updated 7/26/2011 8:18:22 PM ET 2011-07-27T00:18:22

People who live in high latitude regions have bigger eyeballs and brains than other individuals, according to new research.

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."

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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.

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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.

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The findings could help to explain why Neanderthals and their ancestors may have had larger brains than us. Although possibly technically brainier, they were not necessarily any more intelligent.

Since higher latitudes come with colder temperatures, the researchers ruled out increases in eyeball and brain size due to more insulating fat. Only light exposure explained the size differences, even when climate was factored in.

The study demonstrates just how resilient and adaptable humans and other animals can be.

"As we see it," Dunbar said, "populations occupying higher latitudes have three options: keep eyes and the brain the same and accept poorer visual acuity, increase the size of the visual areas to keep visual acuity the same but keep brain size the same, or increase the visual areas, but conserve the size of the frontal parts of the brain, and do this by increasing total brain size."

"It seems we have done the third," he concluded, "since the frontal parts of the brain are the 'smart' areas in terms of IQ. What we seem to have done is to try to keep intelligence constant, rather than sacrificing it."

© 2012 Discovery Channel


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