The Cambrian Explosion

[otseng]

The Cambrian Period

In the sediments of Cambrian age, fossils "suddenly" become common for the first time. This effect, which has come to be known as the "Cambrian Explosion."

Adam Sedgwick named the Cambrian in 1835. He derived the name from ‘Cambria,’ the Roman name for Wales, site of the type area, in which shales and sandstones make up a section about two miles [~3 km] thick. Though these rocks are strongly folded and faulted, some are ossiliferous.

The International Subcommission on Cambrian Stratigraphy has settled on an age of 543 Ma for the Precambrian-Cambrian boundary, and approximately 490 Ma for the Cambrian-Ordovician boundary.

Life on earth has a long and rich history – predating the beginning of the Cambrian by a factor of six or seven. However, much of this early life was microscopic and, microscopic or not, virtually unknown until the 1960s. Prior to then, as far as was widely known even among the scientific community, the fossil record sprang into existence in the Cambrian, already exhibiting a high degree of development and marvellous diversity: the so-called “Cambrian Explosion.”

Today, although the fossil record now extends back 3,465 Ma (± 5 Ma, to the Apex Chert, see Schopf 1999, p. 100) and diverse precursors to the Cambrian biotas are gradually becoming understood, it still appears as if a genuinely rapid diversification of form, particularly among the Metazoa (“animals”), did occur in the Cambrian. During this time, most extant body plans are suddenly found in the fossil record. “Definitive representatives of all readily fossilizable animal phyla (with the exception of bryozoans) have been found in Cambrian rocks, as have representatives of several soft-bodied phyla (Valentine et al. 1991)” (Wray et al. 1996). By way of contrast, “it appears that no [new] phylum-level body plans have arisen in the animal kingdom in the last 500 million years” (Arthur 1997, p. 7).

Extinction

"Three significant paleoceanographic events are juxtaposed in Upper Cambrian (Steptoean Stage) sequences of Laurentia:

1. a mass extinction of trilobites that marks the base of the Steptoean (Marjumiid-Pteroceaphliid biomere);
2. a large positive excursion in carbon isotope values that spans much of the Steptoean (SPICE event); and
3. an imprecisely dated craton-wide drop in sea level (Sauk II – Sauk III event) that was terminated by widespread flooding in the Late Steptoean (mid-late Elvinia Zone time).

The first two events are clearly global in scope, but the scale and timing of the sea level drop is not known in detail outside Laurentia.

Life during the Cambrian period

Almost every metazoan phylum with hard parts, and many that lack hard parts, made its first appearance in the Cambrian. The only modern phylum with an adequate fossil record to appear after the Cambrian was the phylum Bryozoa, which is not known before the early Ordovician. A few mineralized animal fossils, including sponge spicules and probable worm tubes, are known from the Vendian period immediately preceding the Cambrian. Some of the odd fossils of the "Ediacara biota" from the Vendian may also have been animals in or near living phyla, although this remains a somewhat controversial topic. However, the Cambrian was nonetheless a time of great evolutionary innovation, with many major groups of organisms appearing within a span of only forty million years.

MSN Encarta

At the beginning of the Cambrian period (570 million to 500 million years ago) animal life was entirely confined to the seas. By the end of the period, all the phyla of the animal kingdom existed, except for vertebrates.

For debate:

How does the data from the Cambrian Explosion argue for/against the evolutionary model(s)?
How does the data from the Cambrian Explosion argue for/against the creation model(s)?

[Esoteric_Illuminati]

I'll first preface my post by conceding that I'm no biology scholar.

How does the data from the Cambrian Explosion argue for/against the evolutionary model(s)?

First thing, as Norman Geisler points out, is that there is no evidence indicating how approximately 5,000 genetic types of marine/animal life allegedly evolved during these two eras (Precambrian - Cambrian). He goes on to cite a 1995 Time Magazine article which in it states,
Scientists used to think that the evolution of phyla took place over a period of 75 million years, and even that seemed impossibly short. Then two years ago a group of researchers led by John Grotzinger, Samuel Bowring from MIT, and Andrew Knoll [paleontolgist at Harvard University] took this long-standing problem and escalated it into a crisis. First they recalibrated the geological clock, chopping the Cambrian period to about half its former length. Then they announced that the interval of major evolutionary innovation did not span the entire 30 million years, but rather was concentrated in the first thir. "Fast," Harvard's [Stephen Jay] Gould observes, "is now a lot faster than we thought,"... Of course understanding what made the Cambrian explosion possible doesn't address the larger question of what made it happen so fast. Here scientists delicately slide across data-thin ice, suggesting scenarios that are based on intuition rather than solid evidence...
The Cambrian explosion has caused experts to wonder if the twin Darwinian imperatives of genetic variation and natural selection provide an adaquete framework for understanding evolution. "What Darwin described in the _On the Origin of Species_," observes Queen's University paleontologist Narbonne, "was the steady background kind of evolution. But there also seems to be a non-Darwinian kind of evolution that functions over extremely short time periods--and that's where all the action is."

