The increase in brain and eye size allows people to see better in places that receive less light than areas closer to the equator, according to the new study, published in the latest issue of the journal Royal Society Biology Letters.
The effect is most extreme at the poles.
"Someone living on the Arctic Circle would have an eyeball that is 20 percent larger than someone living on the equator," co-author Robin Dunbar told Discovery News.
"People living at high latitudes have greater visual acuity than those who live at the equator," added Dunbar, who is head of the Institute of Cognitive & Evolutionary Anthropology at the University of Oxford.
"The whole point is that they need to have better vision to compensate for the lower light levels at high latitudes, as indicated by the evidence we provide that visual acuity under ambient/natural light conditions remains constant with latitude."
For the study, Dunbar and colleague Eiluned Pearce measured the skulls of 55 individuals from 12 different populations, focusing on the dimensions for orbital volume and cranial capacity. The people lived about 200 years ago. Their skulls are now part of collections housed at the Oxford University Museum of Natural History and the University of Cambridge's Duckworth Collection.
The researchers found significant positive relationships between absolute latitude, orbital volume and brain size. Eyeballs varied in size from around a quarter to a third of an ounce in volume.
The brains, in turn, ranged from about 40.6 ounces for Micronesians, on the low end of the size spectrum, and 50.2 ounces for Scandinavians on the high end.
Skulls from Arctic Circle residents were not included, but the researchers made the "20 percent larger" estimate based on their existing data.
The scientists are quick to point out that brain size isn’t necessarily correlated with intelligence.
"The point we’re trying to make is that the larger brains of high latitude humans doesn’t mean they're smarter, it just means they have increased the size of brain areas dedicated to vision, and this has increased brain size overall," Pearce explained.
As for the larger eyeballs, they permit smaller proportions of images to fall upon each photoreceptor field so that more details can be distinguished. The amount of light hitting Earth's surface as well as minimum day length decrease with increasing absolute latitude, so people living in such areas need the visual boost.
The effect has previously been demonstrated before in birds and other primates, but this new study is the first to show how it affects humans.
More at Discovery News
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