In my last post, I drew on the work of Sarah Hrdy to suggest that cooperative breeding, as practiced by a pre-homo sapiens line of hominids, created the context in which humans’ ability to notice, remember, and recreate patterns of movement proved advantageous to their survival. Infants able to move with and connect with multiple caregivers may have been able to secure better care. This parenting strategy, Hrdy suggests, created the context in which humans could evolve into distinctively big-brained, wholly dependent, slow-growing creatures.

However, the question remains. Why build those brains? What pressures were operating in favor of brains? Wrangham argues persuasively that the calories from cooking also provided the fuel for growing big brains. At 2 percent of our bodyweight, brains consume 20-25 percent of our energy intake. But where was the need?

Robin Dunbar, in his book Grooming, Gossip, and the Evolution of Language (Harvard University Press), suggests that humans needed those brains to monitor the increasing complex social relations of their increasingly complex social groupings. As the story goes, once pre-human primates moved out of the forest and onto the plains, they began to spend more time walking upright. For food, these forest-departing hominids first scavenged prey left by larger animals, and then, by watching these predators, learned to hunt for themselves. They probably ran after game in packs. They controlled fire, cooked and shared their food, and so maximized their calorie load, developing an ability to run for long distances. They cared for their young collectively. They bonded together in groups of up to 150 people for protection at all hours. And to keep track of all of these social relationships, the story goes, they added a large prefrontal or neo- cortex to the typical primate brain—a portion of our noggin that would come to serve as the basis for symbolic action, language, law, and culture (1998: chapter 6, 108-12). As Dunbar asserts, humans boast the “largest brain relative to body size of any species that has ever existed” (1998: 3).

Yet it is important to note that a human brain is not simply big. What is unusual is the relative size of the neocortex compared to the rest of the brain. In support of his thesis about the social pressures on neocortex brain development, Dunbar analyzed the ratios of neocortex to overall brain size for a range of mammals, and then correlated those ratios with group size. Based on the clear pattern that emerged, he concluded that the human ratio of 4:1 predicts an average group size of 150. This size “seems to represent the maximum number of individuals with whom we can have a genuinely social relationship” (1998: 68-77), and it seems to find corroboration in the average size of churches, hunter-gatherer clans, and facebook friends.

However, isn’t it possible that early hominids could have developed some strategy other than increased brain capacity for monitoring their social relations, like the hierarchical ordering practiced by cattle on our farm, where having horns helps? Why grow brains for this purpose?

Another approach to this question would be to root the pressure to grow big brains even farther back in time, before social congregating, in the fact of upright walking itself. As any pregnant woman knows, walking induces labor. When a woman walks, the muscles that hold the infant in are the same muscles that must release to let the infant out. If walking upright induced earlier labors in plains-dwelling humans, than the accentuated dependence of these youngsters would have created the context in which cooperative breeding as a parenting strategy makes a difference. It would also have created a situation in which infants, born early, would have a greater need to create and code patterns of movement that they had not had the time to establish in utero. It is possible, then, that upright walking itself set in motion a vibrant self-reinforcing cycle of increasingly large brains and—given the size and weight of those brains—increasingly earlier births. Babies were born earlier and earlier, with a mental capacity to make up for the difference.

Scientists estimate that the hominid brain stopped growing 195,000 years ago, approximately 50,000 years after homo sapiens first appeared. By that time, human babies, as they are today, even when carried a full forty weeks, are born effectively one year premature relative to other primate babies. They are completely and utterly dependent on other humans for their survival. They cannot feed or transport themselves. They cannot hold up their heads or climb up their mother’s chest hair as baby chimpanzees do. If allowed to develop in the womb to the level of brain and body maturity common among other mammals and primates, a human infant would emerge a full year later, after twenty-one months of womb-care.

This effectively premature birth, called neoteny, creates a situation that is unique among primates. Not only must a newborn infant keep growing its brain while outside the womb, she must do so in an environment that is far more complex and variable than the shaded, padded, muted uterine interior. She must do so in relationship with a cast of mobile human caregivers on whom she depends. And she must do so with the capacity she has when born—the ability to make, notice, remember, and recreate patterns of movement. As a result, the very biology of a human brain—its neuronal structure—emerges as a function of the movements that an infant is making, and the movements that are making her.

The implications of this development are many, and relevant for our understanding of humans and dance. More than any other primate, human infants need to be able to play with movement. On the one hand, they need to learn which movements were best for securing food and hugs. They need to be able to discern how to move in ways that would connect them to others in pleasurable, mutually life-enabling ways. On the other hand, they also need to learn not only which movements to make but how to make new movements. They need to develop a sensory awareness capable of guiding them in inventing movements to respond to an ever shifting panolpy of problems.

All these brain aptitudes, are those that the practice of dancing exercises.

Could it be that dancing is the activity that humans evolved brains to do as the enabling condition of their best bodily becoming?

Next week: What would neuroscientists say?