Humans living at high latitude have bigger eyes and bigger brains to cope with poor light during long winters and cloudy days, UK scientists have said.The Oxford University team said bigger brains did not make people smarter.
Larger vision processing areas fill the extra capacity, they write in the Royal Society's Biology Letters journal.
The scientists measured the eye sockets and brain volumes of 55 skulls from 12 populations across the world, and plotted the results against latitude.
Lead author Eiluned Pearce told BBC News: "We found a positive relationship between absolute latitude and both eye socket size and cranial capacity."
The team, from the Institute of Cognitive and Evolutionary Anthropology, used skulls dating from the 1800s kept at museums in Oxford and Cambridge.
The skulls were from indigenous populations ranging from Scandinavia to Australia, Micronesia and North America.
Largest brain cavities The largest brain cavities came from Scandinavia, while the smallest were from Micronesia.
Eiluned Pearce said: "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."
The work indicates that humans are subject to the same evolutionary trends that give relatively large eyes to birds that sing first during the dawn chorus, or species such as owls that forage at night.
Co author Prof Robin Dunbar said: "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."
The team took into account the overall body size of each individual by measuring the foramen magnum - the hole in the base of the skull that attaches to the spinal column.
They also controlled for the possibility that the larger eye sockets were needed for extra fat around the eyeball to insulate them from freezing temperatures.
The team intends to do more work on establishing a firm link between eyeball size and enhanced visual processing areas in the brain, and to replicate the link found in the 55 original skulls with further study on specimens from other museums.