Decoding the ASPM puzzle

Remember the kerfuffle over ASPM two years ago? ASPM is a gene that regulates brain growth. It evolved considerably in the primate lineage leading to humans and continued to evolve even after the emergence of modern humans, with the latest variant arising about 6000 years ago somewhere in the Middle East. The new variant then proliferated within and outside this region, reaching higher incidences in the Middle East (37–52%) and in Europe (38–50%) than in East Asia (0–25%).

Interest died down when it was found that this variant, despite its apparent selective advantage, does not seem to improve cognitive performance, at least not on standard IQ tests (Mekel-Bobroy et al., 2007; Rushton et al., 2007). Nor do ASPM variants correlate with human brain size variability (Rushton et al., 2007).

Now, new light has been shed on this puzzle by a paper on ASPM in other primates. This gene was initially linked to overall brain size because non-functioning variants cause microcephaly in humans. A comparative study of primate species, however, has shown that evolution of ASPM does not correlate with major changes in whole brain or cerebellum size:

Particularly striking is the result that only major changes of cerebral cortex size and not major changes in whole brain or cerebellum size are associated with positive selection in ASPM. This is consistent with an expression report indicating that ASPM’s expression is limited to the cerebral cortex of the brain (Bond et al. 2002). Our findings stand in contrast to recent null findings correlating ASPM genotypes with human brain size variation. Those studies used the relatively imprecise phenotypic trait of whole brain instead of cerebral cortex size (Rushton, Vernon, and Bons 2006; Woods et al. 2006; Thimpson et al. 2007). Although previous studies have shown that parts of the brain scale strongly with one another and especially with whole brain (e.g., Finlay and Darlington 1995), evidence here suggests that different brain parts still have their own evolutionary and functional differentiation with unique genetic bases. (Ali & Meier, 2008)

This is a point I raised a year ago. If we look at how the new ASPM variant spread geographically and temporally, it seems to match a very specific mental ability, and not general intelligence:

At present, we can only say that it [the new variant] probably assists performance on a task that exhibited the same geographic expansion from a Middle Eastern origin roughly 6000 years ago. The closest match seems to be the invention of alphabetical writing, specifically the task of transcribing speech and copying texts into alphabetical script. Though more easily learned than ideographs, alphabetical characters place higher demands on mental processing, especially under premodern conditions (continuous text with little or no punctuation, real-time stenography, absence of automated assistance for publishing or copying, etc.).

…How well are these tasks evaluated by standard IQ tests? Although most tests involve reading, transcribing, and taking dictation, these abilities are not evaluated over long, uninterrupted time periods. If we look at the two studies that discounted a cognitive advantage for the new ASPM variant, neither tested its participants for longer than 82 min and the tests themselves involved a mix of written and verbal tasks.

It seems premature to conclude that the new ASPM variant is unrelated to cognitive functioning. Current IQ tests do not adequately evaluate mental processing of alphabetical writing, particularly under premodern conditions. Yet this is the cognitive task whose origin and spread most closely coincide with those of the new ASPM variant in human populations. It is also a demanding task that only a fraction
of the population could perform in antiquity, in exchange for privileged status and probably superior reproductive opportunities. (Frost, 2007)

Is this cognitive task localized in a specific part of the brain? There seems to be evidence for such localization … which I will review in my next post.

References

Ali, F. and Meier, R. (2008). Positive selection in ASPM is correlated with cerebral cortex evolution across primates but not with whole brain size. Molecular Biology & Evolution. Advance access

Frost, P. 2007. "The spread of alphabetical writing may have favored the latest variant of the ASPM gene", Medical Hypotheses, 70, 17-20.

Mekel-Bobrov, N., Posthuma D., Gilbert S.L., et al. (2007). The ongoing adaptive evolution of ASPM and Microcephalin is not explained by increased intelligence. Hum Mole Genet, 16, 600–8.

Rushton, J.P., Vernon, PA.., Bons, T.A. (2007). No evidence that polymorphisms of brain regulator genes Microcephalin and ASPM are associated with general mental ability, head circumference or altruism. Biology Letters-UK, 3, 157–60.

The European pattern of skin, hair, and eye color

The following is an executive summary for one of my book proposals. The book itself will probably take me a year to write and I’m sure I’ll have to update the manuscript continually as new information comes in. Comments are welcome.

Humans look strikingly different in Europe, particularly within a zone centered on the East Baltic and covering the north and the east. Here, skin is unusually white. Hair is not only black but also brown, flaxen, golden, or red. Eyes are not only brown but also blue, gray, hazel, or green.

