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Skull measuring continued: Latitude, eye size, and cranial capacity
Pearce, E., & Dunbar, R. (2011). Latitudinal variation in light levels drives human visual system size Biology Letters DOI: 10.1098/rsbl.2011.0570

Skull measuring is all the rage these days. A while back I wrote about a study by Lewis and colleagues, in which they showed that the 19th century anthropologist Samuel George Morton was correct in his assertion that cranial capacity differs between racial groups. This was surprising, because Morton's research had previously been dismissed as a prime example of how racist assumptions can bias results. It was believed, in other words, that Morton had tampered with his data to make sure that Caucasians had the largest cranial capacity of all racial groups.

A globe (source: [url=http://commons.wikimedia.org/wiki/File:Globe.jpg]Wikimedia Commons[/url])Lewis and colleagues were quick to distance themselves from Morton's distinctly racist views, and emphasized that differences in cranial capacity do not reflect differences in intelligence, as Morton had believed. But this, of course, begs the question: If not intelligence, what do differences in cranial capacity reflect?

A recent paper by Pearce and Dunbar in Biology Letters sheds new light on this issue. Pearce and Dunbar measured the size of eye sockets and brains in skulls of people from various parts of the world. Unlike Morton, they did not focus on racial groups, but on the latitude of people's habitat (how far up north people lived). Their finding is straightforward: People that live far up north have larger eyes and correspondingly larger brains. Why? Because near the poles the days are shorter and, more generally, there is less light (they focus on the northern hemisphere, because there are not that many people living near the south pole). And you need big eyes in order to see well under conditions of low light. This had already been shown in birds (nocturnal birds have larger eyes than diurnal birds), but Pearce and Dunbar show that it is true for humans as well.

This explains why Morton found that Caucasians have large skulls: They simply live in habitats with relatively short days and little light. This also explains why Morton found that cranial capacity in Eskimos was a "near approach to the Caucasian mean". (One might argue that Eskimos should have even larger skulls, because they obviously tend to live even farther up north than Caucasians, but I imagine that other factors, such as average body size, also play a role.)

Manga characters live very far up north (source: [url=http://commons.wikimedia.org/wiki/File:Tobidashi-Girl.JPG]Wikimedia Commons[/url])I think, on the whole, the study by Pearce and Dunbar is interesting, particularly in light of the recent stir surrounding the paper by Lewis and colleagues. However, there is one thing that bothers me. Why should larger eyes go hand in hand with larger brains? I concede that it appears to be the case, because there is a high correlation between the two. But Pearce and Dunbar mention this in passing, as though it were obvious, which it isn't. In fact, their reasoning is very strange! They argue that the "close association between retina [part of the eye] and V1 [part of the brain] means that changing retina size directly influences V1 volume." Either I'm missing something or this is completely off the mark. They appear to refer to the fact that the brain is organized retinotopically, which means that adjacent parts of the retina are processed in adjacent parts of the brain. So the overall topography of the retina is preserved in the brain. But the size of the retina is most certainly not. There is no reason whatsoever why a large eye couldn't be handled by a small brain and vice versa.

By analogy (a crude analogy!), you can think of the brain as containing a photo of what you see. Like in a photo, the general layout of the scene is preserved. And, like in a photo, size is not. If you take a photo of something large (or, more accurately, with a large camera), you don't need to print it out in maxi-size. That's silly, and so is the reasoning of Pearce and Dunbar, in this regard.

I think it is far more likely that, in this case, the size of eyes and brains correlate, because there are two distinct ways to improve the quality of vision under low light. One obvious way is to take in more light, by making the eye bigger (and slightly changing its shape). Another way is to devote more processing capacity to vision, to make the most out of what is essentially poor visual input. This would lead to larger brains. It could very well be that a little bit of both is going on, which is why both eye size and brain size increase as people live farther up north.

But, before I get all negative and critical, the paper by Dunbar and Pearce does complement the paper by Lewis and colleagues rather nicely. And it also shows that we don't need to be too afraid of noticing racial differences: Like all animals, humans simply adapt to their environment.

References

Hall, M. I., & Ross, C. F. (2007). Eye shape and activity pattern in birds. Journal of Zoology, 271(4), 437-444.

Lewis, J. E., DeGusta, D., Meyer, M. R., Monge, J. M., Mann, A. E., & Holloway, R. L. (2011). The mismeasure of science: Stephen Jay Gould versus Samuel George Morton on skulls and bias. PLoS Biology, 9(6), e1001071. doi:10.1371/journal.pbio.1001071 [Full text: Open Access]

Pearce, E., & Dunbar, R. (2011). Latitudinal variation in light levels drives human visual system size. Biology Letters. doi:10.1098/rsbl.2011.0570 [Full text: Free]