The Tell-Tale Brain

building on a genetically specified scaffolding unique to humans. Of course, many monkeys and even lower vertebrates may have mirror neurons, but the neurons may need to develop a certain minimum sophistication and number of connections with other brain areas before they can engage in the kinds of abstractions that humans are good at.

What parts of the brain are involved in such abstractions? I already hinted (about language) that the inferior parietal lobule (IPL) may have played a pivotal role, but let’s take a closer look. In lower mammals the IPL isn’t very large, but it becomes more conspicuous in primates. Even within primates it is disproportionately large in the great apes, reaching a climax in humans. Finally, only in humans do we see a major portion of this lobule splitting further into two, the angular gyrus and the supramarginal gyrus, suggesting that something important was going on in this region of the brain during human evolution. Lying at the crossroads between vision (occipital lobes), touch (parietal lobes), and hearing (temporal lobes), the IPL is strategically located to receive information from all sensory modalities. At a fundamental level, cross-modal abstraction involves the dissolution of barriers to create modality- free representations (as exemplified by the bouba-kiki effect). The evidence for this is that when we tested three patients who had damage to the left angular gyrus, they performed poorly on the bouba-kiki task. As I already noted, this ability to map one dimension onto another is one of the things that mirror neurons are thought to be doing, and not coincidentally such neurons are plentiful in the general vicinity of the IPL. The fact that this region in the human brain is disproportionately large and differentiated suggests an evolutionary leap.

The upper part of the IPL, the supramarginal gyrus, is another structure unique to humans. Damage here leads to a disorder called ideomotor apraxia: a failure to perform skilled actions in response to the doctor’s commands. Asked to pretend he is combing his hair, an apraxic will raise his arm, look at it, and flail it around his head. Asked to mime hammering a nail, he will make a fist and bang it on the table. This happens even though his hand isn’t paralyzed (he will spontaneously scratch an itch) and he knows what “combing” means (“It means I am using a comb to tidy up my hair, Doctor”). What he lacks is the ability to conjure up a mental picture of the required action—in this case combing—which must precede and orchestrate the actual execution of the action. These are functions one would normally associate with mirror neurons, and indeed the supramarginal gyrus has mirror neurons. If our speculations are on the right track, then one would expect patients with apraxia to be terrible at understanding and imitating other people’s movements. Although we have seen some hints of this, the matter requires careful investigation.

One also wonders about the evolutionary origin of metaphors. Once the cross-modal abstraction mechanism was set up between vision and touch in the IPL (originally for grasping branches), this mechanism could have paved the way for cross-sensory metaphors (“stinging rebuke,” “loud shirt”) and eventually for metaphors in general. This is supported by our recent observations that patients with angular gyrus lesions not only have difficulty with bouba-kiki, but also with understanding simple proverbs, interpreting them literally rather than metaphorically. Obviously these observations need to be confirmed on a larger sample of patients. It is easy to imagine how cross-modal abstraction 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