Blood and brains

Humans are brainy animals. One way to show that is by looking at brain size: our species has the biggest brains, in relation to body size, of any animal. But there’s more to it than that. An earlier post covered the work of Susan Herculano-Houzel. She developed a technique for counting the number of neurons in a brain, or part of a brain. Among most mammals, big animals have a lower density of brain neurons, so they aren’t actually as brainy (measured by neuron number) as you’d think just based on their brain size. Primates however break the usual mammalian rule. Big primates have the same neuron density as little guys, so they really are quite brainy. And humans, with really big brains and (following primate rules) a high density of neurons, stand out even among primates as exceptionally brainy.

This work isn’t much help if we are looking at extinct hominins, when all we’ve got is their fossil skulls. But now there’s some interesting recent research with a new take on the subject. Brains need to be supplied with blood. The more energy they use, the more blood flow is needed. We can now figure out fairly accurately how much blood flow a brain is getting by looking at the size of the hole that lets the carotid artery in through the base of the skull. And then we can apply this technique to look at humans, and at extinct hominins. It turns out that humans are even more exceptional when we look at blood flow to the brain: we’re getting double the flow that you’d expect based on brain size alone.

Early hominins however, Australopithecus and early Homo, aren’t very impressive upstairs, many with less blood flow to the brain than modern apes. Looking at the graph it looks like there are really two grades of brain evolution. In the lower grade, which includes early hominins and modern apes, there is a gradual increase over millions of years. (I’m just guessing here that the ancestors of chimps and gorillas millions of years ago were about as brainy as contemporary hominins, but we’d still like to find more fossils.) And then there is a big leap up to a higher grade with early Homo erectus, and a rapid increase after that. It looks like something major changed with the appearance of Homo erectus, either on the supply side – improvements in food supply making brains more affordable – or on the demand side – a greater fitness payoff to a high energy brain – or both.