The Origin of the Human Capacity
a James Arthur Lecture on the Evolution of the Human Brain
Just what is it, that strange quality of our consciousness that sets us off from all other living organisms–and which, as importantly, makes us feel so entirely different from them all, even those to whom we know ourselves to be quite closely related? And, whatever it is, how and when did we acquire it? While these questions come close to being unanswerable except in the broadest of terms, they beg to be asked; for they encapsulate the most basic and profound of all the many mysteries posed by our strange and (occasionally) wonderful selves. It is thus natural that they should have been raised–in a variety of different ways and from an equally large number of perspectives–by several earlier James Arthur lecturers on The Evolution of the Human Brain.
My talk today is no exception, and I have even borrowed part of my title from one of my distinguished predecessors as a James Arthur lecturer, Alexander Marshack, who coined the term "the human capacity" to denote this elusive quality that makes humans so distinctive. In his lecture, Marshack (1985) explored the origins of the human capacity in the context of the Ice Age art and symbolism that are its most dramatic early expression. Today I would like to cast my net a little wider, looking at the evidence for the emergence of what we might call cognitive novelties throughout the hominid fossil and archaeological records, and asking whether it is possible to discern any consistent pattern among them. More precisely, I shall ask whether the final step to becoming fully human–the acquisition of the human capacity–was an abrupt or a gradual event, and whether it simply represented the culmination of earlier trends in human evolution, or was an unprecedented, emergent event that could not have been predicted from what went before. And I shall also inquire into possible causes or at least correlates of this fateful step. Before I begin to do this, however, it's necessary to digress for a moment to look briefly at the evolutionary process itself, because the way in which we view this process profoundly affects the manner in which we interpret its manifold results.
Evolution: Expectations from Theory
This may sound odd corning from a lecturer on the evolution of the human brain, but I'll say it anyway. One of the greatest impediments to a rational interpretation of the human fossil record has been the fact that, over the long haul, there has been an undeniable tendency among hominids toward increasing brain size. We'll look later at the details; it's sufficient for the present to note that this "trend" has distracted attention away from diversity in the hominid fossil record, and toward continuity–thereby reinforcing the notion, prevalent from paleoanthropology's earliest days, that our evolutionary history has been one of a slow, singleminded progress from primitiveness to perfection. This transformational bias has proven very congenial to adaptationist paleoanthropologists concerned with projecting the origins of Homo sapiens as far back in time as possible; but the terminology and the mindsets it has fostered are in my view totally inimical to the proper understanding of many important facets of human evolution. And of none of the many aspects of the human story is this truer than it is of the origin of the human capacity. Strangely, perhaps, the first evolutionary biologist to hint at this last point was one of the fathers of the theory of evolution by natural selection, Alfred Russel Wallace. While his elder contemporary Charles Darwin had no doubts whatever that generation-by-generation natural selection fully explained all aspects of human emergence, Wallace, ironically the staunchest advocate of natural selection in all other contexts, simply could not see how this process could have brought into existence the extraordinary consciousness of human beings. "Darwinian theory," he wrote, "shows us how man's body may have developed from that of a lower form under the law of natural selection; but it also teaches us that we possess intellectual and moral faculties which could not have been so developed, but must have had another origin." (Wallace, 1889). Wallace attempted to find this other origin by embracing Spiritualism, a movement that had initially developed in direct opposition to the "materialism" of which evolutionary thought was widely believed to be an excellent example, but which rapidly became rampant with fraud and fakery–which the totally guileless Wallace, as thoroughly decent a human being as ever existed, was ill–equipped to perceive. Wallace's gullibility in this matter greatly annoyed Darwin, and led to a rift between the two men that never completely healed.
The irony, of course, is that both Wallace and Darwin were right. The problem is one of levels of analysis. No reasonable scientist doubts the central role played by natural selection in the evolutionary process, normally at the level of the local population (Tattersall, 1994). But, equally, it is evident that natural selection per se cannot be responsible for the emergence of the evolutionary novelties which it acts to promote or eliminate. A wholly different set of mechanisms, acting at the genomic and developmental levels, comes into play here, and at least potentially these mechanisms are totally independent of those that enter into the selective process. Darwin was certainly correct in surmising that natural selection played a crucial role in establishing enlarged and rewired brains in certain local populations of our ancestors (presumably on the basis of substantial rather than marginal advantages conferred by the resulting behaviors, for the brain, as Bob Martin  pointed out in his James Arthur lecture several years ago, is metabolically a very expensive luxury). But Wallace was equally accurate in his belief that natural selection could not have created human consciousness–that it could not in itself have produced the reconformed or enlarged structures that are responsible for the human capacity, and that it was to favor once they had come into existence. For every organismic attribute has to exist before it can acquire an identifiable function. In this very limited sense all successful evolutionary novelties must necessarily arise as exaptations (Gould and Vrba, 1982): features that appear in one context (in this case, independently of adaptation) before being co-opted by selection in another.
