Planet Earth

Facing the ocean

Gene ExpressionBy Razib KhanApr 29, 2012 3:35 AM

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Halford Mackinder's conceptualization of the world


With the recent publication of the paper on the archaeogenetics of Neolithic Sweden I feel like we're nearing a precipice. That precipice overlooks lands of great richness, filled with hope. It's nothing to fear. It is in short a total re-ordering of our conception of the recent human past, at minimum. The "pots not people" paradigm arose in archaeology over the past few generations due to both scholarly and ideological factors. The scholarly ones being that intellectuals of the 19th and early 20th century made assumptions of extremely tight correspondence between material and cultural characteristics, and demographic dynamics, which seem to have been false. Therefore, the rise of an Anglo-Saxon England and the marginalization of Celtic Britain to the western fringes was not just a cultural reality, but also a fundamentally racial one, as Germans replaced Celts in totality. The ideological problem is that this particular framework was take as a given by the Nazis during World War II, lending a bad odor to the hypotheses of migration which were once so ascendant.

No one could deny that material cultures rise and fall in pulses, and exhibit variation in spatial distribution over the millennia. But by and large scholars large took a very skeptical view of the idea that large scale migrations of populations may have occurred in prehistory, and could have been the underlying causal factors driving the changes in material culture. But a null hypothesis of demographic stasis was in itself a positive statement of beliefs as to the character of the human past. It was no withholding of judgement. Today the results from ancient DNA, and more powerful inferential methods which extract patterns out of extant variation, simply can not be easily fitted into a "pots not people" framework. Nor can we go back to a race-is-culture and culture-is-race model in the vein of the Victorians. Rather, the new order model must take into account the imperfect, but non-trivial, correlation between cultural and genetic variation, and, the differences between patterns of cultural and genetic variation. L. L. Cavalli-Sforza first made famous the correlation between language and genes in History and Geography of Human Genes. But over the past generation a lot of human genetics does seem to confirm that the sharp discontinuity between language families is correlated with discontinuity between genes. I say correlated, because though by and large a demarcated population may have one native language to contrast it with its neighbors, these populations are rarely have one "native genotype" to contrast with their neighbors. Though two populations may exhibit genetic differences, if they have been neighbors for any period of time there is usually gene flow across them which reduces between population difference. A classic case of this can be found in Southern Africa. The Xhosa Bantu ethnic group has long been geographic neighbors with Khoisan populations. This shows in their genomes, which are on the order of 10-20% Khoisan. Additionally, the Khoisan themselves show admixture, from the Bantu, and even Europeans! But though the Xhosa language shows some Khoisan influence, it is certainly not "10-20%", whatever that may be. Similarly, despite the presence of European and Bantu origin ancestry in the Bushmen, they remain culturally distinct. The reason for this is pretty straightforward:

you are 50% genetically derived from each parent, you are not necessarily 50% culturally derived from each parent

. If parents differ in language, religion, and norms, the children may select one particular set of cultural values from one parent, usually in line with the majority culture. In this way individuals who migrate between populations may introduce genetic diversity, but not so much cultural diversity. But this is on a relative small scale, that of the clan or band. What about on a larger structural scale? Over the past 500 years we have seen cultural shifts which had very little to do with genetics. From what I have read Gaelic went from being the majority to the minority language of the Irish Roman Catholic peasantry between 1800 and 1850. Presumably the famine of those years played a role. Whatever the specific historical factors at work, obviously this was a massively significant shift, without much genetic cause. On contrary, previous non-Gaelic speaking transplants either assimilated (e.g., the Old English and the Norse-Gaels), or remained culturally distinct (e.g., the Ulster Scots and the New English). A contrast to this was the Columbian Exchange, which involved either massive cultural and genetic replacement, or substantial cultural replacement and significant genetic amalgamation.