This led Stephen Jay Gould and some others to throw gradualism out the window and posit punctuated equilibrium as the best macroevolution paradigm.

Michael Denton sums up punctuated equilibrium's main criticism, which is the failure to explain the the large systematic gaps in life forms (primititve terrestrial mammal vs whale or primitive terrestrial reptile vs. an Ichtyosaur or mollusks vs. anthropods):
As evidence for the existence of natural links between great divisions of nature, they are only convincing to someone already convinced of the reality of organic evolution.

Truth is, neither paradigm is based on scientific laws and/or observational evidence. Both theories are reduced to philosophical conclusions based on the evidence we have available that needs to be explained, like the Cambrian explosion. That places these theories in the same catagory as Intelligent Design.

I would conclude that the Cambrian explosion has done nothing but cause more problems with traditional macroevolutionary theories.

How does the data from the Cambrian Explosion argue for/against the creation model(s)?

Considering the explanations of ID are in the same philosophical boat as macroevolution, I believe these ID conclusions are just as reasonable, if not moreso.

These are two Young Earth views I consider reasonable explanations for the Cambrian explosion.
1.) The Flood's trauma to the earth and life as we know it created the fossil record. (See Answering Genesis articles.) The "punctuated equilibrium" is real, yet was a result of God. Upon the recession of the flood waters, God created (in Gould's terms) "rapid bursts of speciation."
2.) The fossil record is leftover from the previous creation that was subsequently destroyed upon the fall of the angels. Which I argue in the this debate thread ---> Clicky.

There also is a progressive view of creation. There are different forms of this view, but generally it interprets the Genesis account in which "days" are very long periods of time which allow for evolution to progress and the environment to adapt accordingly. Again, a quote by Norman Geisler in Unshakable Foundations:

The Cambrian explosion most probably would have taken place during these latter stages of creation (Genesis 1:14-19 - volcanic activity subsides, earth cools, CO2 levels decrease, more stable atmosphere clearing up the sky so sun/moon could be seen). The Creator brought forth an influx of aquatic life along with a host of small animal life, and probably toward the end of the stage five. He introduced the "great creatures of the sea," including the reptiles, the explosion of aquatic life, the first true birds apparently were created as the atmosphere and ecosystem reached a somewhat stable temperature (Genesis 1:20-23). The stablization of the atmospheric conditions would have been critically important, because birds are warm-blooded creatures and must generate and maintain heat within their own bodies to counteract temperature fluctuations of the environment."

Now, I personally don't believe in a young earth per se, but I do believe in the Genesis account and that each day was a literal 24 hour period, therefore I don't follow the progressive ID view. Either explanation I provided above for the YEC is reasonable IMO. In either case, I believe God is necessary to reasonably explain the Cambrian explosion. I feel it is more philosophically sound than macroevolution's punctuated equilibrium view.

[youngborean]

An interesting thing I noticed studing this explosion in school. My professor cited a number of Organisms found in the Rocky Shale as primitive ocean bottom dwellers. The question no one asked at the time was what were bottom dwellers doing so high above sea level? Maybe my professor had his shales mixed up but I do believe this expolosion had a variety of sea creatures associated with it. I have always wondered if these creatures weren't found in the rocky shale, how can we prove or talk about associations with the plant fossils found in the mud flats in Africa? I am not an evolutionary biologist, however I studied a good deal of it in my botany degree and would love someone with more expertise to expound on some of these questions.

[perfessor]

An interesting thing I noticed studing this explosion in school. My professor cited a number of Organisms found in the Rocky Shale as primitive ocean bottom dwellers. The question no one asked at the time was what were bottom dwellers doing so high above sea level? Maybe my professor had his shales mixed up but I do believe this expolosion had a variety of sea creatures associated with it.