This pattern also stands out chronologically. It arose very late during the time of modern humans and long after their arrival in Europe some 35,000 years ago. Such is the conclusion now emerging from genetic studies of skin, hair, and eye color.

Europeans owe their light skin to alleles that go back only c. 11,000 years at one gene and 12,000–3,000 years at another. As a Science journalist remarked: “the implication is that our European ancestors were brown-skinned for tens of thousands of years.” They were also uniformly black-haired and brown-eyed. Then, just as recently, their hair and eye color diversified as new alleles began to proliferate at two other genes.

The challenge now will be to narrow the time window. If these changes happened after 7,000 BP, the cause might be northern Europe’s shift from hunting and gathering to cereal agriculture. The change in diet may have reduced the intake of vitamin D, thus favoring the survival of paler Europeans whose skin could synthesize more of this vitamin.

This theory explains how European skin could have turned pale almost at the dawn of history. It leaves unexplained, however, why selection for lighter skin would have multiplied the number and variety of alleles for hair or eye color, especially when so many have little effect on skin color.

If these changes had happened earlier, before 10,000 BP, the cause might involve the last ice age. At that time, the tundra ecozone ran further south in Europe than in Asia, having been pushed down on to the plains of northern and eastern Europe by the Scandinavian icecap. The lower, sunnier latitudes created an unusually bioproductive tundra that could support large herds of game animals and, in turn, a substantial human population—but at the cost of a recurring shortage of male mates. Among present-day hunter-gatherers, similar environments raise the male death rate because the men must cover long distances while hunting migratory herds. The man shortage cannot be offset by more polygyny, since only a very able hunter can provide for a second wife (tundra offers women few opportunities for food gathering, thus reducing their self-reliance in feeding themselves and their children). With fewer men altogether and fewer being polygynous, the sex ratio is skewed toward a female surplus.

In this buyer’s market, men will select those women who look the most feminine. Since human skin color is sexually dimorphic (women are the ‘fair sex’), this sexual selection would eventually whiten the entire population. Where pigmentation has no female-specific form, as with hair and eye color, sexual selection would favor women with color variants that stand out by their novelty, the outcome being an increasingly diverse polymorphism.

Cavalli-Sforza's about-face

The renowned geneticist Luca Cavalli-Sforza is identified with the position that human races do not exist. In his opus The History and Geography of Human Genes, he included a chapter on the ‘failure of the race concept’ and declared that “the classification into races has proved to be a futile exercise".

This position gets much play in the media. An article published in The Economist tells us that the work of Cavalli-Sforza "challenges the assumption that there are significant genetic differences between human races, and indeed, the idea that 'race' has any useful biological meaning at all." This is also how he is seen in an article in The Stanford Magazine:

And he has received another kind of recognition —stacks of hate mail from white supremacists —for his well-publicized insistence that DNA studies can serve as an antidote to racism because they reveal an underlying genetic unity that cuts across racial groupings, making race a scientifically meaningless concept.

Yet not everyone believes he is a convinced antiracist:

How is it, then, that Cavalli-Sforza now finds himself accused of cultural insensitivity, neocolonialism and "biopiracy"? Late in his career, as he struggles to organize his most ambitious project yet -- a sweeping survey of human genetic diversity -- why are some people calling him a racist?

Perhaps because some people feel he is too inconsistent. On this issue, there are really two Cavalli-Sforzas: the one who denounced the race concept in 1994 … and the one who upheld it in 1976:

Today, all continents of the world are inhabited by representatives of the three major human races: African, Caucasian and Oriental. The proportions of the three groups still differ considerably in the various countries, and the migrations are too recent for social barriers between racial groups to have disappeared. The trend, however, seems to be in the direction of greater admixture.

On the most general level. geographic and ecological boundaries (which acted as partial barriers to expansion and migration) help to distinguish three major racial groups: Africans, Caucasians, and a highly heterogeneous group that we may call "Easterners". The Easterners include subgroups that were separated in various older classifications, such as American Natives (American Indians) and Orientals (Chinese, Japanese, Koreans). Some regard Australian aborigines as a separate race, but they do not differ much from Melanesians. From the Melanesians, we can trace a sequence of relativelygradual changes through the transition to Indonesians, then to Southeast Asians, and on to East Asians. American Natives and Eskimos probably both came from a related Northeast Asian stock from (or through) Siberia into North America. Eskimos, however, came much later than American Indians, and they subsequently expanded further eastward to Greenland.

The African continent contains, in the north and east, populations that have various degrees of admixture with Caucasians by all criteria of analysis. In the western, central and southern parts of the continent, Africans are relatively homogeneous - although some isolated groups of hunter-gatherers (like Pygmies and Bushmen) show cultural and physical peculiarities that suggest they should be considered somewhat separately. In fact, the Pygmies at least have attributes that indicate they may be "proto-African" groups — populations that have been the least altered by more recent events.

