Friday, September 07, 2007

The new races of man....   posted by Razib @ 9/07/2007 02:33:00 PM

Over 40 years ago the physical anthropologist C.S. Coon wrote The Origin of Races. Along with The Living Races of Man this book is an excellent source for on all sorts of obscure and hard to find anthropometric data. As Peter Frost has noted it can be rather difficult to find information on the distribution of the most mundane of human characteristics because of the shift away from the descriptive physical anthropology. If you want frequencies on blue eyes in Sweden, your data set is likely to predate World War II.

That being said, the model posited in The Origin of Races is false, Coon hypothesized that six primary races had their origins in independent transitions to sapiency by their hominid precursors. In this model the seeds for racial difference existed on the order of tens of millions of years before the present. Though a great collector of data and a vivid storyteller, like most of the physical anthropologists of the first half of the 20th century Coon's scientific methodology was heavily intuitive and seemed to be guided mostly by gestalt impression.

The subsequent genetic revolution, starting with the successful challenge by the proponents of the molecular clock to the older paleontological model of the origin of hominids which assumed a very early separation from the great apes, and ending with the ascendance of Out-of-Africa over a Multiregional conception of modern human origins, made the older anthropological taxonomy look amateurish. L.L. Cavalli-Sforza's The History and Geography of Human Genes rendered much of the more imaginative anthropological literature on racial history a historical footnote. I've avoided terms like Mongoloid and Caucasoid because of their associations with an older anthropological taxonomical tradition which seemed basically irrelevant in terms of the theoretical framework. Even the early serological work on the Ainu of Japan seemed to show that this putatively Caucasoid population, as determined by older anthropometric methods and description, was more closely related by descent to the populations of East Asia than to Western Eurasians. Analysis of the Indian subcontinent seemed to show that there was more genetic distance between these peoples and those of the Middle East & Europe than some of the older models had projected. I did not see great need to look to the older literature for pointers or guides when genetic markers could tell the full tale.

But lately I've been reconsidering. Though the racial migrations posited by the physical anthropologists of the early 20th century were generally more shaped by their cultural context and imagination than reality, the anthropometric data they collected wasn't manufactured out of whole cloth. The description tracked a real pattern in the data. Though the hypotheses were not very informed by the latest in evolutionary and genetic theory, they were not uninformed by fact. Lately I've been posting a lot about lactase persistence and skin color. These two genetic characteristics have come a long way in terms of their understanding over the past few years. At my other blog I posted yesterday about a study which studied the skin color variation in South Asians. The SNP of SLC24A5 which is fixed in Europeans and not present in Africans and East Asians, seems to exhibit high penetration within South Asia. In fact, it explains about the same proportion of the skin color difference between the lightest and darkest quintile of South Asians as it does in relation to the complexions of Europeans and Africans. Lactase persistence also exhibits a similar pattern, the same allele seems to have swept from the Atlantic to the Bay of Bengal. Its penetration into Sub-Saharan Africa is patchier, and it seems not to have any discernible impact on East Asia.

In The Real Eve, a survey of the literature on neutral genetic markers as proxies for ancestry, Steve Oppenheimer asserted that the basic population groups we see around us today were established and in place by the end of the last Ice Age. In other words, most of the ancestors of people in East Asia, the Middle East, South Asia, Europe and Africa were on those continents 10,000 years ago. Of course there have been changes within the continents, the expansion of the Han within the last 3,000 years in China or the Bantu expansion in Africa are two examples. Additionally, there have been genetic overlays upon the bedrock, for example, the injection of African and Arabian female and male lineages respectively throughout what has become the Arab world over the last 1,500 years. The signature of a Neolithic demic diffusion into Europe from the eastern shores of the Mediterranean starting about 10,000 years ago. Evidence in South Asia of population movements from West and Central Asia from antiquity down to the Islamic period. Nevertheless, in narrative generated by the neutral markers is that the races of man were extant more or less 10,000 years ago and only the margins and details have been affected by subsequent migrations.

I think this is wrong, and Oppenheimer in his chapter on East Asia supplies a clue to why this story isn't complete: he notes that the classical East Asian physical type, that is, Mongoloid, seems to emerge very recently. One could posit that this and artifact due to the paucity of fossil remains in this region for anatomically modern humans. But no, I think the answer is that the Mongoloid physical type emerged within the last 10,000 years due to recent human evolution! We also know from the data that the classical Northern European with fair hair, blue eyes and a taste for milk is also a recent evolutionary development, within the last 10,000 years. In Africa malaria probably resulted in some of the highest selection coefficients known to man, all within the last 5,000 years. In South Asia we know that lactase persistence and lighter skin are due to recent evolution, and likely both are exogenous. In fact, the allele is identitical by descent with that which is common in the Middle East and Europe.

