But what caused the accelerated evolution of the IPL—and the angular gyrus part of it—in the first place? Did the selection pressure come from the need for higher forms of abstraction? Probably not. The most likely cause of its explosive development in primates was the need to achieve an exquisitely refined, fine-grained interaction between vision and muscle and joint position sense while negotiating branches on treetops. This resulted in the capacity of cross-modal abstraction, for example, when a branch is signaled as being horizontal both by the image falling on the retina and the dynamic stimulation of touch, joint, and muscle receptors in the hands.
The next step was criticaclass="underline" The lower part of the IPL split accidentally, possibly as a result of gene duplication, a frequent occurrence in evolution. The upper part, the supramarginal gyrus, retained the old function of its ancestral lobule—hand-eye coordination—elaborating it to the new levels of sophistication required for skilled tool use and imitation in humans. In the angular gyrus the very same computational ability set the stage (became an exaptation) for other types of abstraction as welclass="underline" the ability to extract the common denominator among superficially dissimilar entities. A weeping willow looks sad because you project sadness on to it. Juliet is the sun because you can abstract certain things they have in common. Five donkeys and five apples have “fiveness” in common.
A tangential piece of evidence for this idea comes from my examination of patients who have damage to the IPL of the left hemisphere. These patients usually have anomia. (difficulty finding words), but I found that some of them failed the bouba-kiki test and were also abysmal at interpreting proverbs, often interpreting them literally instead of metaphorically. One patient I saw in India recently got 14 out of 15 proverbs wrong even though he was perfectly intelligent in other respects. Obviously this study needs to be repeated on additional patients but it promises to be a fruitful line of enquiry.
The angular gyrus is also involved in naming objects, even common objects such as comb or pig. This reminds us that a word, too, is a form of abstraction from multiple instances (for example, multiple views of a comb seen in different contexts but always serving the function of hairdressing). Sometimes they will substitute a related word (“cow” for “pig”) or try to define the word in absurdly comical ways. (One patient said “eye medicine” when I pointed to my glasses.) Even more intriguing was an observation I made in India on a fifty-year-old physician with anomia. Every Indian child learns about many gods in Indian mythology, but two great favorites are Ganesha (the elephant-headed god) and Hanuman (the monkey god) and each has an elaborate family history. When I showed him a sculpture of Hanuman, he picked it up, scrutinized it, and misidentified it as Ganesha, which belongs to the same category, namely god. But when I asked him to tell me more about the sculpture, which he continued to inspect, he said it was the son of Shiva and Parvati—a statement that is true for Ganesha, not Hanuman. It’s as if the mere act of mislabeling the sculpture vetoed its visual appearance, causing him to give incorrect attributes to Hanuman! Thus the name of an object, far from being just any other attribute of the object, seems to be a magic key that opens a whole treasury of meanings associated with the object. I can’t think of a simpler explanation for this phenomenon, but the existence of such unsolved mysteries fuels my interest in neurology just as much as the explanations for which we can generate and test specific hypotheses.
LET US TURN now to the aspect of language that is most unequivocally human: syntax. The so-called syntactic structure, which I mentioned earlier, gives human language its enormous range and flexibility. It seems to have evolved rules that are intrinsic to this system, rules that no ape has been able to master but every human language has. How did this particular aspect of language evolve? The answer comes, once again, from the exaptation principle—the notion that adaptation to one specific function becomes assimilated into another, entirely different function. One intriguing possibility is that the hierarchical tree structure of syntax may have evolved from a more primitive neural circuit that was already in place for tool use in the brains of our early hominin ancestors.
Let’s take this a step further. Even the simplest type of opportunistic tool use, such as using a stone to crack open a coconut, involves an action—in this case, cracking (the verb)—performed by the right hand of the tool user (the subject) on the object held passively by the left hand (the object). If this basic sequence were already embedded in the neural circuitry for manual actions, it’s easy to see how it might have set the stage for the subject-verb-object sequence that is an important aspect of natural language.
In the next stage of hominin evolution, two amazing new abilities emerged that were destined to transform the course of human evolution. First was the ability to find, shape, and store a tool for future use, leading to our sense of planning and anticipation. Second—and especially important for subsequent language origin—was use of the subassembly technique in tool manufacture. Taking an axe head and hafting (tying) it to a long wooden handle to create a composite tool is one example. Another is hafting a small knife at an angle to a small pole and then tying this assembly to another pole to lengthen it so that fruits can be reached and yanked off trees. The wielding of a composite structure bears a tantalizing resemblance to the embedding of, say, a noun phrase within a longer sentence. I suggest that this isn’t just a superficial analogy. It’s entirely possible that the brain mechanism that implemented the hierarchical subassembly strategy in tool use became coopted for a totally novel function, the syntactic tree structure.
But if the tool-use subassembly mechanism were borrowed for aspects of syntax, then wouldn’t the tool-use skills deteriorate correspondingly as syntax evolved, given limited neural space in the brain? Not necessarily. A frequent occurrence in evolution is the duplication of preexisting body parts brought about by actual gene duplication. Just think of multisegmented worms, whose bodies are composed of repeating, semi-independent body sections, a bit like a chain of railroad cars. When such duplicated structures are harmless and not metabolically costly, they can endure many generations. And they can, under the right circumstances, provide the perfect opportunity for that duplicate structure to become specialized for a different function. This sort of thing has happened repeatedly in the evolution of the rest of the body, but its role in the evolution of brain mechanisms is not widely appreciated by psychologists. I suggest that an area very close to what we now call Broca’s area originally evolved in tandem with the IPL (especially the supramarginal portion) for the multimodal and hierarchical subassembly routines of tool use. There was a subsequent duplication of this ancestral area, and one of the two new subareas became further specialized for syntactic structure that is divorced from actual manipulation of physical objects in the world—in other words, it became Broca’s area. Add to this cocktail the influence of semantics, imported from Wernicke’s area, and aspects of abstraction from the angular gyrus, and you have a potent mix ready for the explosive development of full-fledged language. Not coincidentally, perhaps, these are the very areas in which mirror neurons abound.