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

When we carefully examined the speech output of many Wernicke’s aphasics, we found that, in addition to the absence of meaning, the most striking and obvious loss was in recursive embedding. Patients spoke in loosely strung together phrases using conjunctions: “Susan came and hit John and took the bus and Charles fell down,” and so forth. But they could almost never construct recursive sentences such as “John who loved Julie used a spoon.” (Even without setting “who loved Julie” off with commas, we know instantly that John used the spoon, not Julie.) This observation demolishes the long-standing claim that Broca’s area is a syntax box that is autonomous from Wernicke’s area. Recursion may turn out to be a property of Wernicke’s area, and indeed may be a general property common to many brain functions. Furthermore, we mustn’t confuse the issue of functional autonomy and modularity in the modern human brain with the question of evolution: Did one module provide a substrate for the other or even evolve into another, or did they evolve completely independently in response to different selection pressures?

Linguists are mainly interested in the former question—the autonomy of rules intrinsic to the module—whereas the evolutionary question usually elicits a yawn (just as any talk of evolution or brain modules would seem pointless to a number theorist interested in rules intrinsic to the number system). Biologists and developmental psychologists, on the other hand, are interested not only in the rules that govern language but also in the evolution, development, and neural substrates of language, including (but not confined to) syntax. A failure to make this distinction has bedeviled the whole language evolution debate for nearly a century. The key difference, of course, is that language capacity evolved through natural selection over two hundred thousand years, whereas number theory is barely two thousand years old. So for what it is worth, my own (entirely unbiased) view is that on this particular issue the biologists are right. As an analogy, I’ll invoke again my favorite example, the relationship between the chewing and hearing. All mammals have three tiny bones—malleus, stapes, and incus—inside the middle ear. These bones transmit and amplify sounds from the eardrum to the inner ear. Their sudden emergence in vertebrate evolution (mammals have them but their reptilian ancestors don’t) was a complete mystery and often used as ammunition by creationists until comparative anatomists, embryologists, and paleontologists discovered that they actually evolved from the back of the jawbone of the reptile. (Recall that the back of your jaw articulates very close to your ear.) The sequence of steps makes a fascinating story.

The mammalian jaw has a single bone, the mandible, whereas our reptilian ancestors had three. The reason is that reptiles, unlike mammals, frequently consume enormous prey rather than frequent small meals. The jaw is used exclusively for swallowing, not chewing, and due to reptiles’ slow metabolic rate, the unchewed food in the stomach can take weeks to break down and digest. This kind of eating requires a large, flexible, multihinged jaw. But as reptiles evolved into metabolically active mammals, the survival strategy switched to consumption of frequent small meals to maintain a high metabolic rate.

Remember also that reptiles lie low on the ground with their limbs sprawled outward, thereby swinging the neck and head close to the ground while they sniff for prey. The three bones of the jaw lying on the ground allowed reptiles to also transmit sounds made by other animals’ nearby footsteps to the vicinity of the ear. This is called bone conduction, as opposed air conduction which is used by mammals.

As they evolved into mammals, reptiles raised themselves up from the sprawling position to stand higher up off the ground on vertical legs. This allowed two of the three jaw bones to become progressively assimilated into the middle ear, being taken over entirely for hearing airborne sounds and giving up their chewing function altogether. But this change in function was only possible because they were already strategically located—in the right place at the right time—and were already beginning to be used for hearing terrestrially transmitted sound vibrations. This radical shift in function also served the additional purpose of transforming the jaw into a single, rigid nonhinged bone—the mandible—which was much stronger and more useful for chewing.