We tend to side with those taxonomists who prefer to group the human species into a few large racial groups (such taxonomists have been called "lumpers"). Others ("splitters") prefer to distinguish a large number of groups differing in relatively subtle ways. (Bodmer & Cavalli-Sforza, 1976, pp. 563-572)

Even later in time, particularly in journal articles, one can find references to race-based thinking:

The first split in the phylo-genetic tree separates Africans from non-Africans, and the second separates two major clusters, one corresponding to Caucasoids, East Asians, Arctic populations, and American natives, and the other to Southeast Asians, (mainland and insular), Pacific islanders, and New Guineans and Australians. Average genetic distances between the most important clusters are proportional to archaeological separation times. (Cavalli-Sforza et al., 1988)

What happened after 1976 to change Cavalli-Sforza’s views on race? Very little in terms of data. Four years earlier, the case against the race concept had already been made in a paper by Harvard geneticist Richard Lewontin. Frank Livingstone, an anthropologist, had even earlier presented similar arguments in his 1962 paper: “On the non-existence of human races”. Both papers had been published in leading journals and were still being widely discussed when Cavalli-Sforza co-authored a genetics textbook in 1976. Evidently, he was not convinced.

At least not then. As one anthropologist told me: “I don't think our perception of the general patterns of genetic variation changed much from '76 to '94, but the intellectual climate that geneticists operate in sure did.”

References

Anon. (2000). The Human Genome Survey, The Economist, 1 July 2000, pg. 11

Bodmer, W.F. and L.L. Cavalli-Sforza. (1976). Genetics, Evolution, and Man. WH Freeman and Company, San Francisco. pp 563-572.

Cavalli-Sforza, L.L., Menozzi, P. & Piazza, A. (1994). The History and Geography of Human Genes. Princeton: Princeton University Press.

Cavalli-Sforza, L.L., Piazza, A., Menozzi, P., and Mountain, J. (1988). Reconstruction of human evolution: Bringing together genetic, archaeological, and linguistic data. Proc. Natl. Acad. Sci. USA, 85, 6002-6006.

Leslie, M. (1999). The History of Everyone and Everything. The Stanford Magazine. May-June.

Lewontin, R.C. (1972). The apportionment of human diversity. Evolutionary Biology, 6, 381-398.

Livingstone, F.B. (1962). On the non-existence of human races. Current Anthropology, 3, 279-281.

Aleksandr Solzhenitsyn RIP

Aleksandr Solzhenitsyn passed away on Monday. He was one of several writers who have truly touched me (George Parkin Grant was another). His main message, for us in the West, is that Communism was not specific to Russian culture and history. This message is summarized by Wikipedia:

It is a popular view that the October revolution of 1917 resulting in a violent totalitarian regime was closely connected to Russia's earlier history of tsarism and culture, especially that of Ivan the Terrible and Peter the Great. Solzhenitsyn claims this is fundamentally wrong and famously denounced the work of Richard Pipes as "the Polish version of Russian history". Solzhenitsyn argues Tsarist Russia did not have the same violent tendencies as the Soviet Union. For instance, in Solzhenitsyn's view, Imperial Russia did not practise censorship; political prisoners were not forced into labour camps and the number of political prisoners was only one ten-thousandth of those in the Soviet Union; the Tsar's secret service was only present in the three largest cities, and not at all in the army. The violence of the Communist regime was in no way comparable to the lesser violence of the Tsars.

He considered it far-fetched to blame the catastrophes of the 20th century on one 16th century and one 18th century czar, when there were many other examples of violence which could have inspired the Bolshevik in other countries earlier in time, especially mentioning similarities with the Jacobins of the Reign of Terror of France.

Communism was a kind of idealism that allowed no dissent. Some debate did exist, but only within narrowly circumscribed bounds. Is abstract art progressive? What is the timetable for building socialism? And so on. There was no substantive debate over core values.

With no debate, there could be no true reform. There could, at best, be tinkering. And such tinkering often made things worse. Eventually, even the Soviet elite realized that fundamental changes were needed. But how?

Imagine a machine with no ‘off’ switch. Imagine a machine that does not hesitate to destroy its own creators. Imagine a machine that can be stopped only by the limitations of reality itself. That was Russia’s nightmare.

And it could become ours. I used to think that such a thing could happen only in Russia. Now, looking around my own country, I must confess I was wrong. It could happen here. And it could happen for the noblest of intentions.

"In our country the lie has become not just a moral category but a pillar of the State."
–Aleksandr Solzhenitsyn, December 29, 1974