Eyes, Brains and Latitude

Two things have been known for some time now: Human brains get bigger as you go north, and the volume of the primate eye and the primate brain are correlated.

This COULD mean, and this may not be true, that as you go north in human populations you'll get larger brains (for thermoregulatory reasons) and you'll therefore get larger eyes (because eyeball and brain size is somehow correlated). But a new paper suggests a different model: Large eyes evolve at high latitudes because there is more dark, and the larger eye demands a larger brain.

Maybe, but I doubt it. the largeness of high latitude brains is way higher in magnitude than could be explained by even a two digit increase in the volume of optical processing of data. But maybe. On the other and, the relationship between orbit size and brain size demonstrated some years ago by John Kappleman (but at first ignored for reasons that I've never understood) is cross species an may not apply within species (many such relationships don't). So, it is all rather mysterious for the present.

In any event, the new research could be of interest.

The farther that human populations live from the equator, the bigger their brains, according to a new study by Oxford University. But it turns out that this is not because they are smarter, but because they need bigger vision areas in the brain to cope with the low light levels experienced at high latitudes.

Scientists have found that people living in countries with dull, grey, cloudy skies and long winters have evolved bigger eyes and brains so they can visually process what they see, reports the journal Biology Letters.

The researchers measured the eye socket and brain volumes of 55 skulls, dating from the 1800s, from museum collections. The skulls represented 12 different populations from across the globe. The volume of the eye sockets and brain cavities were then plotted against the latitude of the central point of each individual's country of origin. The researchers found that the size of both the brain and the eyes could be directly linked to the latitude of the country from which the individual came.

Lead author Eiluned Pearce, from the Institute of Cognitive and Evolutionary Anthropology in the School of Anthropology, 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.'

Co-author Professor Robin Dunbar, Director of the Institute of Cognitive and Evolutionary, 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.'

That the explanation is the need to compensate for low light levels at high latitudes is indicated by the fact that actual visual sharpness measured under natural daylight conditions is constant across latitudes, suggesting that the visual processing system has adapted to ambient light conditions as human populations have moved across the globe.

Read the rest of the press release here.

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I'm from the south, and now live in the grand, light-less state of Washington, so I personally would be interested in any recent correlation of shrinking in the visual part of the brain for those of us who are of North European heritage, but were raised in a sunny place.

I'd reverse the logic. I doubt that there is much selective pressure in favor of larger eye sockets to deal with dimmer light, but instead a selective pressure to limit the size of the eye socket at lower latitudes to deal with the bright sun. With that selective pressure removed, you are allowed a random walk towards larger eye sockets.

(Why else would we invent sun glasses? Northern europeans have a tough time with the bright light of the tropics.)

"But it turns out that this is not because they are smarter, but because they need bigger vision areas in the brain "

Complete non-sequitur. PC strikes again.

The fact better vision has been selected for, resulting in bigger brains says nothing about the extra smarts conferred by bigger brains, as implied by this loopy PR.

Anyway, the stupidity caused by proximity to the equator must be largely environmental - the USA is only recently densely populated and just look at the voting patterns North v. South.

By Vince whirlwind (not verified) on 27 Jul 2011 #permalink

It sounds like a load of codswallop, like something out of "The Bell Curve". I'm not a biologist, but I doubt humans have been around long enough for such evolution to occur or be significant enough to measure.

Here's a better explanation: In the small subgroup of humans which reached the Urals and Caucuses before moving west and east, the small percentage which already had larger eye sockets survived better than those with smaller ones, whether by happenstance, accident or survival of the best adapted. It was because they survived that larger eyes became predominant, not the other way around. As for larger brains, larger sockets require larger skulls and the brain just filled in the space rather than having the brain slosh around in a cavity.

It's likely no more relevant than brunette/blonde/redheads all coming from Europe - again, it sounds like something out of "The Bell Curve".


The visual areas of the brain are easily determined, including with non-invasive methods (e.g. MRI). I remember studies correlating the size of the primary visual area (V1) with various things.

So the good news is, the hypothesis that some populations have larger or smaller visual brain areas (as opposed to merely larger or smaller brains in general) can easily be tested.

There is an idea floating around that the main impetus for the initial impetus for brainification among "higher" primates was driven by the need for better visual abilities, as these ancestors switched from the nocturnal lifestyle of early primates to a diurnal lifestyle. Not sure what status this idea has among paleontologists.

P Smith, that could be, but what you just described is a pretty typical example of evolution.

In any event, you are right to be concerned with echoes of the Bell Curve. I don't think that is what is going on here but you can be sure that any research pointing to human variation will be distorted and turned into "evidence" for the existence of races (even though they don't exist) and for racist thinking and policy.

Toto, I was thinking of that as well while reading about this research: Actually, there are two things going on here at once: Larger eyes and more visual processing go with nocturnal lifeways because of low light, but primate-style color vision and it's associated equipment goes with rainforest frugivory and, later, color-based social signalling.

I think that the observation being made here is a very small scale fine tuning.

My personal null hypothesis is that brains are bigger in higher attitudes for thermal reasons and that for some reason we don't quite get eye size is closely linked to brain size (something developmental, not adaptive).

Always remember that there may be different levels of explanation at work: http://scienceblogs.com/gregladen/2009/05/ultimate_causes_proximate_mec…

What is the size of Neanderthal sockets?

By Henry Harpending (not verified) on 28 Jul 2011 #permalink

Henry, good question. I think John Kappleman knows the answer to that.