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Genetic Variation

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mtDNA haplogroup G1a2

The pith: In this post I examine the most recent results from 23andMe for my family in the context of familial and regional (Bengal) history. I also use these results to offer up a framework for the ethnognesis of the eastern Bengali people within the last 1,000 years, and their relationship to other South Asian and Southeast Asian populations.

Since I received my 23andMe results last May I’ve been blogging about it a fair amount. In a recent post I inferred that perhaps I had a recent ancestor who was an ethnic Burman or some related group. My reasoning was that this explained a pattern of elevated matches on chromosomal segments with populations from southwest China in the HGDP data set. But now we have more than my genome to go on. This week I got the first V3 chip results from a sibling. And finally, yesterday the results from my parents came in. One thing that I immediately found interesting was my father’s mtDNA haplogroup assignment, G1a2. This came from his maternal grandmother, and as you can see it has a distribution which is mostly outside of South Asia. In case you care, I asked my father her background, and like my patrilineage she was a “Khan,” though an unrelated one (“Khan” is just an honorific). I received these results before the total genome assessment, and so initially assumed this confirmed my hunch that my father had some unknown recent ancestry of “eastern” provenance. But it turns out my hunch is probably wrong. In fact, my parents have about the same “eastern” proportion, with my mother slightly more! My expectation was that perhaps my mother would be around 25-30% “Asian,” and my father above 50%. The reality turns out that my father is 38%, and my mother 40%.


Image credit: f_mafra

Below are the “Ancestry Paintings” generated by 23andMe for my family (so far). What you see are the 22 non-sex chromosomes, which have two copies each, and assignments to “Asian,” “European,” and “African,” ancestry groups. The reference populations to generate these assignments come from the HapMap, the northern European sample of white Americans from Utah, Chinese from Beijing, Japanese from Tokyo, and ethnic Yoruba from Nigeria. What the assignment to one of these classes denotes is that that region of the genome is closest to that category in identity. It does not imply that your recent ancestry is European or Asian (African is probably a different matter, but there are many complaints about the results for African Americans and East Africans in the 23andMe forums). This caveat is especially important for South Asians, because we generally find that we’re ~75% European and ~25% Asian. All that means is that though most of our genetic affinity is with Europeans, a smaller fraction seems to resemble Asians more. Via “gene sharing” on 23andMe I can see that the Asian fraction varies from ~35% in South India and Sri Lanka, to ~10% in Pakistan and Punjab. This is not because South Indians have more East Asian ancestry than Punjabis. Rather, to a great extent the South Asian genome can be decomposed into two ancestral elements, one with a distant, but closer, affinity to populations of eastern Eurasia, and one with a close affinity to populations of western Eurasia. What some have termed “Ancient South Indians” (ASI) and “Ancient North Indians” (ANI). ASI ancestry, which is probably just a touch under 50% in South Asians overall, seems to shake out then as somewhat more Asian than European.* The fraction of ASI increases as one moves south and east in South Asia (and as one moves down the caste status ladder).


[zenphotopress album=249 sort=sorder_order number=4]


First, I want to note that I’ll be using abbreviations for my family members now and then (this applies to future posts). My father will be RF, my mother will be RM, and my siblings will be RS, with a number to denote which sibling. So currently we have RS1. As you can see in a gestalt sense we resemble each other a great deal as a family. We’re about 40% Asian, and 60% European. The extent of fragmentation indicates that we’re not that recent of an admixture; otherwise, the Asian and European fragments would cluster on one strand or the other. Some have suggested that my mother does exhibit less fragmentation. A hypothesis for why this may be is that her maternal grandfather was reputedly from a family of Middle Eastern origin who had resettled in South Asia, first in Delhi, and later in southeast Bengal (specifically, the district of Noakhali). Since he presumably would hardly have had any Asian ancestry according to 23andMe’s algorithm the homologs inherited from him would be overwhelmingly European, with only one generation of recombination intervening.

