Wonderful Life: The Burgess Shale and the Nature of History (58 page)

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Invoking another aspect of the same image—the equation of old and extinct with inadequate—Granada exhorts us to rent rather than buy because “today’s latest models could be obsolete before you can say brontosaurus.”

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Wonderfully ironic, since the sequence showed, basically, more effective filters. Evolution, to professionals, is adaptation to changing environments, not progress. Since the filters were responses to new conditions—public knowledge of health dangers—Doral did use the term
evolution
properly. Surely, however, they intended “absolutely better” rather than “punting to maintain profit”—a rather grisly claim in the light of several million deaths attributable to cigarette smoking.

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A properly defined group with a single common ancestor is called monophyletic Taxonomists insist upon monophyly in formal classification. However, many vernacular names do not correspond to well-constituted evolutionary groups because they include creatures with disparate ancestries—“polyphyletic” groups in technical parlance. For example, folk classifications that include bats among birds, or whales among fishes, are polyphyletic The vernacular term
animal
itself probably denotes a polyphyletic group, since sponges (almost surely), and probably corals and their allies as well, arose separately from unicellular ancestors—while all other animals of our ordinary definitions belong to a third distinct group. The Burgess Shale contains numerous sponges, and probably some members of the coral phylum as well, but this book will treat only the third great group—the coelomates, or animals with a body cavity. The coelomates include all vertebrates and all common invertebrates except sponges, corals, and their allies. Since the coelomates are clearly monophyletic (Hanson, 1977), the subjects of this book form a proper evolutionary group.

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This fundamental principle, while true for the complex multicellular animals treated in this book, does not apply to all life. Hybridization between distant lineages occurs frequently in plants, producing a “tree of life” that often looks more like a network than a conventional bush. (I find it amusing that the classic metaphor of the tree of life, used as a picture of evolution ever since Darwin and so beautifully accurate for animals, may not apply well to plants, the source of the image.) In addition, we now know that genes can be transferred laterally, usually by viruses, across species boundaries. This process may be important in the evolution of some unicellular creatures, but probably plays only a small role in the phylogeny of complex animals, if only because two embryological systems based upon intricately different developmental pathways cannot mesh, films about flies and humans notwithstanding.

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Another factual irony: despite the usual picture of horseshoe crabs as “living fossils,”
Limulus polyphemus
(our American East Coast species) has no fossil record whatever. The genus
Limulus
ranges back only some 20 million years, not 200 million. We mistakenly regard horseshoe crabs as “living fossils” because the group has never produced many species, and therefore never developed much evolutionary potential for diversification; consequently, modern species are morphologically similar to early forms But the species themselves are not notably old.

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Twain used Lord Kelvin’s estimate, then current, for the age of the earth. The estimated ages have lengthened substantially since then, but Twain’s proportions are not far off. He took human existence as about 1/3000 of the earth’s age. At current estimates of 250,000 years for the origin of our species.
Homo sapiens
, the earth would be 0.75 billion years old if our span were 1/3000 of totality. By best current estimates, the earth is 4.5 billion years old.

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I have struggled over a proper name for this phenomenon of massive elimination from an initial set of forms, with concentration of all future history into a few surviving lineages. For many years, I thought of this pattern as “winnowing,” but must now reject this metaphor because all meanings of winnowing refer to separation of the good from the bad (grain from chaff in the original)—while I believe that the preservation of only a few Burgess possibilities worked more like a lottery.

I have finally decided to describe this pattern as “decimation,” because I can combine the literal and vernacular senses of this word to suggest the two cardinal aspects stressed throughout this book: the largely random sources of survival or death, and the high overall probability of extinction.

Randomness
. “Decimate” comes from the Latin
decimare
, “to take one in ten.” The word refers to a standard punishment applied in the Roman army to groups of soldiers guilty of mutiny, cowardice, or some other crime. One soldier of every ten was selected by lot and put to death. I could not ask for a better metaphor of extinction by lottery.

