Scientists say people who live at higher latitudes have bigger eyes and brains to help them cope with poor light during long winters and cloudy days.
According to the study published in the Royal Society's Biology Letters journal, larger vision processing areas fill the extra capacity, but bigger brains does not make people smarter.
Scientists from Oxford University's Institute of Cognitive and Evolutionary Anthropology used 55 skulls from 12 populations across the world dating to the 1800s, to measure their eye sockets and brain volumes.
"We found a positive relationship between absolute latitude and both eye socket size and cranial capacity," said Lead author Eiluned Pearce.
The skulls belonged to indigenous people from Scandinavia to Australia, Micronesia and North America.
The largest brain cavities came from Scandinavia, while the smallest were from Micronesia, the state-funded BBC reported.
"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,” explained Pearce.
"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."