Why do people in higher latitudes have bigger brains?

Though pundits have argued that the "bell curve" of human intelligence favors big-skulled northerners, a new study shows definitively that large heads have nothing to do with intelligence.

People in higher latitudes evolved bigger brainpans, but not because they're smarter than their equatorial counterparts. Instead, people living in the far north and far south evolved bigger brains and eyes so that they could cope with living in a world with low light.

Oxford researchers examined the eye sockets and brain volumes of 55 skulls dating back to the 1800s. These skulls came from twelve indigenous populations scattered all over the world, including Australia, Micronesia, North America, and Scandinavia. The researchers plotted the sizes of the brain and eyes against the skulls' latitudes of origins, and they consistently found that, the further away from the equator the skulls came from, the bigger they got.

Scandinavians had the biggest brain cavities, while Micronesians had the smallest. While this is one of those statements that can pretty much be instantly misinterpreted, it should be stressed that we're not talking about the part of the brain that governs intelligence - as far as we know, that's pretty much the same size in any given human population. Instead, the enlarged part of the brain is all to do with vision.

Researcher Eiluned Pearce explains:

"Both the amount of light hitting the Earth's surface and winter day-lengths get shorter as you go further north or south from the equator. We found that as light levels decrease, humans are getting bigger eye sockets, which suggests that their eyeballs are getting bigger. They are also getting bigger brains, because we found this increase in cranial capacity as well. In the paper, we argue that having bigger brains doesn't mean that high-latitude humans are necessarily smarter. It's just they need bigger eyes and brains to be able to see well where they live."

What's particularly interesting is that we see similar evolutionary processes at work in other creatures. Birds, for instance, tend to have larger eyes if they're awake during the night or early morning. But while those birds might have had millions of years to evolve those features, that's hardly the case for humans. Fellow researcher Robin Dunbar points out:

"Humans have only lived at high latitudes in Europe and Asia for a few tens of thousands of years, yet they seem to have adapted their visual systems surprisingly rapidly to the cloudy skies, dull weather and long winters we experience at these latitudes."

In terms of their methodology, the researchers controlled for two main potential confounding variables: they checked the overall body size of the skulls by measuring the hole in the skull that connects with the spinal column, and they also considered and accounted for the possibility that the higher latitude eye sockets were larger because they had to allow more room for insulating fat. Even allowing for both of these possibilities, it does indeed appear that some humans just have naturally bigger brains than others...now let's see how long we can go before that point gets completely misunderstood.

Biology Letters via BBC News.