To assess probabilities of the plausibility of various hypotheses to explain the pattern of the results you need all the non-genomic information. Above is a map of British India. I’ve pointed to the region of Bengal from which my family comes. Of my great-grandparents 7 out of 8 were born in Comilla (which is actually a greater expanse to the southeast of Dhaka than the current Bangladeshi administrative division). 1 grandparent was born in Noakhali, which is just to the southeast of Comilla. 4 out of 8 great-grandparents were born within 5 miles of the town of Chandpur (RF’s grandparents). 3 out of 4 great-grandparents were born within 5 miles of the village of Homna (RM’s grandparents). These two locations are about 30 miles from each other as the crow flies, though transport between them would have been by water in an earlier era (Homna is on the Meghna river, which is actually a more substantial body of water than the Ganges by the time the latter reaches Bangaldesh). This region is bounded on the west by the Padma river, which narrows at Chandpur to about 2 miles in width (average depth ~1,000 feet). To the east is the Indian state of Tripura. This is a relatively porous border, defined on the map, not imposed by geography. You can see that in some regions the Bangaldesh-India border here in the east actually bisects rice paddies.


Tripuri children

Today Tripura state is majority ethnic Bengali due to mass migration of Hindus from what was East Bengal during the 20th century (and later East Pakistan, and now Bangladesh). But its indigenous people are the Tripuri, a tribe whose native language is clearly Tibeto-Burman, and physical type points to their connection with populations to the north and east. At the same time, ~90% of the Tripuri are Hindus, and during the period of Islamic rule in South Asia the rajahs of Tripura styled themselves defenders of Hindu civilization (just as the Tibeto-Burman Ahoms of Assam did). As such, linguistically and genetically the native people of Tripura exhibit a sharp contrast to the Indo-Aryan peoples of the Gangetic Plain, of whom the Bengalis are the easternmost representatives along with the Assamese. But, they have also long been part of the South Asian cultural scene, and can not longer be viewed as purely intrusive (their oral history indicates that they arrived before the Muslims, for one).

Finally, in regards to the detailed backgrounds of my 8 great-grandparents, 2 were of the Khan class. 1 was from a family of Hindu Thakurs who were recently converted to Islam. Another was of the family name Sarkar. 1 was likely from a family of Middle Eastern transplants to South Asia, at least in part. The 4 remaining great-grandparents were Bengali Muslims, with no particular background information beyond that known by my parents.

I gave you all this because genetic variation is strongly conditioned upon geographical and cultural parameters. Water barriers seem to have been particular efficacious in the pre-modern period dividing people culturally and genetically (though ironically water was also a precondition for any bulk trade). Language is also another major parameter of difference. And finally, there is religion. In the last section I would not be surprised if 300 years ago the majority of my ancestors in that generation were Hindus; there is some fluidity in this obviously. I provide the data on radius of place of birth because we know from European results that even villages exhibit genetic clustering. This is mitigated in my family because my father has a diverse background among his grandparents as far as community goes, while my mother has a grandparent who was from a different district, and to a great extent a different ethnic group in biological terms.

When I initially saw that I was ~40% Asian I was little taken aback by the high proportion (remember, the average South Asian is about 25% Asian), but there were two parsimonious explanations, a) I had a lot of ASI, b) I had ancestry which did not seem South Asian as such, but was genuinely from East Asia. To ascertain whether it was the former I began proactively gene sharing with a wide range of South Asians on 23andMe. After dozens of individuals it became clear that I was outside of the normal interval of variation. I was more Asian than individuals from South India or Sri Lanka. Additionally, even these individuals tended to be genetically closest to Central South/Asians in the HGDP data set. I was closest to East Asians. Also, on the two dimensional PCA projected onto Central South/Asians I was definitely outside of the cluster of all the other South Asians. Finally, I did find someone who broke the magic 35% barrier of Asian…and that individual was a Bangladeshi, at 38%. And, like me, he was closer to East Asians on the basic “Global Similarity” match. He also carried a Y chromosomal lineage which was rare in South Asia and common among the Hmong. Finally, when Dienekes started his Dodecad Ancestry Project it was clear that about ~15% of my ancestry clustered with an element which was not South Asian, but East Asian. If one removes this fraction, I would be about 70% European and 30% Asian, absolutely within the normal range for someone with ancestry to the east or south of the subcontinent.

