Sexy Brains

Why do humans have such a need to express themselves? Rodents rarely are seen composing screenplays, but Sunday afternoons at the Santa Monica coffee shop shows a non-trivial portion of the population that feels this inexplicable need to communicate. Seeing all the buddying screenwriters, the throngs of novel-writers, the amateur artists and photographers. Plus the myspacers, bloggers, and chatters. Why are humans so driven to express themselves, even when no one is listening?

Geoffrey Miller, a research fellow at UCL, argues that the human capacity for expression is the result of runaway sexual selection. He proposes that the key mystery in human evolution is why humans encephalized. In the long-run, our brains were obviously useful, but big brains are an incredibly expensive adaptation. Our brains eat up about a fourth of all the energy we use. They require an exceedingly long maturation time. Large-headed infants complicate birthing.

And a key aspect to the mystery-- why only humans? Countless animals seem that they might have encephalized-- ie, lived in similar climates, with similar diets, and faced similar problems-- why just humans?

Miller's argue goes that encephalization, up until tool-use, societies and the Internet, wasn't a useful adaption per se. Rather, just like peacocks choosing mates by size of their plumage, human females selected mates that were big-brained. That metabolically expensive investment was costly, but worth it for the male. And the cost in energy showed prospective females that the male was capable enough to feed that giant brain-- and so likely to have good genetic material. Female humans like mates that can express themselves in some manner. (How many online personal ads look for a mate that can "make me laugh"?)

This makes a prediction-- since females still bear more of the cost of child-rearing, and so should they be more selective of mate. Therefore, the sexual selection should happen predominately in the males of the species. Why then do female humans also pay the developmental and metabolic price of large brains?

One answer may be that along with encephalization came an increase in male parenting requirements, evening out both the per-parent investment in offspring and the degree of sexual dimorphism. I'm not quite satisfied with this explanation though, esp because sexual dimorphism remains prevalent in other features, such as body size.

Still, I think it's a wonderfully interesting idea. And Miller's work reminds us that while natural selection is what gets Darwin most of the kudos, Darwin's brilliant analysis of sexual selection was much more subtle and really the more impressive insight.

Edge Interview with Miller
The Mating Mind