I can think of three possible answers:
1. They had wings to fly above sea level;
2. It was floating shale;
3. THE SEA LEVEL WAS DIFFERENT millions of years ago.

I'm not familiar with the Rock Shale, but it is not uncommon to find sea fossils high above our present sea level.

[perfessor]

[quote=""otseng"]How does the data from the Cambrian Explosion argue for/against the evolutionary model(s)?

First thing, as Norman Geisler points out, is that there is no evidence indicating how approximately 5,000 genetic types of marine/animal life allegedly evolved during these two eras (Precambrian - Cambrian).[/quote]
I would say that the presense of "5,000 genetic types" is indeed evidence. You may not like the "how" explanation, but at least it is based on evidence that we actually have. Creation theory, or ID, are based on evidence that we don't have. It's like, "I don't understand it, it must be wrong." Or, "We can't explain it, so it must have been God's doing."

He goes on to cite a 1995 Time Magazine article which in it states ...{among other things} ... Here scientists delicately slide across data-thin ice, suggesting scenarios that are based on intuition rather than solid evidence...

Bolding by Esoteric. This is a magazine article, not a science paper....

Truth is, neither paradigm (gradual evolution, and punctuated equilibrium) is based on scientific laws and/or observational evidence. Both theories are reduced to philosophical conclusions based on the evidence we have available that needs to be explained, like the Cambrian explosion. That places these theories in the same catagory as Intelligent Design.

This may be your assessment, and you are welcome to it. But to me, it is a specious overstatement. Various evolutionary explanations have been developed based explicitly on observational evidence. Yes I admit that there are gaps; nonetheless, the theory is based on what we do know - not on what we don't know. No way does ID rise to that level of evidentiary basis.

As evidence for the existence of natural links between great divisions of nature, they are only convincing to someone already convinced of the reality of organic evolution.

Guilty as charged. There is in fact lots of evidence for the reality of organic evolution. As I said before, yes there are gaps. The question to you is, do those gaps mean we throw up our hands, stop looking, and just say goddidit, or do we continue our research? I vote for the latter.

I would conclude that the Cambrian explosion has done nothing but cause more problems with traditional macroevolutionary theories.

Yes, and Einstein's theories caused problems with Newton. So what?

Again, a quote by Norman Geisler in Unshakable Foundations:

The Cambrian explosion most probably would have taken place during these latter stages of creation (Genesis 1:14-19 - volcanic activity subsides, earth cools, CO2 levels decrease, more stable atmosphere clearing up the sky so sun/moon could be seen). The Creator brought forth an influx of aquatic life along with a host of small animal life, and probably toward the end of the stage five. He introduced the "great creatures of the sea," including the reptiles, the explosion of aquatic life, the first true birds apparently were created as the atmosphere and ecosystem reached a somewhat stable temperature (Genesis 1:20-23). The stablization of the atmospheric conditions would have been critically important, because birds are warm-blooded creatures and must generate and maintain heat within their own bodies to counteract temperature fluctuations of the environment."

Do you not see this as a series of speculations which are unsupported by scientific observations? Yes supported by passages in Genesis. You can believe it that way if you want. But stop pretending that Genesis is science.

[Jose]

My professor cited a number of Organisms found in the Rocky Shale as primitive ocean bottom dwellers. The question no one asked at the time was what were bottom dwellers doing so high above sea level? Maybe my professor had his shales mixed up but I do believe this expolosion had a variety of sea creatures associated with it.

You're probably thinking of the Burgess Shale in the Rockies. Yes, it has lots of wonderful and weird sea creatures in it. How did it get so high up? Well, the Rockies were formed by standard geological processes, in which prior sea bottom was lifted up to form mountain ranges. As I understand it, part of this was the collision of the North American Plate with additional plates crashing into it from the West.

First thing, as Norman Geisler points out, is that there is no evidence indicating how approximately 5,000 genetic types of marine/animal life allegedly evolved during these two eras (Precambrian - Cambrian).

As perfessor notes, 5,000 genetic types is a lot. Of course, I would quibble with your implication that these life forms appeared suddenly in just "these two eras." The Cambrian used to be defined as the point at which fossils were first found, and the Precambrian was everything before that. Everything before the Cambrian is quite a long time--most of the earth's history, in fact. If many of these life forms actually evolved over millions of years during the Precambrian, then there's no sudden "explosion" to worry about.

