Saturday, 6 August 2011

Brain size, latitude, and ambient light

An Inuk wearing snow goggles. Is ambient light at its lowest in Inuit territory?

The logjam seems to have broken. On the heels of Lewis et al. (2011), we now have another paper on variation in brain size among human populations, this time by Pearce and Dunbar (2011).

Brains vary in size by latitude, being bigger at higher latitudes and smaller at lower ones. This variation seems to reflect an adaptation to climate. But just how, exactly, does climate relate to brain size? How direct or indirect is the relationship?

Pearce and Dunbar (2011) argue that bigger brains are an adaptation to lower levels of ambient light. Specifically, dimmer light requires larger eyes, which in turn require larger visual cortices in the brain. Using 73 adult crania from populations located at different latitudes, the two authors found that both eyeball size and brain size correlate positively with latitude. The correlation was stronger with eyeball size, an indication that this factor was driving the increase in brain size.

How credible is this explanation? First of all, visual cortex size was not directly measured. The authors inferred that this brain area was responsible for the increase in total cranial capacity. Obviously, they couldn’t have done otherwise. They were measuring skulls, not intact brains.

But there’s another problem—one in the realm of logic. A lot of things correlate with latitude: pigmentation, mating systems, rules of descent, degree of paternal investment, and so on. If one of them correlates more strongly with latitude than the others, does it therefore cause the others? Not at all. It may be closer than the others to this shared cause, but it doesn’t necessarily lie on the same causal chain as the others.

In other words, the level of ambient light does not produce a single cascade of consequences, with eyeball size being the first consequence. There are probably many different cascades.

To date, the best map of human variation in brain size is the one by Beals et al. (1984) (see previous post). If dimness of light is the main determinant, brain size should be highest in northwestern Europe, northern British Columbia, the Alaskan panhandle, and western Greenland. These regions combine high latitudes with generally overcast skies. Yet they are not the regions where humans have the biggest brains. Instead, brain size is at its highest among humans from the northern fringe of Arctic Asia and from northeastern Arctic Canada. These regions are, if anything, less overcast than average. They often have high levels of ambient light because of reflection from snow and ice.

The jury is still out on this question. I suspect, however, that the following three factors probably explain variation in brain size with latitude.

1. Among hunter-gatherers, hunting distance increases with latitude because there are fewer game animals per square kilometer (Hoffecker, 2002, pp. 8-9). Hunters must therefore store larger amounts of spatiotemporal information (landmarks, previous hunting itineraries, mental simulations of possible movements by game animals over space and time). This factor might explain why brains have grown smaller since the advent of agriculture.

2. The seasonal cycle matters more at higher latitudes. As a result, northern hunter-gatherers, and northern agriculturalists even more so, must plan ahead for the next season (or even for the season after the next one).

3. Women gather less food at higher latitudes and almost none in the Arctic. They are thus free to specialize in other tasks, such as garment making, food processing, and shelter building. This “family workshop” creates opportunities for greater technological complexity, which in turn increases selection for greater cognitive performance.

I suspect bigger brains provide not so much greater intelligence as greater ability to store information. As such, they nonetheless pre-adapted northern hunter-gatherers for later advances in cultural evolution.

References

Beals, K.L., C.L. Smith, and S.M. Dodd (1984). Brain size, cranial morphology, climate, and time machines, Current Anthropology, 25, 301–330.

Hoffecker, J.F. (2002). Desolate Landscapes. Ice-Age Settlement in Eastern Europe. New Brunswick: Rutgers University Press.

Lewis, J.E., D. DeGusta, M.R. Meyer, J.M. Monge, A.E. Mann, R.L. Holloway. (2011). The Mismeasure of Science: Stephen Jay Gould versus Samuel George Morton on Skulls and Bias, PLoS Biology, 9(6) e1001071

Pearce, E. and R. Dunbar. (2011). Latitudinal variation in light levels drives human visual system size, Biology Letters, doi: 10.1098/rsbl.2011.0570

23 comments:

john said...

As you sort of mention, it seems difficult to accept that light is the dominant factor here. There is much that is different, diet being obvious and perhaps more easily explainable.

UncleTomRuckusInGoodWhiteWorld said...

This is the first time, for example, that I've heard that Europeans have larger eyes on average than, lets say, West Africans?? Asians have larger eyes than Euros?

I realize there are eye lid shape differences, but still. I'm African American and live in Europe, (have lived in East Asia) and although I have not measured eye-balls, it appears this is false. Maybe there sample size is too small?

