Wonderful Life: The Burgess Shale and the Nature of History (48 page)

This means—and we must face the implication squarely—that the origin of
Homo sapiens
, as a tiny twig on an improbable branch of a contingent limb on a fortunate tree, lies well below the boundary. In Darwin’s scheme, we are a detail, not a purpose or embodiment of the whole—“with the details, whether good or bad, left to the working out of what we may call chance.” Whether the evolutionary origin of self-conscious intelligence in any form lies above or below the boundary, I simply do not know. All we can say is that our planet has never come close a second time.

For anyone who feels cosmically discouraged at the prospect of being a detail in the realm of contingency, I cite for solace a wonderful poem by Robert Frost, dedicated explicitly to this concern:
Design
. Frost, on a morning walk, finds an odd conjunction of three white objects with different geometries. This peculiar but fitting combination, he argues, must record some form of intent; it cannot be accidental. But if intent be truly manifest, then what can we make of our universe—for the scene is evil by any standard of human morality. We must take heart in Darwin’s proper solution. We are observing a contingent detail, and may yet hope for purpose, or at least neutrality, from the universe in general.

Homo sapiens
, I fear, is a “thing so small” in a vast universe, a wildly improbable evolutionary event well within the realm of contingency. Make of such a conclusion what you will. Some find the prospect depressing; I have always regarded it as exhilarating, and a source of both freedom and consequent moral responsibility.

In the last chapter I gave the general, abstract brief for contingency. But the case for “just history” cannot rest on mere plausibility or force of argument. I must be able to convince you—by actual example—that honorable, reasonable, and fascinatingly different alternatives could have produced a substantially divergent history of life not graced by human intelligence.

The problem, of course, with describing alternatives is that they didn’t happen—and we cannot know the details of their plausible occurrence. I feel certain, for example, that no Burgess paleontologist could have surveyed the twenty-five possibilities of arthropod design, rejected the most common (and anatomically sleek)
Marrella
, put aside the beautifully complex
Leanchoilia
or the sturdy, workaday
Sidneyia
, and admitted the ecologically specialized
Aysheaia
and the rare
Sanctacaris
to the company of the elect. But even if we could envision a modern arthropod world built by descendants of
Marrella, Leanchoilia
, and
Sidneyia
, how could we specify the forms that their descendants would take? After all, we cannot even make predictions when we know the line of descent: we cannot see the mayfly in
Aysheaia
, or the black widow spider in
Sanctacaris
. How can we specify the world that different decimations would have produced?

I believe that the best response to this dilemma is to adopt a more modest approach. Instead of seeking an illustration based on unknowable descendants of groups that did not in fact survive, let us consider a plausible alternative world different only in the diversity of two groups that graced the Burgess and survive today—for here we need conjecture only about the reasons for relative abundance. Take two groups of modern oceans—one bursting with diversity, the other nearly gone. Would we have known, at the Burgess beginning of both, which was destined for domination and which for peripheral status in the nooks and crannies of an unforgiving world? Can we make a plausible case for a replay with opposite outcome? (Again, as for so much of this book, I owe this example to the suggestion and previous probing of Simon Conway Morris.)

Consider the current distribution of two phyla sharing the most common invertebrate body plan—the flexible, elongate, bilateral symmetry of “worms.” Polychaetes, the major marine component of the phylum Annelida (including earthworms on land), represent one of life’s great success stories. The best modern epitome, Sybil P. Parker’s
McGraw-Hill Synopsis and Classification of Living Organisms
(1982), devotes forty pages to a breathless summary of their eighty-seven families, one thousand genera, and some eight thousand species. Polychaetes range in size from less than one millimeter to more than three meters; they live nearly everywhere, most on the sea floor, but some in brackish or fresh water, and a few in moist earth. Their life styles also span the range of the thinkable: most are free-living and carnivorous or scavenging, but others dwell commensally with sponges, mollusks, or echinoderms, and some are parasites.

By contrast, consider the priapulids, burrowing worms with bodies divided roughly into three parts—a rear end with one or two appendages, a middle trunk, and a retractable front end, or proboscis. Both the form of the proboscis and its power of erection from the trunk inevitably reminded early male zoologists of something else to which they were, no doubt, firmly and fondly attached—hence the burden of nomenclature for these creatures as
Priapulus
, or the “little penis.”

The armature of the priapulid proboscis might give some cause for alarm in unwarranted analogy. In most species the lower portion sports twenty-five rows of little teeth, or scalids, surmounted by a collar, or buccal ring. The upper end contains several inscribed pentagons of teeth surrounding the mouth. Most priapulids are active carnivores, capturing and swallowing their prey whole, although one species may feed on detritus.

But when we turn to Parker’s compendium of living organisms, we find but three pages devoted to priapulids, with a leisurely description of each family. Priapulids just don’t contribute much to an account of organic diversity; zoologists have found only about fifteen species. For some reason, priapulids do not rank among the success stories of modern biology.

