Humans are the brainiest of all mammals. Brain tissue uses a fifth of our energy budget. How do our big brains pay for their upkeep? What are they for?
As with many simple questions, there is not a simple answer (1). We do have a good idea of how our large brains evolved, however, and this is connected to our high-energy diet. As to why we need a very large brain, most scholars assumed that it facilitated general intelligence and helped us to negotiate the complex social relationships found in all human societies. Another hypothesis is that big brains power intelligent conversation and artistic expression that enhance sexual attractiveness.
How Our Large Brains Evolved
Sometimes the historical context in which some trait emerges offers insight into what it is for. Hominins developed their large brains after they left forests in favor of the more open habitat of grasslands dotted with trees. This transition brought a shift away from coarser foods such as fruits and leaves in favor of more energy-dense foods, such as meat, fish, grubs, and honey.
One sign of this dietary change was a reduction in the size of the rib cage. Humans no longer required the large guts needed by great apes to digest a diet heavy in roughage. Another was the reduction in tooth size compared to the large grinding teeth apes needed to chew their high-fiber foods.
As the gut got reduced, there was an increase in brain size. This is explainable in terms of the reduced energy demands of digestion that freed up more metabolic energy for brain function.
Once hearths appeared, some 200,000 years ago, foods that had previously been inedible, such as manioc roots, could now be consumed. Meat also became more easily digestible.
There is a limit to how large brains can become if babies are to be birthed. So the brains of modern humans are not appreciably larger than those of Homo erectus. Nevertheless, an important evolutionary change in brain metabolism allowed the brain to use energy more rapidly so that our brain uses about 10 times as much energy as other bodily tissues (2).
This surge in brain metabolism coincided with the emergence of more complex tools, such as bows and arrows, and fish hooks, suggesting that our ancestors switched to more sophisticated cognition, possibly including the emergence of spoken languages. Each of these facets of the lives of Homo sapiens suggests that our ancestors were making good use of their large brains and enhanced cerebral metabolism. Despite this, many scholars like the notion that our large brains evolved because they help us to attract mates by seeming intelligent.
Could It Be About Sexual Attraction?
This hypothesis is intuitively appealing. After all, successful artists are known to be highly attractive as mates. Male and female rock stars are notorious for their ability to draw groupies who aim to sleep with them.
Yet, the concept of brain capacity generating artistic ability so as to attract mates is problematic as an explanation for the evolution of large brains.
The most obvious problem is the history of brain evolution. Our brains were already very big before artistic activities emerged in the fossil record, as represented, for example, in cave paintings. Big brains are imperfectly related to intelligence. Small-brained birds are among the most intelligent animals on the planet. Some, like the chickadee, achieve this by growing new cells to store new memories about where they have hidden food.
It is probably fair to say that creative individuals are more interesting and attractive to the other gender. This generalization is not peculiar to humans. Birds are justly celebrated for the creativity of their displays, whether it is the mating bower built by a bower bird, the impressive song repertoire of a mocking bird, or the elaborate three-dimensional dances of the manikin bird.
Yet, the argument that intellectual, and artistic ability, emerged to increase sexual attraction encounters some plausibility hurdles. In addition to the fact that our brains were already enlarged long before the creative arts emerged, there is another historical problem: Modern humans are mostly monogamous and sexual selection is strongest when mating competition is most intense. Among highly competitive baboons, only the alpha male gets to reproduce. Male baboons are much larger than females and the dominant male has a conspicuous silver back.
Instead of emphasizing braininess, adolescent women hide their intelligence to appeal to males, even to the extent of underperforming on exams. They spend more time on their appearance than on burnishing their academic credentials. Young men who are actively dating prefer partying over studying and rarely bring their calculus texts on a date.