EVOLUTION IN CONTEXT
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PALEONTOLOGY - The Natural History of Life
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THE CAMBRIAN EXPLOSION
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THE CAMBRIAN EXPLOSION
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Life is far too complex for our theories to predict the specific body plans which should come into existence as the result of evolution. Both Darwinian gradualism and punctuated equilibrium, however, predict that a systematic branching pattern should be evident if all life has arisen from a common ancestor (see Figures 2 and 3). Both of these models are based upon relatively small intra- or inter-species change and, therefore, predict that numerous transitional species must come into existence which gradually diverge to produce the disparity of the higher taxa. Both models predict that as new species emerge and morphological distance between them increases, new genera and eventually new families should appear. As increasing diversity occurs, new orders, new classes, and ultimately new phyla should come into existence. In short, diversity should precede disparity. The gradually increasing diversity of the lower taxa should ultimately result in the disparity of the higher taxa and the appearance of major new body plans. The pattern of geological succession predicted by both of these evolutionary theories is from bottom to top: species to genera to families to orders to classes to phyla.
FIGURE 2: Phyletic gradualism - Through the continuously gradual transformation of species this model predicts that the increasing diversity of the lower taxa should precede the disparity of the higher taxa.
FIGURE 3: Punctuated Equilibrium - Under this model evolutionary change is confined to shorter time spans and small isolated populations in order to account for the lack of transitional forms among the lower taxa.
Lower taxon-level punctuations, however, require numerous transitional steps to produce the disparity of the higher taxa. This model also predicts that the increasing diversity of the lower taxa should precede the disparity of the higher taxa. The pervasive pattern of geological succession is contrary to these theories (see Figure 4). Disparity precedes diversity. The initial appearance of virtually all phyla occurs with very low species diversity. The origin of the major body plans is not the result of the increasing diversity of the lower taxa; the general pattern is not bottom to top. Rather, the dominant pattern is top to bottom, contrary to theory. As paleontologists Douglas Erwin, James Valentine, and John Sepkoski describe the situation: The fossil record suggests that the major pulse of diversification of phyla occurs before that of classes, classes before that of orders, and orders before families. This is not to say that each higher taxon originated before species (each phylum, class, or order contained at least one species, genus, family, etc. upon appearance), but the higher taxa do not seem to have diverged through an accumulation of lower taxa (Erwin, Valentine, and Sepkoski, 1988).
FIGURE 4: Pervasive pattern of natural history: Disparity precedes diversity. "We may acknowledge a central and surprising fact of life's history - marked decrease in disparity followed by an outstanding increase in diversity within the few surviving designs."
Stephen Jay Gould, Wonderful Life, 1989, p. 49
In another article Valentine and Erwin review hypotheses as to the mode of origin of animal body plans for consistency with the fossil evidence. They conclude that both Darwinian gradualism and punctuated equilibrium are inadequate to account for the appearance of invertebrate body plans and their major modifications:
The models we consider are of three sorts: those that extrapolate processes of speciation to account for higher taxa via divergence, those that invoke selection among species, and those that emphasize that many higher taxa originated as novel lineages in their own right, not only as a consequence of species-level processes. It is in this latter class of model that we believe the record favors (Valentine and Erwin, 1985, p. 71).If large populations have gradually evolved there should be unmistakable evidence in the fossil record, yet it is simply not found.
... many of the large populations should have been preserved, yet we simply do not find them. Small populations are called for, then, but there are difficulties here also. The populations must remain small (and undetected) and evolve steadily and consistently toward the body plan that comprises the basis of a new phylum (or class). This is asking a lot. Deleterious mutations would tend to accumulate in small populations to form genetic loads that selection might not be able to handle. Stable intermediate adaptive modes cannot be invoked as a regular feature, since we are then again faced with the problem of just where their remains are. We might imagine vast arrays of such small populations fanning continually and incessantly into adaptive space. Vast arrays should have produced at least some fossil remains also. Perhaps an even greater difficulty is the requirement that these arrays of lineages change along a rather straight and true course --- morphological side trips or detours of any frequency should lengthen the time of origin of higher taxa beyond what appears to be available. Why should an opportunistic, tinkering process set on such a course and hold it for so long successfully among so many lineages?