If you’ve read up to this point, you may be wondering how it is that my father is 38% Asian and my mother is 40% Asian, and I’m 43% Asian. After all, shouldn’t I be an average between the two? Actually, on the PCA scatter plot I am (along with my sibling) exactly between my parents (you can’t see the offspring because the flags are just too large). So why the difference? First, remember that the PCA is projecting you onto a two dimensional axis where the x and y represent the two biggest components of variance in the data set. In other words, it’s yanking out the subset of genetic variance which really stands out in terms of between population difference. This is how an individual who is a first generation Eurasian can be so far from their parents on this plot, but still exhibit a great deal of identity by state in terms of total genome; there’s a lot of variation that the two dimensional plot does not capture (e.g., private variants to family lineages). The Ancestry Painting estimates are different; they’re looking across the whole genome and making assessments for each region as to its genetic affinity between the three reference populations. So to repeat, you have over 50 reference populations vs. 3, and, you have a small proportion of the total genetic variation, vs. the whole genome. Both methods are reporting real and valid results, but they’re somewhat different.

So there are two very simple and methodological explanations for the discrepancy above which I can think of. I’m on V2, while my parents and sibling are on V3. I know this has made a difference in other measurements. Additionally, there’s clearly some “noise” within this algorithm, resulting in people with trace African or Asian ancestry which isn’t real, even if you take into account the kludgey nature of the reference populations. But let’s take the results at face value. With the ancestry painting, recall how the European and Asian components were chunky across the genome? Both of my parents received half their genomes from their parents. My own chromosomes are a mosaic of those of my grandparents. Some of the original linkage between genomic regions because of their physical location on the same strand have been broke apart by recombination in the two generations downstream from my grandparents. Concretely, two instances of meiosis which produced sex cells. Therefore, some of the associations of alleles present in my grandparents have been transformed within me. But even without recombination, it is clear that one homologous chromosome could be more European or Asian than the total genome average. Because only one of these is passed to any given offspring, there is going to be variance from sibling to sibling. Genetics is not a pure blending process. That may be why I am 43% Asian while RS1 is 40% Asian. We’re both sampling from our parents genes, and there’s going be variance in that process (on the chromosomal level you have 22 autosomal draws from each parent where each draw has two outcomes).

An interesting implication of this is that the grandchildren of a multiracial couple will exhibit variance in their ancestral quanta from major racial groups. This is one reason why it is a fallacy to presume that intermarriage will result in the washing away of biological diversity. And processes such as assortative mating could even presumably extract out “pure” individuals from an originally admixed random-mating population.

With all that said, I now believe that with an N = 3 from eastern Bengal that I am not an exception with recent Southeast Asian ancestry, but rather eastern Bengal is part of the gene frequency cline between South Asia and Southeast Asia, and as such has a substantial fraction of eastern ancestry. Zack has my parents’ data, so once the results come back from the first runs of HAP I believe that he will see the same pattern of substantial non-South Asian ancestry in them that Dienekes found in me. The cline here is still sharp. The average Bangladeshi is probably interchangeable with just 10-20% with the average Burmese when it comes to proportions in inference of ancestral quanta algorithms. (remember that the Burmese probably have a small South Asian component too). In contrast, the average Bangladeshi probably can be interchangeable at 80-90% with a resident of Bihar (the closest match in total SNP comparison in 23andMe that I’m sharing with is a Bihar, not the two other ethnic Bengalis). This is clearly a function of geography, the north-south ranges in Burma seal it off from South Asia. In contrast, there are open plains from northern Bangladesh to Bihar. In some ways Burma has more cultural affinity and connection with peninsular South Asia because of the ease of maritime travel. The prevalence of Theravada Buddhism in Burma is a testament to the association of the lower Irrawaddy region with Sri Lanka.


Back to Bangladesh. One aspect of the Indian subcontinent in terms of religious demography is that the heart of Indo-Islam, the Delhi area, never had a Muslim majority. Rather, Muslims were a majority along the northwestern and northeastern fringe (along with a few other districts, such as northern Kerala). The predominance of Islam on the northwest isn’t that surprising, as that region borders upon the Dar-al-Islam proper. But what about Bengal? In the late 19th century the British were apparently surprised that in the united Bengal (which includes roughly the modern state of West Bengal in India, and Bangladesh) had a Muslim majority. Because of differential birth rates and conversion (this second includes sections of my family as I note above) about 2 out of 3 ethnic Bengalis alive today are Muslim, with the balance being Hindu. Bangladesh is estimated to be 90% Muslim, while West Bengal is 25% Muslim. Even today after generations of Hindu outmigration one pattern within Bangladesh is the relative concentration of Hindus to the west and north (also, Hindus in Bangladesh tend to be urban). The “buckle” of the “Koran belt” in Bangladesh is actually the district of Noakhali, on the southeast fringe of Bengal. My mother’s maternal grandfather, who came from a lineage of pirs who had originally settled in the Muslim heartland in Delhi, was from Noakhali. It is apparently said that in Noakhali even the Hindus know proper Islamic forms!**