Tod said...

It is frightening that someone as eminent as Dunbar would put his name to that paper.

Dunbar has a theory that verbal communication is not for exchanging factual information but for the kind of small talk which functions as 'social grooming' to form alliances.

Maybe his ideas about ambient light are the scientific equivalent of small talk; intended to show he is the 'right sort of chap'.

The image says it all.

Anonymous said...

An Inuk wearing snow goggles. Is ambient light at its lowest in Inuit territory?

Is this how the slit-shape eyes of Asians evolved? Did they develop as a kind of natural snow goggles?

Peter Frost said...

John,

Diet? How?

UncleTom,

At high latitudes, the sample is biased toward the northwestern fringe of Europe, i.e., 18 skulls from England, France, and Scandinavia. There are only 3 skulls from northern Asia (China) and none from eastern or central Europe. In my opinion, this sample bias is responsible for the correlation between latitude and eyeball size. It's more of an "overcast sky" effect than a latitude effect.

Tod,

I don't think there's anything underhanded going on. People can be wrong for all kinds of reasons. It's encouraging that the authors of the study are at least looking into this phenomenon and trying to explain it.

Anon,

Yes, that is the most common explanation for the epicanthic eyefold.

Anonymous said...

I suspect that UncleTomRuckus... is the artist formerly known as DragonHorse.

theslittyeye said...

"I suspect bigger brains provide not so much greater intelligence as greater ability to store information"

Greater ability to store information must have something to do with higher intelligence I suppose? Little is understood about how cognition works in our brain though.

I do admit that visual representation is the easiest way for our brain to receive cognitive information, I have written an amateur article about the cognition preference here. I do think there must be a strong relation between the capability of visual information intake and intelligence level...

"Is this how the slit-shape eyes of Asians evolved? Did they develop as a kind of natural snow goggles?"

The explanation for epicanthic eyefold is still debatable. Caucasoid has lived as well in high latitude regions and they do not appear to develop the epicanthic eyefold (one clear example could be the Sami in the north of Scandinavia?) Modern human Mongoloid from archaeological evidence proved to appear the latest among all other races of homo sapiens. There must be a lot of gaps needs to be filled in the evolution of Mongoloid from early homo sapiens (which I consider as the weirdest human evolution).

UncleTomRuckusInGoodWhiteWorld said...

The only other people with eye folds who did not come from Asia are Subsahara Africans...San Bushmen and evwn some East Africans.

Tod said...

If Stephen Jay Gould had never been discredited Dunbar would still have come out with that paper? Implicit in Dunbar's hypothesis is that Gould was basically correct about cranial capacity being no guide to mental functioning. How does he show that?; by using whopping great crania - from NW Europe!

I don't like the sound of the overcast skies hypothesis, it's too like Carlton Coon's argument that blue eyes are an adaptation which helps eysight in low light or foggy conditions.

theslittyeye said...

"The only other people with eye folds who did not come from Asia are Subsahara Africans...San Bushmen and evwn some East Africans."

About those East Africans, I remember I vaguely heard something about their being the descendants of the Chinese crew in Zhenghe expedition in early 15th century.

About the bushmen I have absolutely no idea how they develop epicanthic eyefold. Thing is SEA do not really appear to have that either. epicanthic eyefold features do appear to originate from North Asia.

Anonymous said...

" although I have not measured eye-balls, it appears this is false. Maybe there sample size is too small?"

There's no real difference, as far as I can see, in their set between relative sizes for Indians (Indian) in the tropics and Europeans, and Chinese and other groups with flat midfaces have small eye socket to brain size volume.

My guess is still that it's a "Caucasoids have large eye sockets as a side consequence of selection for a less tranversely flat midface, for reasons purely related to population history and/or climatic selction" issue. Note relative eye socket volume is also fairly high in the relatively tropically adapted (unless I'm wrong) but transversely not flat Abo Australians as well.

Anonymous said...

Pakistanis and Indians Jailed for Starting Birmingham Riots

http://www.birminghammail.net/news/top-stories/2011/08/11/saltley-man-first-to-be-jailed-for-birmingham-riots-full-court-report-from-first-day-of-prosecutions-97319-29216078/




John Derbyshire estimates that blacks comprise about 2% of the British population but 60 - 70% of the rioters.



http://www.nationalreview.com/corner/274226/epitaph-britain-john-derbyshire#



...

Anonymous said...


