"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 the Institute of Cognitive and Evolutionary Anthropology at Oxford University, a lead author on 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."
This suggests that someone from Greenland and someone from Kenya will have the same ability to discern detail, but the person from the higher latitude needs more brainpower and bigger eyes to deal with the lower light levels.
Professor Robin Dunbar, director of the Institute of Cognitive and Evolutionary Anthropology at Oxford University and a co-author of the study, said that people whose ancestors have lived within the Arctic circle, have eyeballs 20% bigger than people whose ancestors lived near the equator. They have an associated increase in the size of the brain's visual cortex, which previous studies have shown correlates with the size of the eyeball.
Brain volume is known to increase with latitude: people living at high latitudes north and south of the equator have bigger brains than people living near the equator and . Dunbar said that scientists have wondered whether these inherited differences in total brain volume were driven by the pressure to adapt to low light levels at high latitudes.
The researchers measured the brain volumes and eye sockets of 55 skulls kept at the Oxford University Museum of Natural History dating from the 19th century. The skulls represented 12 different populations from around the world, including indigenous people from England, Australia, China, Kenya, Micronesia and Scandinavia.
The results, published on Wednesday in the journal Biology Letters, showed that the biggest brains, averaging 1,484 millilitres, were from Scandinavia, while the smallest brains, around 1,200 millilitres, came from Micronesia. Average eye socket size was 27 millilitres in Scandinavia and 22 millilitres in Micronesia.
Dunbar said the increase in brain volume must have evolved relatively recently in human history. "It's only within the last 10,000 years or so that modern humans have occupied all latitudes right up to the Arctic circle. This is, I guess, an adaptation that's happened within the last 10,000 years."
The researchers controlled for possible confounding variables influencing their data, such as the fact that people who live at higher latitudes are physically bigger and the possibility that the size of a person's eye socket in colder climates might be bigger to allow for a thicker layer of insulating fat.
The results for human eyes mirror those found in birds and non-human primates. Bird species that sing earlier in the dawn chorus at high latitudes have bigger eyes than those that sing later, and nocturnal primates have bigger eyeballs than species that are awake during the day.
guardian.co.uk
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