And this last fact brings me to another observation: the patterns of cultural diffusion which seem to characterize the pre-modern world might very well serve as a map for the sweeping of mutants of large effect across Eurasia over the past 10,000 years. The genetic architecture for light skin color are radically different in East Asia as opposed to Europe, the Middle East and South Asia. Similarly, the alleles for lactase persistence that are West Eurasian are found in the same broad swath of land where SLC24A5 seems to have made an impact: from Europe down into North Africa across the Middle East and South Asia. This area is also the classical home of the Caucasoid race, which Coon characterized by its lack of specialized features vis-a-vis Africans and Mongoloids. I am struck by the correspondence here with a particular cultural pattern that reoccurs: memes which originate along the fringe of Eurasia east from India and west to Spain as well as at the nexus with Africa seem to stop at the Sahara and the Himalaya. Consider the scripts dominant from India to Spain, they derive from Aramaic. In contrast, Chinese writing is based on a different model. Consider the Indo-European and Afro-Asiatic languages which span such broad expanses, from the Atlantic to the Bay of Bengal, from the Niger river to borders of Anatolia. Consider that the One True God is worshipped from Europe to Bengal.

In The Human Web the historian William H. McNeil describes the slowly growing networks of information which spread across the globe over the past 3,000 years. He posits the origination of hearths, proto-civilizations, which slowly became entangled in a dense network of trade and political unification. One of the earliest major dynamics which persisted to the relatively recent past was the separation of East Asia from Western Eurasia. Though there were contacts and diffusions, the three major cultural hearths, the West (Europe & the Middle East inclusive), India and China did not have symmetrical relationships. The West and India, though alien and viewing each other as exotics, did have a non-trivial amount of trade and cultural exchange. As early as the Persian Wars soldiers recruited in the Indian provinces could be found involved in European conflicts, while Indian philosophy seems to have had an indirect and diffuse influence upon later Greek philosophy (by which I mean Neo-Platonism and such). Small communities of Christians & Jews and Roman coins in southern India, or the adventuring of Central Asians and Africans in armies of Muslim sultans, attest to South Asia's connections to the West. Even as far back as the Sumerians there was most certainly contact between India and the Middle East (references to Meluhha and trade items found in Harrapa is evidence of this). In contrast, ancient China was a relatively isolated land encircled by barbarians. Granted, some cultural ideas seem to have percolated from the West to the East, and the presence of Christians, Jews and Muslims in China from an early period attest to the more vigorous contact of the last 2,000 years. Nevertheless, the extinction of both the early Jewish and Christian communities attests to the relative isolation from the wider world which still characterized China until the early modern Age of Exploration.

And so it might be with genes. Though there was certainly some traffic between the antipodes of Eurasia, the migration was low enough that independent mutations with similar phenotypic effects seem to have arisen in the different domains. With the rise of agricultural society, enormous populations, increased rate of cultural evolution, I suspect that biological evolution was also cranked up through the simple change of prosaic population genetic parameters (more mutants, more long distance migration, etc.). Alleles of large effect could have swept repeatedly across distinct and long separate populations and bound them together as functional clusters. At the end of the last Ice Age the whole region from the Atlantic to South Asia was characterized by a sparse population just reemerging from their refugia. Long isolated from each other by the Last Glacial Maximum these populations were genetically quite distinct, especially between South Asia and Europe & the Middle East. But just as the rise of long distance trade spread ideas across these disparate zones, so genetic evolution swept along the routes of migration. A group like the Indo-Europeans may be only a small proportion of the ancestry of modern Persians, Indians, Germans and Russians, but they might have been significant in spreading genes of great impact across this broad zone, generating surface commonalities which contradict the deep time cleavages wrought by the isolation of the Ice Age. What I am positing is an inversion of the dynamic which Henry Harpending brought to my attention years ago, he pointed out that some populations exhibit similarity of ancestry but sharp differences of phenotype. Harpending's model was that social selection on particular loci could perpetuate physical differences between groups which intermarried and so equilibrated on most markers. In contrast, the model here that I am proposing is that the differences of ancestry may belie the common ground on major functional loci across several population clusters. And similarly, adaptive evolution may result in divergences in appearance between groups with relatively recent common ancestry, as seems the case in relation to Ainus and other East Asians. All in all, it is a complex & baroque picture, but one which I believe will exhibit interesting systematic patterns. Presenting this in a pithy, precise and accurate manner to the public, well, that's a different story....

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