There's no real difference, as far as I can see, in their set between relative sizes for Indians (Indian) in the tropics and Europeans, and Chinese and other groups with flat midfaces have small eye socket to brain size volume.


It should also be noted that for many southern Chinese their eyeballs jut out of their face, which is likely a function of their skull shape and the lack of space in there for deep eye-sockets.

Chuck said...

Peter,

Off topic but I was thinking some more about "colorism" and the Blacks color-IQ correlation. Reuters (1919) and Jensen (1998) suggest cross assortative mating for IQ -- and other traits -- and light color. Do you have any thoughts on that idea?

Chuck said...

Nix the last comment. This should say:

Bailey and Geary (2010) [6] had a related paper dealing with hominid brain evolution. The variance was explained by: population density, parasite load, latitude, and temperature.

"I suspect bigger brains provide not so much greater intelligence as greater ability to store information. As such, they nonetheless pre-adapted northern hunter-gatherers for later advances in cultural evolution"

So there is a copious amount of research showing 1) that brain size correlates with IQ [1], 2) that there is a sizable genetic correlation between brain size and IQ [2,3, 4], and 3) that g -- which is more than an index of storage capacity (Long term memory? What exactly do you have in mind here?) mediates the IQ-brain volume correlation [5] and yet you "suggest bigger brains provide not so much greater intelligence as greater ability to store information."

Is that the most parsimonious explanation?

Tod said...

Sexual selection reduced the size of the facial bones in Europeans, would that lack of facial bones not mean that that, in Europeans, eyes would have to be set deeper in the skull.

To me it seems that black Africans' eyes are both larger and set further forward in the skull.

Anonymous said...

If we are talking about pure brain size actually explaining IQ variation directly, rather than merely correlating with things that do (different life histories, reproductive and survival adaptations, ill health and so forth), it seems hard to explain male-female differences, or more crucially, the reported absence of any significantly decreased IQ in hypopituitary dwarfs, in whom the head is much smaller in proportion with the entire body (IQ differences proper seem only to hit primordial dwarves).

Information storage would seem like an interesting alternate route, and obviously on a population level might correlate with IQ, but is there any empirical support for it?

Peter Frost said...

Chuck and anon,

There is a correlation of about 0.5between IQ and brain size. That's significant but not high. I agree that brain size contributes to intelligence, but intelligence is not just a matter of having more neural tissue. There must be other factors as well (myelinization?).

Assortative mating has been documented for both intelligence and skin color in many societies. But I doubt that there's a link between the two.

Anonymous said...

Discovered this interesting paper on orbital volume and changes over time in Europe since the paleolithic:

http://www.paleoanthro.org/dissertations/Michael%20Masters.pdf

Thought it would be on interest to the topic (also seems to me to throw further doubt on their theory, based on the trends it outlines, particularly when taken with evidence relating to upper paleolithic limb adapation as an indicator of climate adaption, and other similar lines of data).

Chuck said...

"Chuck

Assortative mating has been documented for both intelligence and skin color in many societies. But I doubt that there's a link between the two."

Peter,

1. The link would be simultaneous assortative mating for intelligence (or some correlate like SES) and light color. Are you saying that such mating patterns are unlikely?
Is there little link between SES and assortative mating for color?

2. I was wondering if you could comment on the following paper:
Ducrest et al., (2010) "Pleiotropy in the melanocortin system, coloration and behavioural syndromes. Donald Templer cites it in "Richard Lynn and the evolution of conscientiousness." Is it plausible that there is such pleiotrophy humans?

Anonymous said...

I thought this would have been the most up to date map on brain size and geography.

http://occidentalascent.files.wordpress.com/2010/12/beals.jpg

This map does not show small brains for Cambodian, which makes more sense to me. It also looks more realistic and less wild extrapolation. The one in this blog looks ridiculous around southern Germany, and the main boundary in Europe looks like an obvious extrapolation. The one I linked also has a centralized biggest brain area, instead of another extrapolated look in northern Asia and NA.

Can someone tell me with map is more accurate and what were the data points? And why do the alps always have bigger brains, who was tested there?

Anonymous said...

Oh ya, I forgot to add that in my map the Japanese don't have smaller brains, which would have gone against the brain size - iq correlation.

But then again i still dont know any of the actual samples.

Drive-By Poster said...

Intelligence is working memory + long-term memory. It ISN'T visual, but can be used for visual tasks just like it can be used for anything else. Psychometrics is in a sorry state.