The Tell-Tale Brain

The lowest savages with the least copious vocabularies [have] the capacity of uttering a variety of distinct articulate sounds and of applying them to an almost infinite amount of modulation and inflection [which] is not in any way inferior to that of the higher [European] races. An instrument has been developed in advance of the needs of its possessor.

2. Crude Oldowan tools—made by just a few blows to a core stone to create an irregular edge—emerged 2.4 million years ago and were probably made by Homo habilis, whose brain size was halfway between that of chimps and modern humans. After another million years of evolutionary stasis, aesthetically pleasing symmetrical tools began to appear which reflected a standardization of production technique. These required switching from a hard hammer to a soft, perhaps wooden, hammer while the tool was being made, so as to ensure a smooth rather than a jagged, irregular edge. And lastly, the invention of stereotyped assembly-line tools—sophisticated symmetrical bifacial tools that were hafted to a handle—took place only two hundred thousand years ago. Why was the evolution of the human mind punctuated by these relatively sudden upheavals of technological change? What was the role of tool use in shaping human cognition?

3. Why was there a sudden explosion—what Jared Diamond, in his book Guns, Germs, and Steel, calls the “great leap”—in mental sophistication around sixty thousand years ago? This is when widespread cave art, clothing, and constructed dwellings appeared. Why did these advances come along only then, even though the brain had achieved its modern size almost a million years earlier? It’s the Wallace problem again.

4. Humans are often called the “Machiavellian primate,” referring to our ability to predict other people’s behavior and out-smart them. Why are we humans so good at reading one another’s intentions? Do we have a specialized brain module, or circuit, for generating a theory of other minds, as proposed by the British cognitive neuroscientists Nicholas Humphrey, Uta Frith, Marc Hauser, and Simon Baron-Cohen? Where is this circuit and when did it evolve? Is it present in some rudimentary form in monkeys and apes, and if so, what makes ours so much more sophisticated than theirs?

5. How did language evolve? Unlike many other human traits such as humor, art, dancing, and music, the survival value of language is obvious: It lets us communicate our thoughts and intentions. But the question of how such an extraordinary ability actually came into being has puzzled biologists, psychologists, and philosophers since at least Darwin’s time. One problem is that the human vocal apparatus is vastly more sophisticated than that of any other ape, but without the correspondingly sophisticated language areas in the human brain, such exquisite articulatory equipment alone would be useless. So how did these two mechanisms with so many elegant interlocking parts evolve in tandem? Following Darwin’s lead, I suggest that our vocal equipment and our remarkable ability to modulate our voice evolved mainly for producing emotional calls and musical sounds during courtship in early primates, including our hominin ancestors. Once that evolved, the brain—especially the left hemisphere—could start using it for language.

But an even bigger puzzle remains. Is language mediated by a sophisticated and highly specialized mental “language organ” that is unique to humans and that emerged completely out of the blue, as suggested by the famous MIT linguist Noam Chomsky? Or was there a more primitive gestural communication system already in place that provided scaffolding for the emergence of vocal language? A major piece of the solution to this riddle comes from the discovery of mirror neurons.

I HAVE ALREADY alluded to mirror neurons in earlier chapters and will return to them again in Chapter 6, but here in the context of evolution let’s take a closer look. In the frontal lobes of a monkey’s brain, there are certain cells that fire when the monkey performs a very specific action. For instance, one cell fires during the pulling of a lever, a second for grabbing a peanut, a third for putting a peanut in the mouth, and yet a fourth for pushing something. (Bear in mind, these neurons are part of a small circuit performing a highly specific task; a single neuron by itself doesn’t move a hand, but its response allows you to eavesdrop on the circuit.) Nothing new so far. Such motor-command neurons were discovered by the renowned Johns Hopkins University neuroscientist Vernon Mountcastle several decades ago.

While studying these motor-command neurons in the late 1990s, another neuroscientist, Giacomo Rizzolatti, and his colleagues Giuseppe Di Pellegrino, Luciano Fadiga, and Vittorio Gallese, from the University of Parma in Italy, noticed something very peculiar. Some of the neurons fired not only when the monkey performed an action, but also when it watched another monkey performing the same action! When I heard Rizzolatti deliver this news during a lecture one day, I nearly jumped off my seat. These were not mere motor-command neurons; they were adopting the other animal’s point of view (Figure 4.1). These neurons (again, actually the neural circuit to which they belong; from now on I’ll use the word “neuron” for “the circuit”) were for all intents and purposes reading the other monkey’s mind, figuring out what it was up to. This is an indispensable trait for intensely social creatures like primates.

It isn’t clear how exactly the mirror neuron is wired up to allow this predictive power. It is as if higher brain regions are reading the output from it and saying (in effect), “The same neuron is now firing in my brain as would be firing if I were reaching out for a banana; so the other monkey must be intending to reach for that banana now.” It is as if mirror neurons are nature’s own virtual-reality simulations of the intentions of other beings.

In monkeys these mirror neurons enable the prediction of simple goal-directed actions of other monkeys. But in humans, and in humans alone, they have become sophisticated enough to interpret even complex intentions. How this increase in complexity took place will be hotly debated for some time to come. As we will see later, mirror neurons also enable you to imitate the movements of others, thereby setting the stage for the cultural “inheritance” of skills developed and honed by others. They may have also propelled a self-amplifying feedback loop that kicked in at one point to accelerate brain evolution in our species.

FIGURE 4.1 Mirror neurons: Recordings of nerve impulses (shown on the right) from the brain of a rhesus monkey (a) watching another being reach for a peanut, and (b) reaching out for the peanut. Thus each mirror neuron (there are six) fires both when the monkey observes the action and when the monkey executes the action itself.

As Rizzolatti noted, mirror neurons may also enable you to mime the lip and tongue movements of others, which in turn could provide the evolutionary basis for verbal utterances. Once these two abilities are in place—the ability to read someone’s intentions and the ability to mimic their vocalizations—you have set in motion two of the many foundational events that shaped the evolution of language. You need no longer speak of a unique “language organ,” and the problem doesn’t seem quite so mysterious anymore. These arguments do not in any way negate the idea that there are specialized brain areas for language in humans. We are dealing here with the question of how such areas may have evolved, not whether they exist or not. An important piece of the puzzle is Rizzolatti’s observation that one of the chief areas where mirror neurons abound, the ventral premotor area in monkeys, may be the precursor of our celebrated Broca’s area, a brain center associated with the expressive aspects of human language.

Language is not confined to any single brain area, but the left inferior parietal lobe is certainly one of the areas that are crucially involved, especially in the representation of word meaning. Not coincidentally, this area is also rich in mirror neurons in the monkey. But how do we actually know that mirror neurons exist in the human brain? It is one thing to saw open the skull of a monkey and spend days or weeks probing around with a microelectrode, but