Despite having large brains our ancestors left few signs of intelligent behavior. A million years ago, their tools were primitive and unvaried. Many anthropologists now believe our oversized noggins were the result of improved diet, rather than fancy craftwork.
The role of diet in brain size comes into sharper focus by considering another explanation for our large brains, the fact that humans mature more slowly than other primates.
Do large-brained adults preserve the large-brained trait of juveniles
Ever notice that toddlers have disproportionately large heads making them top heavy and liable to fall on their heads? What if the infantile trait of a relatively large head were preserved into maturity (a process called neoteny)? One reason this happens is when development is slowed to prolong the juvenile period. Slowing development can be an advantage if it increases body size, or if it permits us to learn more.
The clearest evidence for neoteny in head shape is the fact that adult humans have a marked resemblance to young chimps in the proportions of the head and face. This phenomenon has impressed comparative anatomists at least since the time of Darwin. Our heads look too big for an adult primate, but not for a juvenile.
The neoteny hypothesis is intriguing but suffers from a fatal weakness. Brains are terrifically expensive to run: they require ten times as much energy per pound as the rest of the body. The neoteny theory cannot explain why humans would preserve such a large energetically expensive brain. Under this scenario, slightly smaller brains would be selected for over many generations until the brain was no larger than it needed to be. This weakness is tackled head on by the dietary theory, known as the expensive tissue hypothesis (1).
The expensive tissue hypothesis
The expensive tissue hypothesis is rather like the chalk line drawn around a homicide victim - everything gets reduced to a single dimension: dietary energy. Taking the body as a whole, two tissues use up a lot of energy, the gut and the brain. When humans are compared to a typical primate, most tissue types use about the same amount of energy with two striking exceptions. The human gut (intestines) uses less energy than a typical primate and the human brain uses far more.
What really excites physical anthropologists is the fact that the energy subtracted from the gut's energy budget gets added, almost exactly to the brain's budget. What can this mean? Evidently brain size is normally held in check by its great energy cost. For humans, though, reduction in the size of the gut created "surplus" energy that permitted an increase in the size of the brain.
So why did the human gut get reduced? Our ancestors switched to a more refined diet that required both less chewing and less digesting. Greater consumption of meat is a plausible explanation here. It is no accident that hunting mammals such as lions and African hunting dogs have comparatively large brains and comparatively small guts. It all fits together like the body lying inside its chalk line.
One of the hardest aspects of human brain evolution to explain is why it has been so rapid, almost doubling in a million years. It is as though brain size were promoting itself like a wild fire rushing through a forest. Why might brain expansion feed itself? When brains get bigger, and humans get smarter, they are more successful at figuring out how to trap game animals, so the proportion of refined food increases and the size of the gut declines. Energetically speaking, this makes room for further expansion in brain size. This is positive feedback.
If it were a car, we might say that the expensive tissue hypothesis was a sweet ride. Yet, it cannot go everywhere or do everything. In particular, it doesn't explain why humans suddenly began getting so much smarter than other primates some 200,000 years ago. This enigma is taken up in another post.
1. Aiello, L. C., & Wheeler, P. (1995). The expensive tissue hypothesis. Current Anthropology, 36, 199-221.