Scientists at the Oxford University have revealed that people living at high latitudes have bigger eyes and brains to cope with poor light during long winters and cloudy days.
They measured the eye sockets and brain volumes of 55 skulls from 12 populations across the world, and plotted the results against latitude.
"We found a positive relationship between absolute latitude and both eye socket size and cranial capacity," lead author Eiluned Pearce told BBC News.
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.
The largest brain cavities came from Scandinavia, while the smallest were from Micronesia.
"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," Pearce said.
"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," he stated.
However, the team said bigger brains did not make people smarter.
"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," he added.
The scientists published their finding in the Royal Society's Biology Letters journal.