A key point is that massive cultural change does not always correlate with massive genetic change. The reason that "pots not people" became popular is that earlier scholarship was predicated on an assumption of perfect correspondence, rather than correlation of patterns. As an example of what I mean, assume that you are a given a population of predominantly African origin, with a European minority ancestral component. The population speak a Germanic language, and adhere to Protestant Christianity. What region of Europe would you assume that the European ancestral contribution derived from? Of course it would be Northwest Europe, and that is exactly what you see in black Americans, and what you do not see in black Brazilians, who speak a Southwest European language and are predominantly Roman Catholics. Despite the European language and religion of the African Diaspora of the New World, the dominant genetic contribution remains African. But, you can derive from the cultural identity of these groups which European populations amalgamated with them genetically, or inversely, you can make reasonable inferences of cultural state simply by observing the character of European ancestry on a population level! In other words, robust correlations between genes and culture can occur, even if the two do not exhibit the same weight from population to population (i.e., black Americans are mostly European culturally, but genetically mostly African). Ruminations on these issues are important when taking into account the latest findings of genome bloggers. As has been noted, trying to reconstruct past populations as combinations of present populations is going to miss something. That is, there is no guarantee that the present captures all the variation of the past. Additionally, the analytic tools must always be used judiciously. The model based clustering algorithms return "ancestral components," but only in some specific cases do they truly return proportions which align with admixtures between real concrete populations. Rather, you get back patterns of variation and relationships which are visualized for you, but must not be taken in a literal fashion. In regards to the hypothesis free method of principal component analysis, the independent dimensions of variation are still conditional upon the variation you put into the algorithm. By this, I mean that if you overload the data set with one particular population, then the largest explanatory dimensions of variation are going to be dictated by that population.

With all due caution entered into the record, it does seem that some "mysteries" are going to solved in the near future, at least to a first approximation. As far back as the mid-2000s it was apparent in the early STRUCTURE analyses using microsatellites that some European populations, the French Basque and Sardinians, lacked an affinity which spanned Central Eurasia, and reached deep into South Asia. Adding Finns into the mix seems to have confirmed that they too seem to lack it. The main "twist" on this picture though seems to be that Lithuanians and Latvians also have very little of it too. But there is also earlier uniparental work which suggests marginal differences between Indo-European and Finnic populations in the northeast Baltic. To-be-published work on Indian genetics does seem to imply that there was significant in-migration of Indo-European populations into the subcontinent within the last 4,000 years. And an earlier publication by the same group confirmed the old intuition that South Asians seem to exhibit significant, though not total, affinity to West Eurasians. That affinity being a product of recent descent in part from West Eurasians, via a hybridization event. But perhaps the biggest general takeaway, as opposed to specific inference, is that the humans who inhabit the southern and western reaches of "Rimland" are syntheses which emerged in the last 10,000 years. Southeast Asians are derived from several pulses of farmers from the fringes of what became southern China, but absorbing an ancient earlier substratum. South Asians are the product of a fusion of West Eurasians and the western elements of that same substratum, a Paleolithic population of South Eurasians who once spanned the Indus to the South China Sea, and extant today in "pure" form only in the Andaman Islands and the Negritos of the Malay peninsula.* At minimum Europe looks be a synthesis of least two pulses from outside or the margins of Europe (i.e., Southwest Asia and West Central Eurasia), as well as possible expansions from within the subcontinent, not to mention assimilation of the deeply rooted populations by an within the newcomers. As far as the Middle East and North Africa, I think there is enough circumstantial evidence that there too there have been significant population movements since the Neolithic. This is perhaps why many of the earlier genetic analyses using particular Southwest Asian populations as "references" for the original Middle Eastern farmers who moved into Europe may have yielded misleading results. The major lacunae in my model is in East Asia, in particular China. The Han Chinese do not seem to have much West Eurasian admixture at all (in contrast to the Mongolians and the Hui). Additionally, the data do seem to point to some amalgamation of non-Han populations in southern China. But overall I don't have a good gasp of how the landscape of East Asia came to be be. Both archaeology and genetics point us to the likelihood that phenotype which we associate with East Asians became overwhelmingly dominant only recently. It is here that the strongest instance of the "first farmers" hypothesis may be found. Sometimes it is good to live in interesting times. * The Negritos of Malaya are not pure from the samples I have seen. And those of the Philippines seem to be very distinct the more westerly South Eurasians.

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