Why Only Humans Have Large Brains

on Aug 11, 2010

We often think that having a [proportionally] large brain is something very central to being human. While this is certainly an attribute unique to humans, it’s not the cause of our uniqueness. Think about it. If having a large brain is so advantageous, then how come other animals haven’t evolved to have it? As you’ve probably guessed, there are tradeoffs to having a large brain.

Consider the human brain. Although it’s proportionally humongous compared to the brain of other animals, it takes about 2-3% percent of our body mass. But look at the energy usage. Our large brains use about 20% of the calories we consume. That’s pretty darn expensive. At that price, we better be getting a lot out of it. We do, as humans, because we have culture. Through culture, we’re exposed to a staggering amount of information from others. Consider everything you’ve learned from friends, teachers, and extended family. That large brain is put to use because we have so much useful and trustworthy information to put in it.

Now consider the case of non-human animals. They don’t have anything close to the vast culture that we humans do. Their culture is limited to immediate family members because non-human animals have no way to control conflicts of interest between non-kin. This statement reveals a couple of interesting things. First, that it’s actually disadvantageous for non-human animals to have expensive, large brains because they have very little to put in them. Second, that the ability to control conflicts of interest between non-kin is essential to our humanity. While I discuss its importance in our uniquely large brains, managing conflicts of interest is central to our unique language abilities.

So in a nutshell:
Large brains require culture.
Culture requires trust in information from others.
Trust requires controlling conflicts of interest.

On a side note, while culture is a requirement for large brains, it is not sufficient. As hinted above, large brains require a consistent source of rich food to meet energy needs. Also, large brains take a longer time to develop and thus require that individuals have access to protection during this development time. These needs are afforded by the human village, another consequence of solving the conflict of interest problem.

For an extended discussion of large brains, culture, conflicts of interest, and human uniqueness, please see my colleagues’ book, Death from a Distance and the Birth of a Humane Universe.