The "explosion" is actually the result of the first appearance of hard parts--shells, for example--that could be fossilized easily. It's very difficult to preserve soft tissues. As more and more sites have been discovered that do preserve soft-bodied Precambrian animals, however, we're learning a lot more. The Ediacaran Hills of Australia were among the first locations of such fossils. Many more are being reported from China.

From the new data, it looks like the evolution of the various body plans was well under way before the beginning of the Cambrian. There were also many odd creatures that we don't recognize--things that went extinct. The current thinking is that the "Cambrian explosion" of hard-shelled creatures was likely a result of increased selection pressure by predators. Soft-bodied things got eaten. Things with shells were protected, and lived to reproduce. It wouldn't take long for the shelled creatures to take over the ecological niches of the shell-less creatures that were being eaten.

Since we defined the beginning of the Cambrian as the place where we first see lots of shelled fossils, of course it will look like a sudden appearance of organisms. We now know that it is not the sudden appearance of organisms, just the point in evolutionary time at which the protection of shells became essential.

[youngborean]

I can think of three possible answers:
1. They had wings to fly above sea level;
2. It was floating shale;
3. THE SEA LEVEL WAS DIFFERENT millions of years ago.

I'm not familiar with the Rock Shale, but it is not uncommon to find sea fossils high above our present sea level.

Right. These explanations are certainly as possible as the flood mentioned in your #3. Truly only explanations.

[youngborean]

You're probably thinking of the Burgess Shale in the Rockies. Yes, it has lots of wonderful and weird sea creatures in it. How did it get so high up? Well, the Rockies were formed by standard geological processes, in which prior sea bottom was lifted up to form mountain ranges. As I understand it, part of this was the collision of the North American Plate with additional plates crashing into it from the West.

Yeah this seems like a good explanation, consistent with geological explanation of mountain formation. Why don't we look for more fossils on the ocean floor? I suppose it's probably really expensive. So the assumption here is that first these fossils (If I remember correctly Burgess is one of the largest collections) are Cambrian? If our main source (Burgess) of defining Cambrian is on the Surface of the Rockies (erosion would obviously affect this thought) how do we arrive at an accurate date for the Cambrian explosion in geological terms? Thanks for your first answer, very insightful, made try to remember some geology instruction I had.

[Jose]

Why don't we look for more fossils on the ocean floor?

It turns out that much of the ocean floor is quite new. At the spreading centers, like the mid-Atlantic ridge, it is being formed. At the subduction zones--the trenches--it is sliding under the continents, beting melted, and converted into volcanoes (hence, the Ring of Fire around the Pacific). Melting and being reborn as volcanic eruptions is hard on fossils... The good fossil sites are on the continents, where for one reason or another, they were pressed down low enough to have ocean above them. It North America, this was pretty common around the edges, but not in the part of the "stable craton" that we might think of as the center of bouyancy. That's around southern Canada, Minnesota, etc.

If our main source (Burgess) of defining Cambrian is on the Surface of the Rockies (erosion would obviously affect this thought) how do we arrive at an accurate date for the Cambrian explosion in geological terms?

There are lots of Cambrian sites. I've collected at the "bottom of the Cambrian" in California and Nevada, for example. The Burgess Shale is special in that it had the first examples of some of the soft-bodied Cambrian creatures, and really good preservation of many different things.

As for the date, that depends on radiometric dating. Basically (I may need some help here from those who are better versed in geology than I am), one finds appropriate rock types that contain appropriate minerals for which the technique is possible, and one assays the relative proportions of radioactive elements and their breakdown products. Given the measured half-lives of the elements, one can calculate how long it has been since the rock was formed. There are creationist arguments that "carbon dating" is unreliable, so you will be certain to find those who don't like this method. Of course, we are talking about different elements than carbon, which have longer half-lives. And, of course, we're also talking about being really careful to avoid contamination and sample mis-handling, etc.

Usually, fossils are in rock that can't be dated directly. One must rely on dating the "datable" strata above and below, which gives an age-range for the fossil. There are creationists who say this isn't valid either, because it doesn't date the fossil...but it's the same principle as finding a house on a street when you know the addresses two blocks north and two blocks south. Interpolation is valid methodology.

