords, no one would expect a chordate to evolve into Phylum Cnidaria. A jelly might evolve into another type of jelly, but not into a bird. Nor will the chordate bird evolve into a cnidarian.

Explore Evolution assumes that such transformations should occur, yet this evolutionary route has never been claimed by scientists.