Humans, specifically Homo sapiens, have entrenched themselves at the top of the food chain, something most people take for granted and ascribe to the powers of the human brain. But for all that, scientists don't really understand why our brains evolved to be as large and powerful as they are. A popular theory, which has been around for a long time, is called the cognitive buffer hypothesis, and it says large brains evolved to deal with fast or unexpected changes to the environment, thereby helping with survival. In other words, a variable environment would lead to larger brains among species that live there. But it is difficult to verify this theory using humans, since us modern humans are the only survivors of the Homo genus, and we have nothing to compare with.
It turns out that in larger human brains, regions involved in bringing together information are hyperexpanded – but we don't know what affect this might have on intelligence yet. Armin Raznahan at the US National Institute of Mental Health in Maryland and his colleagues discovered this by comparing brain images from around 3,000 people. They compared the area of 80,000 points across the cortex – the large part of our brains that is involved in higher functions like thinking. Analysing these, they found that some particular areas expanded more than others in people who had an overall larger brain size. These regions seem to be involved in integrating information from across the brain, he says.
Contrary to popular belief, we don't have a limited supply of brain cells. Sure, the majority of them grow early in life, but some areas of the brain continue to grow them into adulthood and beyond -- a process called neurogenesis. The hippocampus is one of these areas, and it's also among the most important areas of the brain, playing roles in memory, emotion, and learning. But as we grow older, a number of factors compete with neurogenesis and kill brain cells. No, they're not smoking weed and drinking alcohol; instead, they're a little more common than you might expect.
In the past two million years, humans have experienced a massive increase in brain size, one not seen in any other species. This rapid evolution gave us brains roughly triple the volume of those of our pre-human ancestors. But the intelligence we enjoy as a result would seem to be advantageous for all sorts of species, not just us. So why was ours the only line to go down this route? The social brain hypothesis was a popular answer.