THIS VIEW OF LIFE

Of Tongue Worms, Velvet Worms, and Water Bears

Fresh evidence confirms the uniqueness of the Cambrian explosion

by Stephen Jay Gould

I belong to a generation of students formally educated, in large measure, by the practice of rote memorization. Hence, I know the Gettysburg Address by heart (and who can ever expunge the arcana learned at age ten, while who, at age fifty, can retain the important items encountered last week). At least I know Lincoln's line for paleontologists: ". . . we here highly resolve that these dead shall not have died in vain." And, when Stephen Dedalus, in Joyce's novels, routes "on old Olympus's topmost tops" into his stream of consciousness, I know that he is musing upon the standard mnemonic for remembering the names of cranial nerves in proper sequence front to back--olfactory, optic, oculomoter. . . .

Among the classical items of rote memorizatior in early schooling, two stand out for later utility to paleontologists like me--the geological time scale and the list of animal phyla, the major taxonomic divisions of life in our kingdom (some twenty to forty depending on the version you learned). Most of my fellow students didn't complain too much about the dozen or so major groups, for everyone should know a vertebratte from an arthropod from a mollusk from an echinoderm, if only because we do encounter such creatures in our daily lives. But for the larger number of so-called minor phyla-the unrememberable whatchamacallits with such funny names as Ctenophora (comb jellies) and Priapulida (little penis worms)--most of us had only contempt and loathing, for we couldn't recall them on exams, and we never encountered them in Central Park or at Jones Beach ("nature" to New York City kids).

Yet these "minor" phyla embody some of the most fascinating problems of natural history and should not be ranked with the unknown and the unloved. They are, first of all, minor only in the sense of current membership (few species alive today); although some, brachiopods and bryozoans in particular, dominated the early fossil record of multicellular animal life. Moreover, these groups are decidedly not minor in degree of anatomical distinctness, for they are as different, one from the next, as a fish from a fly or a clam from a sea cucumber.

The minor phyla must play a crucial role in unraveling the greatest of all mysteries surrounding the history and fossil record of animal life. I have often written in these essays about the "Cambrian explosion," the extremely restricted time that encompasses the first appearance in the fossil record of nearly all basic anatomical designs for animal life. According to a recent study, the first ever based upon rigorously determined radiometric age dates, this episode lasted an astonishingly short 5 million years, from about 535 to 530 million years ago.

Since then, only one new phylum with a prominent fossil record has been added to life's archives--the Bryozoa, a group of small colonial organisms, which, like reef-building corals, secrete calcified skeletons surrounding the individual animals of a colony. (The bryozoans arose at the beginning of the very next, or Ordovician, period and their Cambrian absence may be an artifact of our failure to find earlier representatives.) One might argue without great exaggeration that 530 million years of subsequent evolution has produced no more than a set of variations upon themes established during this initial explosion--although some of these little fillips, including human consciousness and insect flight, have had quite an impact upon the history of life!

The minor phyla provide a key to the Cambrian explosion because they-represent a potential exception and softening. This episode is enormously puzzling and contrary to preferred assumptions about the generally slow and steady character of evolutionary change. Therefore, paleontologists have sought (largely unconsciously, for thus do we act upon our deepest biases) mitigating circumstances or arguments that might either diminish or spread out the Cambrian explosion.

Among such sops to our uniformitarian preferences, none has been more common--I can hear the words in my mind as stated by a bevy of professors and read in dozens of books--than following potential invocation of minor phyla to make the Cambrian explosion merely an intensification of ordinary possibilities, rather than an exclusivity: "But how can you claim that all phyla originated during this tiny beginning interval? After all, about half the animal phyla contain no hard parts at all and therefore have no fossil record. How do you know that these groups haven't been arising throughout the 530 million years since the Cambrian explosion? Moreover, most of these phyla contain very few species. Doesn't their rarity indicate a potentially recent origin, leaving little time for their gradual spread and speciation?"

This argument is not unreasonable and seems particularly strong under certain

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