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.
Pearce, E., & Dunbar, R. (2011). Latitudinal variation in light levels drives human visual system size Biology Letters DOI: 10.1098/rsbl.2011.0570
10 comments:
aren't northerners just bigger in general?
did they control for that?
Not exactly. I think they only had skulls to work with. They controlled for foramen magnum cross-sectional area which is a measure of brain-stem size, which they say is a proxy for body size. I don't know anything about how good a proxy it is.
I can see the north being less bright, and have less light in the winter, but they also have the midnight sun in the summer. Are there actually fewer hours of sunlight in polar areas?
Insulation versus heat dispersion? How much of this brain mass is actually neurons and affiliate cells? Any variation in neuronal density between populations?
It could make sense if you squint.
@ northerner:
"Are there actually fewer hours of sunlight in polar areas?"
Certainly less useful hours for medium latitudes.
If you go polar, you will be overstimulated in summer (hard to sleep!) and understimulated in winter. Add that UV from sun+snow reflection is harmful for "large eyes".
A linear correlation would be suspect.
@ pulnimar:
I thought of the fat ratio too!
Another problem can be that moderns have smaller brains (even when corrected with body size I believe). Then this could also be explained by population size I assume, as the shrink played out over different populations.
We need better data.
The idea that northern peoples are brainier unfortunately has a long history.
It's unfortunate that a fact that doesn't align with modern political sensibilities used to be acknowledged?
northerner: I think the total hours of sunlight per year are constant but the minimum number of hours in the shortest day of the year varies with latitude.
And arguably, for survival, it's the minimum i.e. the hardest challenge, that counts.
I was interested in the reports of this study in the newspapers until I read the details of the methodology and small sample size. Like you, I was amazed that in an age when we have literally thousands of MRI scans available that anyone should undertake (or agree to publish) such an outdated and underpowered study.
At the equator, sunlight averages slightly above twelve hours a day—the sun sinks rapidly, and twilight is only a few minutes. The further you go north, the more twilight there is, eating away at both periods of total darkness and full daylight.
In particular, since eyes do adjust to twilight, there's a longer period of useful light the further north (well, pole-wards) you go.
None of these are huge effects, and there are further confounders (in particular, storms in the tropics can make days black as night, and I don't think that happens in, say, Canada), but the narrative that those Africans have an easy life and we Northerners have to work so hard is, well, racist bullshit.
To the racists, it doesn't matter whether data is sound or nonexistent: in this case, there is minimal data but maximal reporting of it, and that difference is due to racism. Some of it is because racism is fascinating even to non-racists, but most of it is because racists like racism and the journalists provide.
Racism is any act which treats a person's race as more important than than the individual.
That is as true of people who say that there can be no difference between various groups as those who say there must be a difference.
The simple truth is that there is likely to be a difference in brain size and it will help medical research understand the causes and implications of head/brain size if people were allowed to do research.
But there are people who stop sensible research because they think that promoting some ridiculous idea that everyone is the same (even against the evidence) is somehow going to make people respect each other.
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