Having said this, the Cambrian Explosion is often (erroneously) thought of as a very sudden appearance, all at once, of many species. At the level of resolution that we are talking about, "all at once" is probably at least 10 million years. If we include the rapidly-increasing knowledge of Precambrian fossils, then we must increase the length of time for this "explosion" considerably. Maybe we're talking 50 or 100 million years. I dunno...that seems like a long time to me.

[otseng]

How does the data from the Cambrian Explosion argue for/against the evolutionary model(s)?

What is special about the Cambrian explosion is the relatively short time in which life seems to have evolved. The question for evolutionists is, how did such a wide range of life evolve in so short a period of time?

The earliest morphological evidence for life is 3.5 billion years old, fossils of stromatolites (colonies of cyanobacteria) and single, undifferentiated cells, or Prokaryotes. For 1.6 billion years these simple cells were the only kind of living organism, until the arrival of Eukaryotes, or single cells with differentiated nuclei and cell organelles. Although representing a large leap in complexity, the Eukaryotes were still only single cells or cell aggregates. It was another 1.4 billion years before complex, multicellular life made an appearance in the form of the Ediacaran faunas (see fossils), followed by all the variety of the true Cambrian animals about 550 million years ago.

Therefore 80% of the history of life on Earth is exclusively single or undifferentiated multi-cellular. 3 billion years went by before complex multicellular life appeared, but when it did it only took between 5 and 10 million years for all the basic body plans of the organisms we see around us today to be established. This is why the origin of multicellular life, in particular the metazoans or large animals with complex body plans, is termed the Cambrian explosion.

Source: The Cambrian explosion

Though dates vary, the Cambrian layer roughly started 550 MYA and lasted roughly 50 million years. Within the Cambrian period is the Cambrian explosion that is estimated to have lasted between 5 and 17 million years. In this time, most all the modern phyla are found in the Cambrian. The estimates of the number of phyla vary, but generally is around 50.

The Cambrian Explosion is the radiation of animal phyla that started about 570 million years ago, which is 30 million years after the beginning of the Cambrian geologic period, and lasted between 5 and 17 million years through much of the early Cambrian.

Source: Wikipedia

The Cambrian is a geologic period that began around 542 million years ago and ended about 490 million years ago. The Cambrian Period is the earliest period in whose rocks are found numerous large, distinctly-fossilizable multicellular organisms more complex than sponges or medusoids. During this time, roughly fifty separate major groups of organisms or "phyla" (including almost all the basic body plans of modern animals) emerged suddenly, in most cases without evident precursors. This radiation of animal phyla is referred to as the Cambrian Explosion

Source: Free Dictionary

50 is the number of phyla found in the Cambrian layer, but only 38 exist currently. So, there was actually more phyla in the past than now. This poses a serious anomaly for proponents of common descent. Common descent should predict that more phyla would evolve over time, not an immediate explosion of phyla and a decrease over time.

A simple way of putting it is that currently we have about 38 phyla of different groups of animals, but the total number of phyla discovered during that period of time (including those in China, Canada, and elsewhere) adds up to over 50 phyla. That means [there are] more phyla in the very, very beginning, where we found the first fossils [of animal life], than exist now.

Source: Explosion of Life

Probably the only peer-reviewed article challenging evolutionary theories in regards to the evidence of the Cambrian explosion was just published this year by SC Meyer - The origin of biological information and the higher taxonomic categories Proceedings of the Biological Society of Washington, 117[2]:213-239, August 4, 2004.

It makes for interesting reading and has (predictably) caused quite a reaction among evolutionary proponents.

The "Cambrian explosion" refers to the geologically sudden appearance of many new animal body plans about 530 million years ago. At this time, at least nineteen, and perhaps as many as thirty-five phyla of forty total (Meyer et al. 2003), made their first appearance on earth within a narrow five- to ten-million-year window of geologic time. Many new subphyla, between 32 and 48 of 56 total (Meyer et al. 2003), and classes of animals also arose at this time with representatives of these new higher taxa manifesting significant morphological innovations. The Cambrian explosion thus marked a major episode of morphogenesis in which many new and disparate organismal forms arose in a geologically brief period of time.

He argues that it is highly improbable for random mutations alone to account for the development of complex life forms in the period of the Cambrian explosion. Life would have had to have evolved in many different ways, including protein evolution, cell evolution, and body plan evolution.

Regarding protein evolution, the chance of just a 150 residue protein producing a functional protein by mere chance is 1 in 10E77.

