Friday, July 29, 2011

What Big Eyes You Have

According to the BBC, a new study has found that northern peoples have bigger eyes - and bigger brains.

Actually, the paper in question talked about eyes but didn't make much of the brain finding, which is confined to the Supplement. Nonetheless, they did find an effect on brain size too. Peoples living further from the equator have larger eye sockets and also larger total cranial capacity (brain volume), apparantly. The authors include Robin Dunbar of "Dunbar's Number" fame.

Their idea is that humans evolved larger eyes because further from the equator, there's on average less light, so you need bigger eyes to collect more light and see well.

They looked at 19th century skulls stored in museum collections, and measured the size of the eye sockets (orbits). They did this by filling them with a bunch of little glass balls and counting how many balls fit. They had a total of 73 "healthy adult" skulls from 12 different places, ranging from Scandinavia to Kenya.

Latitude essentially meant northern-ness because only one population (Australian Aborigines) were from far south of the equator.

Total brain size also increased with latitude, but eye size increased even faster, so the eye:brain ratio increased. They don't really discuss the brain size finding, except to suggest that it might be accounted for by increased visual cortex (though there's no direct evidence of that), but here it is, showing latitude vs. cranial capacity in ml.

The idea that northern peoples are brainier unfortunately has a long history. For example, it's been suggested that the coldness of northern climes meant that life was harder, so people evolved to be smarter to survive.

The heat of the Sahara was easy living compared to the deadly horrors of an English winter, in other words. Hmm.

The idea that higher latitudes are darker, so you'd need bigger eyes, and then a bigger brain (at least the visual parts of the brain) to process what you see, is certainly more plausible than that theory. However, the data in this paper seem pretty scanty.

Measuring skulls by filling them with little balls was cutting edge neuroscience in the 19th century. However, nowadays, we have MRI scanners. Although usually intended to image the brain, many MRI scans of the head also give an excellent image of the skull and eyes. Millions of people of all races get MRI scans every year.

Nowadays, people have medical records, so we can tell exactly how healthy people are. The people who became these skulls in a museum were said to be healthy, but how healthy a 19th century Indian or Kenyan could hope to be, by modern standards, I'm not sure. Certainly there's an excellent chance that they were malnourished and I suspect this would make your eyes and skull smaller.

ResearchBlogging.orgPearce, E., & Dunbar, R. (2011). Latitudinal variation in light levels drives human visual system size Biology Letters DOI: 10.1098/rsbl.2011.0570

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