Human beings have come a long way since parting company with the ancestral African ape seven or eight million years ago. But that long road has been far from the straight one that most of us were taught about in school. Research over the past few decades has shown that there is a lot more to the evolutionary process than the guiding hand of natural selection, by which the best-adapted individuals reproduce more successfully than the rest.
For example, scientists now acknowledge that such factors as mere random chance—as well as competition among entire species jostling for ecological space in constantly and unpredictably changing environments—also play important roles in determining evolutionary outcomes. As we learn more about genetics at the molecular level, we’ve been able to explain how significant evolutionary “leaps” may occur: structurally tiny changes in DNA can reap huge developmental consequences. Impressive additions to the human fossil record over the past half-century have demonstrated beyond a doubt that the history of the human family has been one of vigorous evolutionary experimentation, rather than a stately progression from primitiveness to perfection.
Over the course of the Pliocene and Pleistocene epochs (the past six million years), an amazing diversity of hominids competed robustly for space on the ecological stage. Indeed, several species of hominid typically existed at any one time. Two million years ago, for instance, at least seven hominid species shared the Old World. And that is just in the known record, which vastly underrepresents the actual diversity. For modern Homo sapiens to be the world’s only hominid is highly unusual. This plain fact alone indicates just how unique we are, even compared with our most immediate predecessors.
Ever since its origin a little less than two million years ago, the genus Homo has shown a tendency toward species proliferation; around nine species are now recognized within it, even by fairly conservative paleoanthropologists. At the same time, species within our genus generally have shown a strong common tendency toward larger brains over time. On the face of it, this propensity might seem to argue for the process of steady modification; but if you look more closely, this same trend occurred independently in at least three separate lineages within Homo: namely, those leading to Homo neanderthalensis in Europe, to Homo sapiens in Africa, and to late Homo erectus in eastern Asia. Therefore, we have to seriously consider the possibility that this tendency was due not to the greater reproductive success of larger-brained individuals, but to the greater competitive success of larger-brained species. Nonetheless, because brain enlargement exists throughout the genus Homo, paleoanthropologists will need to know what underlies it before we can fully explain the amazing phenomenon that is the cognitively modern Homo sapiens.
We human beings today are most strongly distinguished by our ability to disassemble the world around us into a vocabulary of abstract symbols, which we can rearrange in our minds to produce notions of alternative worlds. The acquisition of this faculty was not a simple consequence of increasing brain size; the recently extinct Neanderthals had brains as large as ours or even larger, but did not show the same proclivities. What’s more, judging by a striking absence of representative objects in the early archaeological record, our symbolic capacity was a very late acquisition, substantially postdating the appearance of hominids whose bony skeletons looked exactly like ours. Evidently, early Homo sapiens needed time to discover its symbolic potential, through the action of a cultural stimulus, not a biological one.
Almost certainly, that stimulus was the invention of language, which is after all the ultimate symbolic activity, and which we know can be spontaneously invented by human groups. But whatever the case, the biological potential that underpins symbolic reasoning was acquired in an exaptive, rather than in an adaptive, context. Our unique intelligence was not driven into existence by natural selection; it was, instead, a purely emergent acquisition. But, once acquired, it made Homo sapiens a competitor of unprecedented ferocity. The previously diverse hominid competition, including those big-brained Neanderthals, quickly disappeared.
Even though only we possess reason, we modern Homo sapiens are far from perfected. We are still poor judges of risk, for example, and prone to making bad decisions. The average brain size of a member of Homo sapiens has declined by well over 10 percent since the peak of the last Ice Age, yet it would be foolish to associate this reduction with any loss of brain power. The explanation of the amazing shrinking brain is probably due to a combination of factors. Because brain tissue is metabolically expensive, there is no advantage in having any more of it than necessary. Most likely, the new symbolic way of dealing with information is more algorithmically efficient than the different mode that preceded it, reducing the volume of brain needed for adequate function. Armed with these streamlined brains, humans probably won’t acquire more raw brain power in the future.
Still, since we are the product of a long process of mental refinement among our forebears (who, despite lacking symbolic cognition, nonetheless had formidable intuitive intelligence), to hope for further refinement is only natural. It has often been predicted, for instance, that our descendants will be supremely rational, possessing huge, brainy heads perched atop puny bodies because strenuous work would no longer be required.
Alas, this prediction derives from the erroneous selection-driven model of our evolution. In reality, our forebears lived in small groups, sparsely scattered over vast landscapes that were subject to rapid climatic variation on huge scales. These are precisely the circumstances in which you would expect swift evolutionary change, as tiny, genetically unstable populations became stressed, isolated, and thrown together again in rapid iteration. But today we live cheek by jowl in a crowded world, in huge populations too large to allow meaningful genetic novelties to become incorporated in them. In short, the conditions necessary for meaningful change don’t exist right now. Short of some cataclysmic change in our circumstances, we shall have to learn to live with ourselves as we are.
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