Darwinian Magic: Another Question-Begging Darwinian Explanation of the Cambrian Explosion
Why did 20 or more separate animal phyla suddenly appear in the geological blink of an eye? Because "Clades reach highest morphological disparity early in their evolution," answer Martin Hughes, Sylvain Gerber, and Matthew Albion Wills, writing in the Proceedings of the National Academy of Sciences. That's all, folks. The Cambrian animals exploded onto the scene because evolution works fast sometimes. It seems to be a pattern that high disparity occurs early on. Must be a law of nature. What, you've got a problem with that?
These three biologists from the University of Bath took a bath in their own assumptions. They don't deny that the animals appear suddenly. They just believe that when opportunity arises, evolution works fast to fill up the landscape with endless forms most beautiful. Under the section, "Why Do Clades Have Early High Disparity?" they say:
What might explain the prevailing pattern of early high disparity in clade evolution? Both ecological and developmental explanations have been proposed, and our results remain consistent with both. The "empty ecospace" model predicts that clades will radiate and diversify more rapidly when colonizing a new environment. This colonization may occur because ecospace has been vacated by other occupants (e.g., in the wake of some other extinction, typically the result of external, physical factors) or because a hitherto inaccessible environment or other resource has been rendered viable by the acquisition of some novel, "key" adaptation or series of characters (an intrinsic, biological trigger). Morphological change under these circumstances may be rapid either because transitions are unusually large or because rates of cladogenesis are unusually high (even with "normal" step sizes at each splitting event). (Emphasis added, reference numbers omitted.)
Jaws should drop at that "explanation." If Hughes, Gerber and Wills were simply describing what is found in the fossil record -- the sudden appearance of complex animals -- it would be one thing. But they attempted to explain how these complex animals appeared, so suddenly as Jonathan Wells described in the film Darwin's Dilemma, that it's comparable to less than two minutes on a twenty-four-hour clock.
Look carefully at the wording above. The authors are very careful to hide their faith in Darwinian evolution by using indirect language. What is "the acquisition of some novel, 'key' adaptation or series of characters" but a cover for some unexplained, one might say miraculously improbable innovation? Was it a trilobite eye? Was it a jointed appendage? Was it a digestive tract? And pray tell, what is "cladogenesis" if not a hand-waving dodge, magic word for the evolutionary "creation" of a whole new body plan, like a phylum? They provide an example of what they mean: birds --
...birds are defined in the manner they are because they acquired a distinctive suite of apomorphies pertaining to the evolution of flight; key innovations, in this case, that also enabled them to exploit a new environment.
Time to watch Flight: The Genius of Birds again.
If evolution rushes in to fill an "ecospace [that] has been vacated" with "key innovations" that "enable them to exploit a new environment," then why don't we see Mars filled with life? Mars is a whole planet waiting to be filled. The "Vacancy" sign has been up for billions of years. What's evolution waiting for?
It's absurd to assume that opportunity necessarily leads to fulfillment. As humans we sometimes say, "If you build it, they will come," but Darwinian evolution has no motivation to come. It is aimless, blind and purposeless. You can wait till the cows come home and nothing will happen unless beneficial mutations arise out of sheer dumb luck (perhaps by cosmic rays), in the right order, each one contributing to survival with no supervising intention steering them to work together.
To see the absurdity of expecting evolution to fill an ecospace with cladogenesis, run the experiment Jonathan Wells proposed: rupture a living cell in a sealed test tube and wait. Rupture a million different kinds of cells, if you prefer. The tube has the perfect environment, because all the ingredients are present -- they were once part of living cells. Does anyone really believe "cladogenesis" will occur, to fill in the vacated ecospace? The late triple-PhD organic chemist A. E. Wilder-Smith use to give another pithy example: a sardine can. It's another perfect environment for the emergence of life, because the sardines were once alive, and all their parts are intact. There are millions of sardine cans around the world. If anyone thought for a minute that a new life form would emerge inside a sardine can (absent any living spores), the food industry would panic.
Hughes, Gerber and Wills might respond that they are not assuming the origin of life, but the adaptation of existing life. Natural selection is available with existing life where it is not in chemical evolution. That would be a weak defense. Available ecospace existed in the Precambrian seas, filled with life, for over a billion years. The authors do not mention any plausible trigger that made complex multicellular animals appear simultaneously when they did. They speculate about ocean chemistry, sea level and climate change, but those things presumably varied throughout Precambrian times.
