People from northern parts of the world have evolved bigger brains and larger eyes to help them to cope with long, dark winters and dim skies.
Researchers from Oxford University studied the eye sockets and brain capacity of 55 human skulls from 12 different populations across the world and found that the further human populations live from the equator, the bigger their brains.
Those populations need bigger vision areas in the brain to cope with the low light levels at high latitudes, the scientists said in a report of their findings in the journal Biology Letters.
'As you move away from the equator, there's less and less light available, so humans have had to evolve bigger and bigger eyes', said Eiluned Pearce from Oxford's School of Anthropology, who led the study.
'Their brains also need to be bigger to deal with the extra visual input.
'Having bigger brains doesn't mean that higher latitude humans are smarter, it just means they need bigger brains to be able to see well where they live.'
The skulls used in the study dated back to the 1800s and included samples from indigenous populations of England, Australia, Canary Islands, China, France, India, Kenya, Micronesia, Scandinavia, Somalia, Uganda and the United States.
The researchers plotted the volume of the eye sockets and brain cavities against the latitude of the central point of each individual's country of origin and found that the size of both the brain and the eyes could be directly linked to the latitude of the country.
Oxford's Robin Dunbar, who also worked on the study, said the results showed the speed at which humans had evolved to cope with the challenges of new habitats.
'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,' he said.
The researchers said that from measuring the brain cavity, the study suggested the biggest brains belonged to populations who lived in Scandinavia, and the smallest belonged to Micronesians.