The Tell-Tale Brain: A Neuroscientist's Quest for What Makes Us Human

THIS LIBERATION FROM the constraints of a strictly gene-based Darwinian evolution was a giant step in human evolution. One of the big puzzles in human evolution is what we earlier referred to as the “great leap forward,” the relatively sudden emergence between sixty thousand and a hundred thousand years ago of a number of traits we regard as uniquely human: fire, art, constructed shelters, body adornment, multicomponent tools, and more complex use of language. Anthropologists often assume this explosive development of cultural sophistication must have resulted from a set of new mutations affecting the brain in equally complex ways, but that doesn’t explain why all of these marvelous abilities should have emerged at roughly the same time.

One possible explanation is that the so-called great leap is just a statistical illusion. The arrival of these traits may in fact have been smeared out over a much longer period of time than the physical evidence depicts. But surely the traits don’t have to emerge at exactly the same time for the question to still be valid. Even spread out, thirty thousand years is just a blip compared to the millions of years of small, gradual behavioral changes that took place prior to that. A second possibility is that the new brain mutations simply increased our general intelligence, the capacity for abstract reasoning as measured by IQ tests. This idea is on the right track, but it doesn’t tell us much—even leaving aside the very legitimate criticism that intelligence is a complex, multifaceted ability which can’t be meaningfully averaged into a single general ability.

That leaves a third possibility, one that brings us back full circle to mirror neurons. I suggest that there was indeed a genetic change in the brain, but ironically the change freed us from genetics by enhancing our ability to learn from one another. This unique ability liberated our brain from its Darwinian shackles, allowing the rapid spread of unique inventions—such as making cowry-shell necklaces, using fire, constructing tools and shelter, or indeed even inventing new words. After 6 billion years of evolution, culture finally took off, and with culture the seeds of civilization were sown. The advantage of this argument is that you don’t need to postulate separate mutations arriving nearly simultaneously to account for the coemergence of our many and various unique mental abilities. Instead, increased sophistication of a single mechanism—such as imitation and intention reading—could explain the huge behavioral gap between us and apes.

I’ll illustrate with an analogy. Imagine a Martian naturalist watching human evolution over the last five hundred thousand years. She would of course be puzzled by the great leap forward that occurred fifty thousand years ago, but would be even more puzzled by a second great leap which occurred between 500 B.C.E. and the present. Thanks to certain innovations such as those in mathematics—in particular, the zero, place value, and numerical symbols (in India in the first millennium B.C.E.), and geometry (in Greece during the same period)—and, more recently, in experimental science (by Galileo)—the behavior of a modern civilized person is vastly more complex than that of humans ten thousand to fifty thousand years ago.

This second leap forward in culture was even more dramatic than the first. There is a greater behavioral gap between pre–and post–500 B.C.E. humans than between, say, Homo erectus and early Homo sapiens. Our Martian scientist might conclude that a new set of mutations made this possible. Yet given the time scale, that’s just not possible. The revolution stemmed from a set of purely environmental factors which happened fortuitously at the same time. (Let’s not forget the invention of the printing press, which allowed the extraordinary spread and near universal availability of knowledge that usually remained confined to the elite.) But if we admit this, then why doesn’t the same argument apply to the first great leap? Maybe there was a lucky set of environmental circumstances and a few accidental inventions by a gifted few which could tap into a preexisting ability to learn and propagate information quickly—the basis of culture. And in case you haven’t guessed by now, that ability might hinge on a sophisticated mirror-neuron system.