As Stephen Meyer shows in Darwin's Doubt, prior to the Cambrian explosion there were only microbes, sponges and the enigmatic Ediacaran fauna that most paleontologists do not consider related to the Cambrian animals. There were no jointed appendages, eyes, guts, nervous systems or any of the other organs, tissue types and body plans that characterize Cambrian phyla. What kind of Darwinian "miracle" would it take to get all those complex traits in blink of an eye? Calling it "cladogenesis" is like calling the simultaneous chance appearance of cars, boats, and airplanes "vehicle-o-genesis."
The model has striking similarities to the old notion of Punctuated Equilibrium discussed in Chapter 7 of Darwin's Doubt. In fact, the PNAS paper was edited by Steven Stanley, "an early advocate of the punctuated equilibrium model" according to Meyer (p. 137). As such, this new model is really a throwback to the old "punk eek," and suffers from the same drawbacks: it's an attempt to explain away the absence of evidence for evolution by claiming, without any mechanism better than old-fashioned neo-Darwinism, why the fossil record shows a discontinuous pattern, not a record of a gradually branching tree.
In addition to cladogenesis, the authors entertain the idea that developmental changes (like mutations in Hox genes) can lead to innovations:
We stress that ecological and developmental explanations for early high disparity are not mutually exclusive; neither do our results allow us to distinguish between them. The hypothesis of increasing developmental constraint predicts that the increasing complexity and interdependence of ontogenetic processes with evolutionary time effectively lock down the potential for subsequent morphological innovation. Such mechanisms purportedly explain why body plans become invariant and inflexible with time, although mechanisms by which these constraints may be lifted have been posited.
This is off topic for the Cambrian explosion. It's the origin of the body plans, not their subsequent invariance, that is the issue. Besides, the authors leave it off, saying, "Testing this hypothesis would require detailed ontogenetic data far beyond the scope of this study."
Explosion as a Law of Nature
Basically, the authors try to present "high disparity" not just as a pattern, but a rule -- a way that evolution works. For support of their evolutionary "rule" they point to other instances in the fossil record of rapid ecospace filling after mass extinctions, concluding:
Overall, there is no clear temporal trend in disparity profile shapes from the Cambrian to the Recent, and early high disparity is the predominant pattern throughout the Phanerozoic.
One appreciates the concession. Adding miracles to miracles doesn't help Darwinian evolution. Furthermore, assuming Darwinism was responsible is known as begging the question. Their statement also avoids the fact that nearly all animal phyla -- the highest taxonomic level of disparity -- appeared in the Cambrian, not in subsequent epochs of the fossil record.
Hughes, Gerber and Wills begin their paper with this damaging admission for a theory that still rules biology 154 years after it was proposed:
There are few putative macroevolutionary trends or rules that withstand scrutiny.
The authors of this paper did not want scrutiny. They were under the spell of seduction:
Evolution is usually characterized as an essentially contingent and unpredictable process. This makes it very difficult to identify general rules comparable to those that typify the other natural sciences. Nonetheless, the prospect of formulating and testing macroevolutionary generalities is extremely seductive, because they seem to offer fundamental insights into the manner in which evolutionary processes operate throughout Earth's history.
You want to be charitable to these authors, but it's challenging. They know full well that they were using Darwinian evolution as filler in the absence of data:
Early high patterns inevitably imply an unsampled period of cladogenesis (or the existence of ghost ranges) at the base of the clade, but this either occurs too fast for the available stratigraphic resolution or is not fossilized.
They also know full well what they were up against:
Among the first questions to be addressed using disparity indices was the perceived magnitude of the Cambrian "explosion." From Charles Darwin onward, evolutionary biologists have been perplexed by the apparently instantaneous first appearances of numerous phyla (a highly disparate sample of species) in the Cambrian fossil record. The subsequent discovery of hitherto unknown fossil groups from the Cambrian Burgess Shale and similar localities added to the enigma, prompting the radical hypothesis that the disparity of metazoans peaked in the Cambrian and subsequent extinctions winnowed this down to much more modest levels soon thereafter. Surprisingly, a relatively small number of studies have tested this hypothesis directly in focal clades. These predominantly conclude that Cambrian animal groups had a disparity comparable to that of their modern counterparts. This nonetheless suggests that metazoans reached high levels of disparity relatively early in their history, the phenomenon of early high disparity. Unfortunately, such analyses are limited for two reasons. First, they discount the intervening trajectory of clade evolution. Second, the clade history is truncated both by the present and by a Precambrian fossil record that is enigmatic at best. As a result, the focus of disparity studies has increasingly turned to clades that both originate and go extinct within the Phanerozoic. Once again, there is a purported tendency for clades to evolve their most disparate forms relatively early in their histories. However, the validity of this early high disparity model has never been tested systematically.
So they tested it and could only come up with "cladogenesis" to fill "vacant ecospace" with "key innovations." That, it seems, is another way of saying Darwinian magic.
Image credit: JD Hancock/Flickr.