We conclude that the extrapolation of microevolutionary rates to explain the origin of new body plans is possible, but does not accord with the primary evidence (Valentine and Erwin, 1985, pp. 95, 96).
The model of punctuated equilibrium or species selection attempts to account for the lack of evidence by relying primarily on the evolution of small isolated populations which would have a diminished chance of leaving a fossil record. This scenario has its difficulties, however, as Valentine and Erwin point out:
The required rapidity of the change implies either a few large steps or many and exceedingly rapid smaller ones. Large steps are tantamount to saltations and raise the problems of fitness barriers; small steps must be numerous and entail the problems discussed under microevolution. The periods of stasis raise the possibility that the lineage would enter the fossil record, and we reiterate that we can identify none of the postulated intermediate forms. Finally, the large numbers of species that must be generated so as to form a pool from which the successful lineage is selected are nowhere to be found. We conclude that the probability that species selection is a general solution to the origin of higher taxa is not great, and that neither of the contending theories of evolutionary change at the species level, phyletic gradualism or punctuated equilibrium, seem applicable to the origin of new body plans (Valentine and Erwin, 1985, p. 96).This evidence further substantiates the proposition that minor lower-level evolutionary change cannot be extrapolated to account for major evolutionary change. This appears to be true for the both the tortoise and the hare, Darwinian gradualism and punctuated equilibrium. The tortoise is far too slow to account for the fossil evidence and the hare spends far too much time in stasis.
Darwin admitted that the geological evidence was the "most obvious and gravest objection which can be urged against my theory". At the time, he was primarily concerned with the lack of transitional forms in the fossil record. Today, those concerns are compounded by fewer transitional forms than Darwin had in his day, and by the systematically upside-down order of geological succession. Darwin was ironically prophetic in stating that the facts can lead to conclusions directly opposite to those at which he arrived.
Roger Lewin further describes the origin of most major body plans in the Science Research News report, "A Lopsided Look at Evolution":
Described recently as "the most important evolutionary event during the entire history of the Metazoa," the Cambrian explosion established virtually all the major animal body forms -- Bauplane or phyla -- that would exist thereafter, including many that were 'weeded out' and became extinct. Compared with the 30 or so extant phyla, some people estimate that the Cambrian explosion may have generated as many as 100. The evolutionary innovation of the Precambrian/Cambrian boundary had clearly been extremely broad: "unprecedented and unsurpassed," as James Valentine of the University of California, Santa Barbara, recently put it (Lewin, 1988).Lewin then asked the all important question:
"Why, in subsequent periods of great evolutionary activity when countless species, genera, and families arose, have there been no new animal body plans produced, no new phyla?" (Lewin, 1988).If neo-Darwinian theory is true, why should the Cambrian contain a greater number of body plans than exist today, particularly with such low species diversity? Figures 5 and 6 graphically illustrate the situation.
FIGURE 5: The Cambrian Explosion The sudden appearance of between 50 and 100 disparate body plans with extremely low species diversity supports the conclusion that neither gradual Darwinian evolution nor lower taxon-level punctuations can account for the origin of the higher taxa and the major body plans. In the history of life on earth, disparity typically precedes diversity.
FIGURE 6: The Present "We may acknowledge a central and surprising fact of life's history - marked decrease in disparity followed by an outstanding increase in diversity within the few surviving designs." (Stephen Jay Gould, 1989)
Lewin's question leads us to an even more important question. What mechanisms have prevented major evolutionary change from occurring over the past 500 million years? Why did the origin of the phyla appear to have stopped first, followed by classes and then orders?
One rather convincing explanation that has been offered for the pattern is based upon the fact that any major novelty that arises relatively quickly through non-selective processes is likely to be poorly adapted. The more novel the body plan is, the more susceptible it will be to elimination by competition and therefore the more adaptive space will be required for it to become established. In short, competition tends to inhibit the establishment of higher taxa. Natural selection tends to inhibit major evolutionary change.
Michael J. Behe
Professor of Biochemistry
Lehigh UniversityMichael J. Denton
Professor of Biochemistry
University of OtagoPhillip E. Johnson
Professor of Law
UC BerkeleyJames W. Valentine
Professor of Integrative Biology
UC Berkeley
Video material courtesy Access Research Network.
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