Too darn hot?

Does higher IQ correlate with colder temperatures? Not among people belonging to the same cultural system, such as the Chinese. (source)

Big brains are costly, not only because of their high energy consumption but also because many genes have to interact to create neural tissue. The bigger and more complex the brain, the more it is vulnerable to accidents at the gene level, like random mutations.

Mutations happen more often at warmer temperatures. In Drosophila, an increase of 10ºC will double or triple the mutation rate. Tight underwear has probably done more to harm the human genome than fallout from nuclear testing (Sutton, 1975, p. 318).

Drawing on these two points, Greg Cochran is now suggesting that large human brains are a precarious outcome of evolution (here and here). However strong the natural selection may be for a bigger brain, the mutation rate is pushing back in the opposite direction. Beyond a certain size, big brains are possible only where the mutation rate is relatively low—in cooler regions at higher latitudes.

This is a seductive way of explaining why brain size correlates with latitude. And, yes, such a correlation does exist. So thought most 19th-century physical anthropologists, notably Samuel George Morton, but Stephen J. Gould (1978) concluded otherwise after a reanalysis of Morton’s data that became the centerpiece of his book The Mismeasure of Man. A team of physical anthropologists has since located and remeasured Morton’s skulls. Their conclusion? The original measurements had few errors, and the errors were distributed randomly. There was, in fact, a non-significant tendency by Morton to overestimate African skull size (Lewis et al., 2011).

Brain size and latitude also seem to correlate among ancestral hominids from A. Afarensis to H. sapiens (Henneberg & Miguel, 2004). The correlation remains even if time period is controlled. It thus cannot be due to the overall rise in cranial capacity over time and the parallel expansion of ancestral hominids into higher latitudes.

Is this correlation adequately explained by the ‘Too Darn Hot’ theory? The main supporting evidence is the finding that ‘loss of function’ mutations are much more common in sub-Saharan Africans than in other humans (MacArthur & Tyler-Smith, 2010; Tennessen et al., 2012). Natural selection seems to have more trouble weeding them out in the tropics than elsewhere.

But do these mutations need to be weeded out? Are they in fact deleterious? Some authors think so. Most don’t, including the ones of a study that Greg cites:

[…] the implicit assumption that LOF variants (and indeed other changes predicted to be damaging to the protein) are necessarily deleterious to human health is a dangerous one, especially when such an assumption is used to infer disease causality for a novel variant. In fact, the studies reviewed above demonstrate that healthy humans carry many dozens of LOF variants, most of which have little or no effect on health (at least in the heterozygous state). (MacArthur & Tyler-Smith, 2010)

In general, these mutations seem to involve genes of very low selective value, so they could very well hang around indefinitely if natural selection against them is weak enough and if the population is large enough.

Here we come to the usual explanation for Africa’s large number of ‘loss of function’ mutations. There are so many of them because Africans have largely stayed put in the same place with the same population base. In contrast, non-Africans are descended from small founder groups that took only a small portion of this junk variability on their way out of Africa:

The gene-diversity results presented here are consistent with one another and with those of many previous studies in showing higher levels of diversity in African populations than in non-African populations […] A higher level of African diversity supports the hypothesis that modern humans first arose in Africa and then colonized other parts of the world (Stoneking 1993), but genetic diversity is related not just to a population’s “age” but also to demographic events in a population’s history, such as bottlenecks and effective population size. (Jorde et al., 2000)

Alternate theories

Do we have other theories for latitudinal variation in brain size? To date, there seem to be three:

Need to reduce heat loss at higher latitudes

According to Beals, Smith & Dodd (1984), heads have grown larger at higher latitudes as a way to reduce heat loss. An object will lose less heat if it has a high ratio of volume to surface area. Natural selection has thus favored more globular heads at higher latitudes. The increase in brain size is thus incidental.

This explanation was challenged in the comments section of the above paper. Iwatoro Morimoto pointed out that "in recent centuries, brachycranic skulls show a considerable increase in frequency in Eurasian populations, including the Japanese." Since mean temperatures have changed little in recent centuries, there must have been another factor at work.

Another commenter, Erik Trinkaus, similarly pointed out that Neanderthal cranial capacity was no bigger during glacial periods than during interglacials. The same was true for early modern humans. For populations already established at northern latitudes, cranial capacity shows no evidence of rising and falling with mean temperature.

Finally, if the increase in brain size was driven by the need for a more globular head, that goal can be met by filling the extra head space with non-neural tissue, like bone or cartilage. Neural tissue has a high maintenance cost. Why maintain something at great expense if you don’t really need it?