Magnitude
. But the literal meaning might suggest the false implication that chances for death, though applied equally to all, are rather low—only about 10 percent. The Burgess pattern indicates quite the opposite. Most die and few are chosen—a 90 percent chance of death would be a good estimate for major Burgess lineages. In modern vernacular English, “decimate” has come to mean “destroy an overwhelming majority,” rather than the small percentage of the ancient Roman practice. The
Oxford English Dictionary
indicates that this revised usage is not an error or a reversed meaning, but has its own pedigree—for “decimation” has also been used for the taking of nine in ten.

In any case, I wish to join the meaning of randomness explicit in the original Roman definition with the modern implication that most die and only a few survive. In this combined sense, decimation is the right metaphor for the fate of the Burgess Shale fauna—random elimination of most lineages.

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There are two in jokes in this line:
orogeny
is standard geological jargon for mountain building;
Paleobscene
is awfully close to the epoch’s actual name—Paleocene.

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The “lowest Silurian” refers to rocks now called Cambrian, a period not yet codified and accepted by all in 1859. Darwin is discussing the Cambrian explosion in this passage.

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Although the 12C/13C ratio in the Isua rocks is indicative of organic fractionation, the excess of 12C is not so high as for later sediments. Schidlowski argues that the subsequent metamorphism of the Isua rocks lowered the ratio (while leaving it within the range of organic values), and that the original ratio probably matched that of later sediments.

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Burgess himself was a nineteenth-century governor general of Canada; Walcott named the formation not for him but for Burgess Pass, which provided access to the quarry from the town of Field.

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Much material in this section comes from my previous essay on Walcott’s discovery (Gould, 1988).

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I know this so well from personal experience. People ask me all the time what 1 was thinking when Niles Eldredge and I first developed the theory of punctuated equilibrium in the early 1970s. I tell them to read the original paper, for I don’t remember (or at least cannot find those memories amidst the jumble of my subsequent life).

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These outer coverings were, of course, harder than the soft organs below. But the carapaces of most Burgess organisms were not mineralized, and therefore not formed of conventional “hard parts” that fossilize easily. These carapaces were rather like the exoskeletons of modern insects, stiffened but not mineralized. “Lightly sclerotized” might be a better term than “soft-bodied,” but the potential for conventional fossilization is nearly nil in either case.

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I asked Whittington why so little work had been done before his redescriptions, for Walcott’s specimens had always been available at the Smithsonian. He cited a number of reasons, all no doubt contributory, but not enough in their ensemble to explain this curious fact. Walcott’s wife, for one, was quite possessive and discouraging, though she held no proprietary power over the specimens. She hated Percy Raymond for collecting again at the Burgess so soon after her husband’s death in 927. Raymond, in his turn, had been no fan of Walcott’s, and taunted him as “the great executive paleontologist” for letting administrative work absorb all his time, thus precluding a proper study of the Burgess fossils. (This was an unusually acerbic assessment for Raymond, who was the most mild-mannered of men. Al Romer, who knew him well, once told me that Raymond was at the bottom of a familial pecking order, with his wife, children, and dog above him. His favorite hobby, collecting pewterware, definitely contributed to his non-macho image.) While Walcott lived, no one else would work on the specimens, for he always intended to do a proper study himself, and no one dared upstage the most powerful man in American science. (Such proprietary claims are traditionally honored in paleontology, even for scientists low on the totem pole; discovery implies the right of description, with a statute of limitation often construed as extending for a lifetime.) Walcott’s wife, and the memory of his power, managed to extend a reluctance for work on Burgess material even beyond Walcott’s grave. Moreover, as Whittington reports, although the “type” specimens were accessible (the few used in the original descriptions of the species), almost all the material resided in drawers placed high in cabinets, and therefore unavailable for casual browsing-the serendipitous mode of origin for many paleontological studies. They also were housed in a building without air conditioning (now remedied). Most paleontologists work in universities, and have substantial free time only during the summer. Need I say more to anyone who has experienced the pleasures of our nation’s capital in July or August!

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Walcott, of course, had not failed to note this prominent organ, and its uniqueness did pose a problem for his conclusion that
Yohoia
was a branchiopod. Walcott evaded this dilemma by arguing that the great appendage was a male “clasper,” or structure used to hold females during mating (and present in many branchiopods). But Whittington determined that all specimens bore great appendages, and disproved Walcott’s rationale.