Wonderful Life: The Burgess Shale and the Nature of History (52 page)

Our hypothetical advocate of the cone and ladder might be willing to give ground on these first two incidents from the dim mists of time, but he might then be tempted to dig his entrenchment across the Cambrian boundary. Surely, once the great explosion occurs, and traditional fossils with hard parts enter the record, then the outlines must be set, and life must move upward and outward in predictable channels.

Not so. As noted in chapter II, the initial shelly fauna, called Tommotian to honor a famous Russian locality, contains far more mysteries than precursors. Some modern groups make an undoubted first appearance in the Tommotian, but more of these fossils may represent anatomies beyond the current range. The story is becoming familiar—a maximum of potential pathways at the beginning, followed by decimation to set the modern pattern.

The most characteristic and abundant of all Tommotian creatures, the archaeocyathids (figure 5.6), represent a long-standing problem in classification. The familiar litany plays again. These first reef-forming creatures of the fossil record are simple in form, usually cone-shaped, with double wall—cup within cup. In the traditional spirit of the shoehorn, they have been shunted from one modern group to another during more than a century of paleontological speculation. Corals and sponges have been their usual putative homes. But the more we learn about archaeocyathids, the stranger they appear, and most paleontologists now place them in a separate phylum destined to disappear before the Cambrian had run its course.

5.6. An archaeocyathid, showing the basic organization of cup within cup.

Even more impressive is the extensive disparity just now being recognized among organisms of the “small shelly fauna.” Tommotian rocks house an enormous variety of tiny fossils (usually one to five millimeters in length) that cannot be allied with any modern group (Bengtson, 1977; Bengtson and Fletcher, 1983). We can arrange these fossils by outward appearance, as tubes, spines, cones, and plates (figure 5.7 shows a representative sample), but we do not know their zoological affinities. Perhaps they are merely bits and pieces from an era of early, still imperfect skeletonization; perhaps they covered familiar organisms that later developed the more elaborate shells of their conventional fossil signatures. But perhaps—and this interpretation has recently been gaining favor among aficionados of the small shelly fauna—most of the Tommotian oddballs represent unique anatomies that arose early and disappeared quickly. For example, Rozanov, the leading Russian expert on this fauna, concludes his recent review by writing:

5.7. Representative organisms of unknown affinity from the Cambrian “small shelly fauna” (Rozanov, 1986). (A)
Tommotia
. (B)
Hyolithellus
. (C)
Lenargyrion
.

Once again, we have a Christmas tree rather than a cone. Once again, the unpredictability of evolutionary pathways asserts itself against our hope for the inevitability of consciousness. The Tommotian contained many modern groups, but also a large range of alternative possibilities. Rewind the tape into the early Cambrian, and perhaps this time our modern reefs are built by archaeocyathids, not corals. Perhaps no Bikini, no Waikiki; perhaps, also, no people to sip rum swizzles and snorkle amidst great undersea gardens.

Our traditionalist is now beginning to worry, but he will grant this one last point
pour mieux sauter
. OK, the very first Cambrian fauna included a plethora of alternative possibilities, all equally sensible and none leading to us. But, surely, once the modern fauna arose in the next phase of the Cambrian, called Atdabanian after another Russian locality, then the boundaries and channels were finally set. The arrival of trilobites, those familiar symbols of the Cambrian, must mark the end of craziness and the inception of predictability. Let the good times roll.

This book is quite long enough already, and you do not want a “second verse, same as the first.” I merely point out that the Burgess Shale represents the early and maximal extent of the Atdabanian radiation. The story of the Burgess Shale is the tale of life itself, not a unique and peculiar episode of possibilities gone wild.

Our traditionalist is now reeling. He is ready to abandon virtually all of life to contingency, but he will make his last stand with vertebrates. The game, after all, centers on human consciousness as the unpredictable product of an incidental twig, or the culmination of an ineluctable, or at the very least a probable, trend. To hell with the rest of life; they aren’t on the lineage leading to consciousness in any case. Surely, once vertebrates arose, however improbable their origin, we could then mount confidently from ponds to dry land to hind legs to big brains.

I might grant the probability of the most crucial environmental transition—from water to land—if the characteristic anatomy of fishes implied, even for incidental reasons, an easy transformation of fins into sturdy limbs needed for support in the gravity of terrestrial environments. But the fins of most fishes are entirely unsuited for such a transition. A stout basal bar follows the line of the body axis, and numerous thin fin rays run parallel to each other and perpendicular to the bar. These thin, unconnected rays could not support the weight of the body on land. The few modern fishes that scurry across mud flats, including
Periophthalmus
, the “walking fish,” pull their bodies along and do not stride with their fins.