Increase in visual cortex at higher latitudes

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, visual cortex size was not directly measured. The authors inferred that this brain area was responsible for the increase in total cranial capacity. Of course they couldn’t have done otherwise. They were measuring skulls, not intact brains.

To date, the best map of human variation in brain size is by Beals et al. (1984). 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, brains are biggest 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.

Increase in cognitive demands at higher latitudes

Finally, brain size may have increased at higher latitudes because of an increase in cognitive tasks, specifically foresight. As ancestral humans spread out of the tropics and into latitudes with a predictable summer/winter cycle, it became much more advantageous to simulate the future consequences of present actions.

This point is discussed by Hoffecker (2002, p. 135). Among early modern humans, tools and weapons were more complex at arctic latitudes than at tropical latitudes. “Technological complexity in colder environments seems to reflect the need for greater foraging efficiency in settings where many resources are available only for limited periods of time.” Arctic humans planned ahead to cope with resource fluctuations and high mobility requirements, such as by developing untended devices (e.g., traps and snares) and means of food storage.

Colder environments imposed even higher cognitive demands when hunting and gathering gave way to agriculture. Food had to be grown not only for present needs but also for the next cold season. As late as the 18th century, farm families often faced starvation in early spring—when their winter provisions had run out and their spring crop had not yet come in.

The yearly cycle and the need to plan ahead thus preadapted early non-tropical humans for later cultural developments, such as invention of writing and bookkeeping, complexification of social relations, creation of towns and cities, systems of military defense, roads and highways, etc. At that point, cognitive demands were no longer driven by the yearly cycle, at least not primarily. They were now being driven by an increasingly complex cultural environment—what we call ‘civilization.’

Conclusion

How does the ‘Too Darn Hot’ theory stack up against these alternate theories? The main contender seems to be the last one, i.e., the increase in cognitive demands at higher latitudes. According to that theory, the yearly cycle has given way to gene-culture co-evolution as the main driving force behind increases in intellectual capacity.

Thus, if people live within the same cultural system and are exposed to similar cognitive demands, they should on average have the same intellectual capacity … regardless of the mean temperature of their particular locality.

Conversely, the ‘Too Darn Hot’ theory would predict the existence of a north-south cline in IQ even among people of a similar cultural background, since people at more tropical latitudes should have a higher incidence of deleterious mutations.

China, for example, covers a wide range of latitudes from the sub-Arctic to the tropics. Although the Chinese have occupied this latitudinal range for some 2,500 years, i.e., about 100 generations, their mean IQ doesn’t seem to vary along a north-south cline (see above map).

Perhaps 100 generations isn’t long enough. But what about the Amerindians? They’ve inhabited a full range of latitudes from the Arctic to the equator for some 12 to 15 thousand years. That’s 480 to 600 generations. Is there a difference in mean IQ between the Naskapi of northern Labrador and the Yanomamo of Amazonia? I’d be surprised.

References

Anon. (2011). IQ geography in China, November 19, The Slitty Eye,

http://theslittyeye.wordpress.com/2011/11/19/iq-geography-in-china/

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

Cochran, G. (2012). Changes in attitudes, West Hunter, July 18

http://westhunt.wordpress.com/2012/07/18/changes-in-attitudes/

Cochran, G. (2012). Too darn hot? West Hunter, July 14

http://westhunt.wordpress.com/2012/07/14/too-darn-hot/

Gould S.J. (1981). The Mismeasure of Man. New York: W. W. Norton and Company.

Gould , S.J. (1978). Morton’s ranking of races by cranial capacity: unconscious manipulation of data may be a scientific norm, Science, 200, 503–509.

Henneberg, M. and C. de Miguel. (2004). Hominins are a single lineage: brain and body size variability does not reflect postulated taxonomic diversity of hominins, Journal of Comparative Human Biology, 55, 21–37

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

Jorde, L.B., W.S. Watkins, M.J. Bamshad, M.E. Dixon, C.E. Ricker, M.T. Seielstad, & M.A. Batzer. (2000). The distribution of human genetic diversity: a comparison of mitochondrial, autosomal, and Y-chromosome data, American Journal of Human Genetics, 66, 979–988.

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

MacArthur, D.G., & C. Tyler-Smith. (2010). Loss-of-function variants in the genomes of healthy humans, Human Molecular Genetics, 19, R125-R130.

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

Sutton, H.E. (1975). An Introduction to Human Genetics, New York: Holt, Rinehart and Winston.

Tennessen, J.A., A.W. Bigham, T.D. O’Connor, W. Fu, E.E Kenny, et al. (2012). Evolution and functional impact of rare coding variation from deep sequencing of human exomes, Science, 337, 64-69.