An examination of priapulid distribution provides a clue to their relative failure. All priapulids live in unusual, harsh, or marginal environments—as if they cannot compete in the shallow, open environments frequented by most “standard” marine organisms, and can hang on only where ordinary creatures don’t bother. Two priapulid families include worms grown so small that they live among sand grains in the rich and fascinating (but decidedly “unstandard”) world of the so-called interstitial fauna. Most priapulids belong to the family Priapulidae, larger worms (up to twenty centimeters) of the sea bottom. But these priapulids do not inhabit the richest environments of the shallow-water tropics. They live in the coldest realms—at great depths in tropical regions, and in shallow waters in the frigid climates of high latitudes. They can also tolerate a variety of unusual conditions—low oxygen levels, hydrogen sulfide, low or sharply fluctuating salinity, and unproductive surroundings that impose long periods of starvation. It does not strain the boundaries of reasonable inference to argue that priapulids have managed to keep a toehold in a tough world by opting for difficult places devoid of sharp competition.

We might assume that these striking differences between modern polychaetes and priapulids indicate something so intrinsic about the relative mettle of these two groups that their geological history should be an uninterrupted tale of polychaete prosperity and priapulid struggle. If so, we are in for yet another surprise from the redoubtable Burgess fauna. This first recorded beginning of modern soft-bodied life contains six genera of polychaetes and six or seven genera of priapulids. (See Conway Morris’s monographs on priapulids, 1977d and polychaetes, 1979.)

Furthermore, the Burgess priapulids are numerically a major component of the fauna and, along with anomalocarids and a few arthropods, the earth’s first important soft-bodied carnivores.
Ottoia prolifica
(figure 5.1), most common of the Burgess priapulids, swallowed its prey whole. Hyolithids (conical shelled creatures of uncertain affinity) were favored as food. Thirty-one specimens have been found in the guts of
Ottoia
, most swallowed in the same orientation (and, therefore, almost certainly hunted and consumed in a definite style). One
Ottoia
had six hyolithids in its gut. Another specimen had eaten some of its own—the earliest example of cannibalism in the fossil record.

By contrast, polychaetes (figure 5.2), though equal to priapulids in taxonomic diversity, are much rarer numerically. Conway Morris remarks: “In comparison with the situation in many modern marine environments, the Burgess Shale polychaetes had a relatively minor role.”

Obviously, something dramatic (and disastrous) has happened to priapulids since the Burgess. Once, they had no rivals for abundance among soft-bodied forms, exceeding even the proud polychaetes of current majesty. Now, they are few and forgotten, denizens of the ocean’s spatial and environmental peripheries. The entire modern world contains scarcely more genera of priapulids than the single Burgess fauna from one quarry in British Columbia—while Burgess priapulids occupied center stage, not the tawdry provinces. What happened?

5.1. The Burgess priapulid
Ottoia
in its burrow, with its proboscis half extended. Drawn by Marianne Collins.

We do not know. It is tempting to argue that polychaetes had some biological leverage from the start and were destined for domination, however modest their beginning. But we have no idea what such an advantage might be. Conway Morris makes the intriguing observation that Burgess polychaetes had no jaws and that these organs of successful polychaete predators did not evolve until the subsequent Ordovician period. Perhaps the origin of jaws gave polychaetes their edge over the previously more abundant priapulids?

This supposition is plausible and may be correct, but we do not know; and a correlation (jaws with the beginning of dominance) need not imply a cause. In any case, our hypothetical Burgess geologist would not have known that the modest polychaetes would evolve jaws fifty million years hence.

5.2. The Burgess polychaete
Canadia
. Drawn by Marianne Collins.

The distribution and scarcity of modern priapulids, relative to Burgess abundance, does indicate a basic failure, but who can reconstruct the whys or wherefores? And who can say that a replay of life’s tape would not yield a modern world dominated by priapulids, with a few struggling jawless polychaetes at a tenuous periphery? What did happen makes sense; our world is not capricious. But many other plausible scenarios would have satisfied any modern votary of progress and good sense, and priapulid dominance lies firmly among the might-have-beens.

Are these Burgess fancies common to life’s history throughout or an oddity of uncertain beginnings, superseded by later inexorability? Consider one more might-have-been: When dinosaurs perished in the Cretaceous debacle, they left a vacuum in the world of large-bodied carnivores. Did the current reign of cats and dogs emerge by predictable necessity or contingent fortune? Would an Eocene paleontologist, surveying the vertebrate world fifty million years ago, have singled out for success the ancestors of Leo, king of beasts?

I doubt it. The Eocene world sported many lineages of mammalian carnivores, only one ancestral to modern forms and not especially distinguished at the time. But the Eocene featured a special moment in the history of carnivores, a pivot between two possibilities—one realized, the other forgotten. Mammals did not hold all the chips. In 1917, the American paleontologists W. D. Matthew and W. Granger described a “magnificent and quite unexpected” skeleton of a giant predacious bird from the Eocene of Wyoming,
Diatryma gigantea: