Humans who live further from the equator have rapidly evolved bigger eyes and larger brains to better process the low light levels at those lofty latitudes.

Anthropologists at Oxford University collected 55 skulls, dating from the 1800s, that represented 12 different populations from around the globe. The researchers measured the eye socket and brain volumes and plotted them against the latitude of each individual’s country of origin.

The team, lead by the Institute of Cognitive and Evolutionary Anthropology’s Eiluned Pearce, found a significant positive correlation between the size of brain and the latitude of the country. People from the northern-European countries of Scandinavia had the biggest brains, while Micronesians, from just north of the equator, had the smallest.

The larger brains don’t necessarily relate to intelligence — bigger vision areas in the brain are required to help cope with the low light conditions caused by cloudy skies and long winters in northern territories. The study also notes that actual visual sharpness, measured under natural daylight conditions, is constant all around the globe, which further suggests that the visual processing system has adapted to ambient light conditions as humans colonized the Earth.

Co-author Professor Robin Dunbar, Director of the Institute of Cognitive and Evolutionary, said in a press release: “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 other animals, observations have shown the links between eye size and light levels. Birds with relatively bigger eyes are the first to sing at dawn in low light, and primates with the largest eyes are those who eat and forage at night.