Thus, the probability of finding a functional protein among the possible amino acid sequences corresponding to a 150-residue protein is similarly 1 in 10E77.

Given these odds, just for a 150 residue protein to have evolved in less than 50 million years is highly improbable. Of course, life contains proteins much more complicated than this, so the odds increases even more to explain what we see in the Cambrian explosion.

Some can argue that random mutations will have mostly neutral effects and the functional mutations will gradually increase over time. However, mutagenesis experiments show that there is more functional loss over time than functional gain.

Thus, Axe's results imply that, in all probability, random searches for novel proteins (through sequence space) will result in functional loss long before any novel functional protein will emerge.

Evolving genes and proteins will range through a series of nonfunctional intermediate sequences that natural selection will not favor or preserve but will, in all probability, eliminate.

For a new cell to arise, new combinations of proteins must arise simultaneously from either existing proteins or new proteins. This further decreases the statistical probability of the origin of new cells.

Thus random variations must, again, do the work of information generation--and now not simply for one protein, but for many proteins arising at nearly the same time. Yet the odds of this occurring by chance alone are, of course, far smaller than the odds of the chance origin of a single gene or protein--so small in fact as to render the chance origin of the genetic information necessary to build a new cell type (a necessary but not sufficient condition of building a new body plan) problematic given even the most optimistic estimates for the duration of the Cambrian explosion.

So, he states that the Cambrian explosion does not give evolution enough time to produce all the life that is found in the Cambrian layer.

The sensitivity of proteins to functional loss, the need for long proteins to build new cell types and animals, the need for whole new systems of proteins to service new cell types, the probable brevity of the Cambrian explosion relative to mutation rates--all suggest the immense improbability (and implausibility) of any scenario for the origination of Cambrian genetic information that relies upon random variation alone unassisted by natural selection.

He also brings up that genetic mutation is not sufficient for morphological changes.

Significant morphological change in organisms requires attention to timing. Mutations in genes that are expressed late in the development of an organism will not affect the body plan. Mutations expressed early in development, however, could conceivably produce significant morphological change (Arthur 1997:21). Thus, events expressed early in the development of organisms have the only realistic chance of producing large-scale macroevolutionary change (Thomson 1992). As John and Miklos (1988:309) explain, macroevolutionary change requires alterations in the very early stages of ontogenesis.

Yet recent studies in developmental biology make clear that mutations expressed early in development typically have deleterious effects (Arthur 1997:21). For example, when early-acting body plan molecules, or morphogens such as bicoid (which helps to set up the anterior-posterior head-to-tail axis in Drosophila), are perturbed, development shuts down (Nusslein-Volhard & Wieschaus 1980, Lawrence & Struhl 1996, Muller & Newman 2003).

This problem has led to what McDonald (1983) has called "a great Darwinian paradox" (p. 93). McDonald notes that genes that are observed to vary within natural populations do not lead to major adaptive changes, while genes that could cause major changes--the very stuff of macroevolution--apparently do not vary. In other words, mutations of the kind that macroevolution doesn't need (namely, viable genetic mutations in DNA expressed late in development) do occur, but those that it does need (namely, beneficial body plan mutations expressed early in development) apparently don't occur.6 According to Darwin (1859:108) natural selection cannot act until favorable variations arise in a population. Yet there is no evidence from developmental genetics that the kind of variations required by neo-Darwinism--namely, favorable body plan mutations--ever occur.

Developmental biology has raised another formidable problem for the mutation/selection mechanism. Embryological evidence has long shown that DNA does not wholly determine morphological form (Goodwin 1985, Nijhout 1990, Sapp 1987, Muller & Newman 2003), suggesting that mutations in DNA alone cannot account for the morphological changes required to build a new body plan.


These considerations pose another challenge to the sufficiency of the neo-Darwinian mechanism. Neo-Darwinism seeks to explain the origin of new information, form, and structure as a result of selection acting on randomly arising variation at a very low level within the biological hierarchy, namely, within the genetic text. Yet major morphological innovations depend on a specificity of arrangement at a much higher level of the organizational hierarchy, a level that DNA alone does not determine. Yet if DNA is not wholly responsible for body plan morphogenesis, then DNA sequences can mutate indefinitely, without regard to realistic probabilistic limits, and still not produce a new body plan. Thus, the mechanism of natural selection acting on random mutations in DNA cannot in principle generate novel body plans, including those that first arose in the Cambrian explosion.