CAMBRIAN EXPLOSION / ORIGIN OF THE PHYLA


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"When Charles Darwin wrote "The Origin of Species " in 1859, the sudden appearance of animal fossils at the beginning of the Cambrian was of particular concern to him. It was at odds with his view that the diversification of life on earth through natural selection had required a long period of time. Darwin's theory predicted that the major groups of animals should gradually diverge during evolution. He knew that the sudden appearance of fossils would be used by his opponents as a powerful argument against his theories of descent with modification and natural selection. Consequently, he argued that a long period of time, unrepresented in the fossil record, must have preceded the Cambrian to allow the various major groups of animals to diverge. At that time the strata that we now regard as Cambrian were subsumed within the concept of the Silurian, so Darwin wrote,

'I cannot doubt that all the Silurian trilobites have descended from some one crustacean, which must have lived long before the Silurian age....Consequently, if my theory be true, it is indisputable that before the lowest Silurian strata was deposited, long periods elapsed, as long as, or probably longer than, the whole interval from the Silurian to the present day.....The case must at present remain inexplicable; and may be truely urged as a valid argument against the views here entertained'

The Origin of Species, 1859, pp. 313 - 314

"On the sudden appearance of groups of Allied Species in the lowest known fossiliferous strata"

Consequently, if my theory be true, it is indisputable that before the lowest Silurian stratum was deposited, long periods elapsed, as long as, or probably far longer than, the whole interval from the Silurian age to the present day; and that during these vast, yet quite unknown periods of time, the world swarmed with living creatures. To the question why we do not find records of these vast primordial periods, I can give no satisfactory answer.

Note: In Darwin's time, the "Silurian" was the name given the oldest known fossil-bearing strata. "Cambrian" does not occur as an index entry in this edition of the Origin.

Most families, orders, classes, and phyla appear rather suddenly in the fossil record, often without anatomically intermediate forms smoothly interlinking evolutionarily derived descendant taxa with their presumed ancestors.

The record jumps, and all the evidence shows that the record is real: the gaps we see reflect real events in life's history -- not the artifact of a poor fossil record.

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).

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?"

The Meanings of Diversity and Disparity

"I must introduce at this point an important distinction that should allay a classic source of confusion. Biologists use the vernacular term diversity in several different technical senses. They may talk about "diversity" as number of distinct species in a group: among mammals, rodent diversity is high, more than 1,500 separate species; horse diversity is low, since zebras, donkeys, and true horses come in fewer than ten species. But biologists also speak of "diversity" as difference in body plans. Three blind mice of differing species do not make a diverse fauna, but an elephant, a tree, and an ant do -- even though each assemblage contains just three species.

The revision of the Burgess Shale rests upon its diversity in this second sense of disparity in anatomical plans. Measured as number of species, Burgess diversity is not high. This fact embodies a central paradox of early life. How could so much disparity in body plans evolve in the apparent absence of substantial diversity in number of species? -- for the two are correlated, more or less in lockstep, by the iconography of the cone (see figure 1.16). ...

Several of my colleagues (Jaanusson, 1981; Runnegar, 1987) have siggested that we eliminate the confusion about diversity by restricting this vernacular term to the first sense -- number of species. The second sense--- difference in body plansshould then be called disparity. Using this terminology, 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."

[G]aps between higher taxonomic levels are general and large.

Evidence of gradualism between phyla, classes and even orders is either non-existent or is much disputed. Certainly, no pervasive pattern of gradualism exists. George Gaylord Simpson acknowledged this decades ago as he described the situation in these terms:

"This is true of all thirty-two orders of mammals...The earliest and most primitive known members of every order already have the basic ordinal characters, and in no case is an approximately continuous sequence from one order to another known. In most cases the break is so sharp and the gap so large that the origin of the order is speculative and much disputed...

This regular absence of transitional forms is not confined to mammals, but is an almost universal phenomenon, as has long been noted by paleontologists. It is true of almost all classes of animals, both vertebrate and invertebrate...it is true of the classes, and of the major animal phyla, and it is apparently also true of analogous categories of plants.



[T]he fossil record itself provided no documentation of continuity -- of gradual transitions from one kind of animal or plant to another of quite different form.


The gaps in the fossil record are real, however. The absence of a record of any important branching is quite phenomenal. Species are usually static, or nearly so, for long periods, species seldom and genera never show evolution into new species or genera but replacement of one by another, and change is more or less abrupt.

[T]he origin of no innovation of large evolutionary significance is known.

[L]arge evolutionary innovations are not well understood. None has ever been observed, and we have no idea whether any may be in progress. There is no good fossil record of any.

Taxa recognized as orders during the (Precambrian-Cambrian) transition chiefly appear without connection to an ancestral clade via a fossil intermediate. This situation is in fact true of most invertebrate orders during the remaining Phanerozoic as well. There are no chains of taxa leading gradually from an ancestral condition to the new ordinal body type. Orders thus appear as rather distinctive subdivisions of classes rather than as being segments in some sort of morphological continuum.

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)