The Human Capacity
So what happened? [...] Consciousness is the product of our brains, which are in turn the product of a long evolutionary history. But the properties of the modem human brain are quite clearly emergent, the result of a chance coincidence of acquisitions. For, while natural selection plays an essential role in the evolutionary process, it is not a creative force; it has to act on variations that come into existence spontaneously. Nothing arises for anything, and natural selection can only work on variations presented to it. We must thus conclude that the immediate ancestor of modem humans possessed a brain that had, for whatever reason, evolved to a point where a single developmental change, or genetically related group of changes, was sufficient to create a structure with an entirely new potential.
But this is probably not the whole story. Recall that the earliest humans who looked exactly like us behaved, as far as can be told, pretty much like Neanderthals, for upward of 50 kyr. These humans had brains that were externally like our own, but that evidently did not function in the way that the Cro-Magnons' did in later times. What happened? Did the earliest anatomically modern and the earliest behaviorally modern humans represent separate but skeletally identical species, the latter eventually replacing the former? This scenario seems inherently improbable, since any such dramatic Old World-wide replacement would have had to have taken place in an implausibly short window of time; and there is, in any event, no direct evidence for it. The only evident (and as we've seen, in terms of evolutionary mechanisms far from unusual) alternative is that the potential for the unique human capacity was born with our species Homo sapiens as a byproduct of some other change, and that it lay fallow, as it were, until unleashed by a cultural (rather than biological) stimulus. This capacity, once declenched, would then have spread quite easily by cultural contact among populations that already possessed the latent ability to acquire it.
What might that releasing stimulus have been? Like many others, I am almost sure that it was the invention of language. We must bear in mind here that by the time Homo sapiens came on the scene the peripheral equipment that allows articulate speech bad been around for several hundred thousand years, having emerged initially for other purposes entirely. The archaeological record is but a dim record of the full panoply of behaviors of any early hominid, but if it shows us anything at all it is the starkness of the contrast between the torrential outpouring of symbolic behaviors by the Cro-Magnons and the essentially symbol-free behaviors of their predecessors. The fundamental innovation that we see with the Cro-Magnons, underwriting all their varied achievements, is that of symbolic thought, with which language is virtually synonymous. Like thought, language involves forming and manipulating symbols in the mind, and our capacity for symbolic reasoning is virtually inconceivable in its absence. Imagination and creativity are part of the same process, for only when we create mental symbols can we combine them in new ways and ask "What if?" Nonverbal intuitive reasoning can, of course, take one a long way (recall the toolmaking experiment); and indeed, we can probably look upon the considerable achievements of the Neanderthals as the ultimate example of what intuition can do; but it is almost certainly symbolic thought that, above all, differentiates us from them–and from every other hominid, indeed every other organism, that has ever existed. For it is surely symbolic reasoning that makes consciousness as we know it possible, given that only by symbolically recreating the world in our minds can we objectify not only ourselves but our own positions in that world.
The strong signal from the behavior record, then, is that our ac quisition of the human capacity was a recent, and emergent, happening. Much as paleoanthropologists like to think of our evolution as a linear affair, a gradual progress from primitiveness to perfection, this received wisdom is clearly in error. We are not simply the inevitable result of a remorseless process of fine-tuning over the eons, any more than we are the summit of creation. For all of our remarkable–and recently acquired–cognitive attributes, we are but one of numerous evolutionary experiments spawned by our diverse family Hominidae. Yes, there has been an overall trend within Hominidae toward an increase in relative brain size over the past 2 million years or so, suggesting that there is something about being hominid (or at least a member of Homo) that predisposes to this trend. And yes, it is unquestionably the unique cognitive features resulting from our own particular experiment that account for the fact that we are the lone hominid in the world today, as for so much else. But we risk deceiving ourselves if we try to link such observations too closely. The archaeological record of human behavioral evolution shows clearly that the human capacity, whatever the neural mechanisms and cultural stimuli underlying it, is the end result of a long and untidy history of sporadic accretion. More significantly, however, it powerfully suggests that the final acquisition of this remarkable capacity was not simply a gradual extrapolation of what had gone before. Instead, this event of events was a sudden and emergent one–whose roots, perhaps ironically, were firmly embedded in the prosaic evolutionary phenomenon of exaptation.
I. Tattersall, The Origin of the Human Capacity, Sixty-eighth James Arthur Lecture on the Evolution of the Human Brain 1998 (New York: American Museum of Natural History, 1998), pp. 1-3, 23-25.