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.
THE ORIGIN OF Homo sapiens
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.
Run the tape again, and let the tiny twig of Homo sapiens expire in Africa. Other hominids may have stood on the threshold of what we know as human possibilities, but many sensible scenarios would never generate our level of mentality. Run the tape again, and this time Neanderthal perishes in Europe and Homo erectus in Asia (as they did in our world). The sole surviving human stock, Homo erectus in Africa, stumbles along for a while, even prospers, but does not speciate and therefore remains stable. A mutated virus then wipes Homo erectus out, or a change in climate reconverts Africa into inhospitable forest. One little twig on the mammalian branch, a lineage with interesting possibilities that were never realized, joins the vast majority of species in extinction. So what? Most possibilities are never realized, and who will ever know the difference?
Arguments of this form lead me to the conclusion that biology’s most profound insight into human nature, status, and potential lies in the simple phrase, the embodiment of contingency: Homo sapiens is an entity, not a tendency.
By taking this form of argument across all scales of time and extent, and right to the heart of our own evolution, I hope I have convinced you that contingency matters where it counts most. Otherwise, you may view this projected replaying of life’s tape as merely a game about alien creatures. You may ask if all my reveries really make any difference. Who cares, in the old spirit of America at its pragmatic best? It is fun to imagine oneself as a sort of divine disk jockey, sitting before the tape machine of time with a library of cassettes labeled “priapulids,” “polychaetes,” and “primates.” But would it really matter if all the replays of the Burgess Shale produced their unrealized opposite—and we inhabited a world of wiwaxiids, a sea floor littered with little penis worms, and forests full of phororhacids? We might be shucking sclerites instead of opening shells for our clambakes. Our trophy rooms might vie for the longest Diatryma beak, not the richest lion mane. But what would be fundamentally different?
Everything, I suggest. The divine tape player holds a million scenarios, each perfectly sensible. Little quirks at the outset, occurring for no particular reason, unleash cascades of consequences that make a particular future seem inevitable in retrospect. But the slightest early nudge contacts a different groove, and history veers into another plausible channel, diverging continually from its original pathway. The end results are so different, the initial perturbation so apparently trivial. If little penis worms ruled the sea, I have no confidence that Australopithecus would ever have walked erect on the savannas of Africa. And so, for ourselves, I think we can only exclaim, O brave—and improbable—new world, that has such people in it!
AN EPILOGUE ON PIKAIA
I must end this book with a confession. I pulled a small, and I trust harmless, pedagogical trick on you. In my long discussion of Burgess Shale organisms, I purposely left one creature out. I might offer the flimsy excuse that Simon Conway Morris has not yet published his monograph on this genu—for he has been saving the best for last. But that claim would be disingenuous. I forbore because I also wanted to save the best for last.
In his 1911 paper on supposed Burgess annelids, Walcott described an attractive species, a laterally compressed ribbon-shaped creature some two inches in length (figure 5.8). He named it Pikaia gracilens, to honor nearby Mount Pika, and to indicate a certain elegance of form. Walcott confidently placed Pikaia among the polychaete worms. He based this classification on the obvious and regular segmentation of the body.
Simon Conway Morris therefore received Pikaia along with his general thesis assignment of the Burgess “worms.” As he studied the thirty or so specimens of Pikaia then known, he reached a firm conclusion that others had suspected, and that had circulated around the paleontological rumor mills for some time. Pikaia is not an annelid worm. It is a chordate, a member of our own phylum—in fact, the first recorded member of our immediate ancestry. (Realizing the importance of this insight, Simon wise
ly saved Pikaia for the last of his Burgess studies. When you have something rare and significant, you must be patient and wait until your thoughts are settled and your techniques honed to their highest craft; for this is the one, above all, that you must get right.)
The structures that Walcott had identified as annelid segments exhibit the characteristic zigzag bend of chordate myotomes, or bands of muscle. Furthermore, Pikaia has a notochord, the stiffened dorsal rod that gives our phylum, Chordata, its name. In many respects Pikaia resembles, at least in general level of organization, the living Amphioxus—long used in laboratories and lecture rooms as a model for the “primitive” organization of prevertebrate chordates. Conway Morris and Whittington declare:
5.8. Pikaia, the world’s first known chordate, from the Burgess Shale. Note the features of our phylum: the notochord or stiffened rod along the back that evolved into our spinal column, and the zigzag muscle bands. Drawn by Marianne Collins.
The conclusion that it [Pikaia] is not a worm but a chordate appears inescapable. The superb preservation of this Middle Cambrian organism makes it a landmark in the history of the phylum to which all the vertebrates, including man, belong (1979, p. 131).
Fossils of true vertebrates, initially represented by agnathan, or jawless, fishes, first appear in the Middle Ordovician, with fragmentary material of uncertain affinity from the Lower Ordovician and even the Upper Cambrian—all considerably later than the Burgess Pikaia (see Gagnier, Blieck, and Rodrigo, 1986).
I do not, of course, claim that Pikaia itself is the actual ancestor of vertebrates, nor would I be foolish enough to state that all opportunity for a chordate future resided with Pikaia in the Middle Cambrian; other chordates, as yet undiscovered, must have inhabited Cambrian seas. But I suspect, from the rarity of Pikaia in the Burgess and the absence of chordates in other Lower Paleozoic Lagerstätten, that our phylum did not rank among the great Cambrian success stories, and that chordates faced a tenuous future in Burgess times.
Pikaia is the missing and final link in our story of contingency—the direct connection between Burgess decimation and eventual human evolution. We need no longer talk of subjects peripheral to our parochial concern—of alternative worlds crowded with little penis worms, of marrelliform arthropods and no mosquitoes, of fearsome anomalocarids gobbling fishes. Wind the tape of life back to Burgess times, and let it play again. If Pikaia does not survive in the replay, we are wiped out of future history—all of us, from shark to robin to orangutan. And I don’t think that any handicapper, given Burgess evidence as known today, would have granted very favorable odds for the persistence of Pikaia.
And so, if you wish to ask the question of the age—why do humans exist?—a major part of the answer, touching those aspects of the issue that science can treat at all, must be: because Pikaia survived the Burgess decimation. This response does not cite a single law of nature; it embodies no statement about predictable evolutionary pathways, no calculation of probabilities based on general rules of anatomy or ecology. The survival of Pikaia was a contingency of “just history.” I do not think that any “higher” answer can be given, and I cannot imagine that any resolution could be more fascinating. We are the offspring of history, and must establish our own paths in this most diverse and interesting of conceivable universe—one indifferent to our suffering, and therefore offering us maximal freedom to thrive, or to fail, in our own chosen way.
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