Populations in the north have evolved to cope with dull, cloudy skies and short periods of daylight.
Good news for people who are from the north: you are likely to have a bigger brain than your southern counterparts. Unfortunately, that doesn’t necessarily mean you are more intelligent than people from the south – just that you have evolved to cope with the longer winters and greyer skies in northern climates.
A study of populations across both hemispheres has shown people from countries further from the equator have more grey matter and larger eyes than those from sunnier parts.
This is because living in low light conditions means the eyes and brain need to work harder in order to process images to a good level of detail, or “high resolution”.
Researchers said the findings, published in the Royal Society’s Biology Letters journal, could theoretically even apply to different communities within a particular country, meaning people in the north of Britain might have larger brains than those in the south.
Northerly populations have evolved to cope with dull, cloudy skies and short periods of daylight since migrating to Europe and northern Asia as many as 40,000 years ago.
Analysis of 55 skulls from 12 countries, dating from the 1800s, showed that people in northern areas have evolved to grow larger eyes and visual areas of the brain.
Having wider pupils allows the eyes to take in more light, while a bigger retina is able to distinguish more detail, producing a higher-resolution image.
A larger optic nerve and a more sizeable area of the brain dedicated to processing sight are also needed for northern populations to see to the same level of detail as people from further south.
Researchers from Oxford University examined the volume of eye sockets and brain cavities in the skulls and found a direct link to the latitude of their country of origin.
While English people had an average brain size of 1,416ml, the figure in Micronesia, which lies very near the equator, was just 1,200ml.
In contrast Scandinavians, the most northerly population tested, had a brain capacity of 1,484ml – more than 20 per cent higher than Micronesia.
A similar difference was seen in eye volume, with English skulls indicating an average of 26.22ml, compared with 26.83 in Scandinavia and 21.83 in Micronesia.
Eiluned Pearce, a postgraduate student who led the research, said: “As you move away from the equator, there’s less and less light available, so humans have had to evolve bigger and bigger eyes. 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.”
Although the tests did not examine differences between skulls from different parts of the same country, Ms Pearce said the data could in theory mean someone with an entirely Scottish or northern English heritage would have a larger brain and eyes than someone whose ancestors were all from the south of England.
Any difference would likely be minimal because the difference in light levels between the north and south is slight and interbreeding within Britain would dilute any evolutionary change, she added.
Co-author Professor Robin Dunbar, Director of the Institute of Cognitive and Evolutionary Anthropology at Oxford, 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 study took into account the fact that people living in more northerly areas are generally bigger overall and the possibility that eye socket size could also be linked to the need to have more fat to insulate the eyeball in cold weather.
Other research has shown that birds which sing earliest in the dawn chorus have relatively bigger eyes, and animals which are nocturnal have larger eyes than those that eat and forage during the day.