Terrestrial vertebrates could arise because a relatively small group of fishes, only distantly related to the “standard issue,” happened, for their own immediate reasons, to evolve a radically different type of limb skeleton, with a strong central axis perpendicular to the body, and numerous lateral branches radiating from this common focus. A structure of this design could evolve into a weight-bearing terrestrial limb, with the central axis converted to the major bones of our arms and legs, and the lateral branches forming digits. Such a fin structure did not evolve for its future flexibility in permitting later mammalian life; (this limb may have provided advantages, in superior rotation, for bottom-dwelling fishes that used the substrate as an aid in propulsion). But whatever its unknown advantages, this necessary prerequisite to terrestrial life evolved in a restricted group of fishes off the main line—the lungfish-coelacanth-rhipidistian complex. Wind the tape of life back to the Devonian, the so-called age of fishes. Would an observer have singled out these uncommon and uncharacteristic fishes as precursors to such conspicuous success in such a different environment? Replay the tape, expunge the rhipidistians by extinction, and our lands become the unchallenged domain of insects and flowers.

Can we not grant the traditionalist some solace? Let contingency rule right to the origin of mammals. Can we not survey the world as mammals emerged into the realm of dinosaurs, and know that the meek and hairy would soon inherit the earth? What defense could large, lumbering, stupid, cold-blooded behemoths provide against smarts, sleekness, live birth, and constant body temperature? Don’t we all know that mammals arose late in the reign of dinosaurs; and did they not then hasten the inevitable transition by eating their rivals’eggs?

This common scenario is fiction rooted in traditional hopes for progress and predictability. Mammals evolved at the end of the Triassic, at the same time as dinosaurs, or just a tad later. Mammals spent their first hundred million year—two-thirds of their total history—as small creatures living in the nooks and crannies of a dinosaur’s world. Their sixty million years of success following the demise of dinosaurs has been something of an afterthought.

We have no indication of any trend toward mammalian hegemony during this initial hundred million years. Quite the reverse—dinosaurs remained in unchallenged possession of all environments for large-bodied terrestrial creatures. Mammals made no substantial moves toward domination, larger brains, or even greater size.

If mammals had arisen late and helped to drive dinosaurs to their doom, then we could legitimately propose a scenario of expected progress. But dinosaurs remained dominant and probably became extinct only as a quirky result of the most unpredictable of all event—a mass dying triggered by extraterrestrial impact. If dinosaurs had not died in this event, they would probably still dominate the domain of large-bodied vertebrates, as they had for so long with such conspicuous success, and mammals would still be small creatures in the interstices of their world. This situation prevailed for a hundred million years; why not for sixty million more? Since dinosaurs were not moving toward markedly larger brains, and since such a prospect may lie outside the capabilities of reptilian design (Jerison, 1973; Hopson, 1977), we must assume that consciousness would not have evolved on our planet if a cosmic catastrophe had not claimed the dinosaurs as victims. In an entirely literal sense, we owe our existence, as large and reasoning mammals, to our lucky stars.

I will not carry this argument to ridiculous extremes. Even I will admit that at some point in the story of human evolution, circumstances conspired to encourage mentality at our modern level. The usual scenario holds that attainment of upright posture freed the hands for using tools and weapons, and feedback from the behavioral possibilities thus provided spurred the evolution of a larger brain.

But I believe that most of us labor under a false impression about the pattern of human evolution. We view our rise as a kind of global process encompassing all members of the human lineage, wherever they may have lived. We recognize that
Homo erectus
, our immediate ancestor, was the first species to emigrate from Africa and to settle in Europe and Asia as well (“Java Man” and “Peking Man” of the old texts). But we then revert to the hypothesis of global impetus and imagine that all
Homo erectus
populations on all three continents moved together up the ladder of mentality on a wave of predictable and necessary advance, given the adaptive value of intelligence. I call this scenario the “tendency theory” of human evolution.
Homo sapiens
becomes the anticipated result of an evolutionary tendency pervading all human populations.

In an alternative view, recently given powerful support by reconstructions of our evolutionary tree based on genetic differences among modern groups (Cann, Stoneking, and Wilson, 1987; Gould, 1987b),
Homo sapiens
arose as an evolutionary item, a definite entity, a small and coherent population that split off from a lineage of ancestors in Africa. I call this view the “entity theory” of human evolution. It carries a cascade of arresting implications: Asian
Homo erectus
died without issue and does not enter our immediate ancestry (for we evolved from African populations); Neanderthal people were collateral cousins, perhaps already living in Europe while we emerged in Africa, and also contributing nothing to our immediate genetic heritage. In other words, we are an improbable and fragile entity, fortunately successful after precarious beginnings as a small population in Africa, not the predictable end result of a global tendency. We are a thing, an item of history, not an embodiment of general principles.

This claim would not carry startling implications if we were a repeatable thing—if, had
Homo sapiens
failed and succumbed to early extinction as most species do, another population with higher intelligence in the same form was bound to originate. Wouldn’t the Neanderthals have taken up the torch if we had failed, or wouldn’t some other embodiment of mentality at our level have originated without much delay? I don’t see why. Our closest ancestors and cousins,
Homo erectus
, the Neanderthals, and others, possessed mental abilities of a high order, as indicated by their range of tools and other artifacts. But only
Homo sapiens
shows direct evidence for the kind of abstract reasoning, including numerical and aesthetic modes, that we identify as distinctively human. All indications of ice-age reckoning—the calendar sticks and counting blade—belong to
Homo sapiens
. And all the ice-age art—the cave paintings, the Venus figures, the horse-head carvings, the reindeer bas-relief—was done by our species. By evidence now available, Neanderthal knew nothing of representational art.