The Tell-Tale Brain

TLDR: One of the big puzzles in human evolution is the relatively sudden emergence between sixty thousand and a hundred thousand years ago of a number of traits the authors regard as uniquely human: fire, art, constructed shelters, body adornment, multicomponent tools, and more complex use of language.

Abstract: ion might work for bouba-kiki, but how do you explain metaphors that combine very abstract concepts like “it is the east, and Juliet is the sun” given the seemingly infinite number of such concepts in the brain? The surprising answer to this question is that the number of concepts is not infinite, nor is the number of words that represent them. For all practical purposes, most English speakers have a vocabulary of about ten thousand words (although you can get by with far fewer if you are a surfer). There may be only some mappings that make sense. As the eminent cognitive scientist and polymath Jaron Lanier pointed out to me, Juliet can be the sun, but it makes little sense to say she is a stone or an orange juice carton. Bear in mind that the metaphors that get repeated and become immortal are the apt ones, the resonant ones. In doggerel, comically bad metaphors abound. Mirror neurons play another important role in the uniqueness of the human condition: They allow us to imitate. You already know about tongue protrusion mimicry in infants, but once we reach a certain age, we can mime very complex motor skills, such as your mom’s baseball swing or a thumbs-up gesture. No ape can match our imitative talents. However, I will note as an interesting aside here, the ape that comes closest to us in this regard is not our nearest cousin, the chimpanzee, but the orangutan. Orangutans can even open locks or use an oar to row, once they have seen someone else do it. They are also the most arboreal and prehensile of the great apes, so their brains may be jam-packed with mirror neurons for allowing their babies to watch mom in order to learn how to negotiate trees without the penalties of trial and error. If by some miracle an isolated pocket of orangs in Borneo survives the environmental holocaust that Homo sapiens seems hell-bent on bringing about, these meek apes may well inherit the earth. Miming may not seem like an important skill—after all, “aping” someone is a derogatory term, which is ironic given that most apes are actually not very good at imitation. But as I have previously argued, miming may have been the key step in hominin evolution, resulting in our ability to transmit knowledge through example. When this step was taken, our species suddenly made the transition from gene-based Darwinian evolution through natural selection —which can take millions of years—to cultural evolution. A complex skill initially acquired through trial and error (or by accident, as when some ancestral hominid first saw a shrub catching fire from lava) could be transmitted rapidly to every member of a tribe, both young and old. Other researchers including Merlin Donald have made the same point, although not in relation to mirror neurons. 3 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. A caveat is in order. I am not arguing that mirror neurons are sufficient for the great leap or for culture in general. I’m only saying that they played a crucial role. Someone has to discover or invent something—like noticing the spark when two rocks are struck together—before the discovery can spread. My argument is that even if such accidental innovations were hit upon by chance by individual early hominins, they would have fizzled out were it not for a sophisticated mirror-neuron system. After all, even monkeys have mirror neurons, but they are not bearers of a proud culture. Their mirror-neuron system is either not advanced enough or is not adequately connected to other brain structures to allow the rapid propagation of culture. Furthermore, once the propagation mechanism was in place, it would have exerted selective pressure to make some outliers in the population more innovative. This is because innovations would only be valuable if they spread rapidly. In this respect, we could say mirror neurons served the same role in early hominin evolution as the Internet, Wikipedia, and blogging do today. Once the cascade was set in motion, there was no turning back from the path to humanity.