An explanation for this pattern is that the religious influence and power of Hindu elites declined as a direct function of distance from the regions of West Bengal, which were closer to the core Aryavarata, and had traditionally been the locus of power of Hindu dynasties before the rise of Islam. Additionally, Bengal was the last region of the mainland subcontinent with a robust Buddhist society during the flowering of the Pala Empire around the year 1000. It is therefore suggested that many Bengali Muslims were converted directly from Buddhism, not Hinduism (there remains even today a small minority of ethnic Bengali Buddhists, who carry the surname “Baura.” This is in distinction to the descendants of Tibeto-Burman people who now speak Bengali, but retain a tribal identity and Theravada Buddhist religion). Also, it may be that eastern Bengal was populated mostly by animist tribes before the arrival of Muslims, and just as European colonial powers were more successful in Asia at spreading their religion among marginalized people (e.g., tribal peoples in northeast India and Southeast Asia are often Christian), so Islam found purchase among those outside of the Hindu caste system.

These models are broadly persuasive to me. But, I still am suspicious that there was such a strong disjunction in the depth of Hindu institutions in western vs. eastern Bengal; after all, the kings of Tripura to the east were Hindu when Islam was new in South Asia. If being tribal and marginal to the core Hindu civilization was one of the grounds for susceptibility to Islam it is peculiar that it is precisely many tribal people in modern Bangladesh who are not Muslim. Indeed, the Tibeto-Burman populations nearer to Indian groups in eastern South Asia are Hindu or Buddhist, not Muslim (those further in the hinterlands were not integrated into any South Asian religion, but converted to Christianity by Western missionaries within the last century).

Instead, I find the model espoused in The Rise of Islam and the Bengal Frontier, 1204-1760 broadly plausible as a complement, or even substitute, to the above hypotheses. Additionally, it has the utility of making sense of the genetic data which I have presented here so far. The author argues that eastern Bengal, most of Bangladesh, was very lightly populated before the conquest of Bengal by Muslims in the 13th century. During the modern era the western region of Bengal, in India, has tended to have issues with the moribund nature of many of the water courses. But one thousand years ago this region was more active in terms of sedimentation, while eastern Bengal was a wilderness. Over the centuries there has been a shift of large rivers to the east, opening up that area to cultivation because of improved transport. Additionally, the arrival of Muslims also resulted in the spread of new techniques of land clearing and settlement. The rough model is that eastern Bengal is in fact a relatively newly settled territory in terms of its current demographic density. As the clearance and settlement operations were performed by Muslim elites, many of the peasants who settled these lands were either Muslim, or more likely, adopted the religion of their landlords. Because of the virgin nature of the territory these original settlers entered into a phase of massive demographic expansion, to the point where eastern Bengal (Bangladesh) is now today twice as populous as western Bengal (West Bengal). The key here is that there need not be a massive conversion of the enormous masses of marginal animist, Hindu or Buddhist peasants. Rather, all one needs is a modest number of converted Bengali peasants to enter into exponential population growth until the land is “filled.” (interestingly, one sees similar patterns between descendant populations in both the USA and among Koreans. The religions in the “core” homelands are very different in constitution from the Diaspora)

I find this persuasive for two major reasons. First, Peter Bellwood’s First Farmers documents the difficulties of populations which have not been engaged in intensive farming to switch to that modality. At least back to the Mughal period Bengal was a densely settled land from which one could extract massive rents simply due to aggregate productivity. Today a united Bengal would have a population of 240 million, making it the fourth most populous nation in the world, below the USA, and just above Indonesia. In hindsight I find it less likely that the peasants of eastern Bengal descend from tribal peoples who had been practicing extensive agriculture, but were introduced to new techniques, than that western populations already habituated to the grinding expectations of intensive farming colonized the “empty” lands (in fact, Bengali peasants migrate to Assam in part because of the perception of land surplus there, even though Assam has 30 million inhabitants). But this initial phase of colonization would entail relatively few peasants, and probably exhibit some male bias. Therefore, this can to explain a substantial fraction of the eastern ancestry among Bangladeshis, as in the first generations the Bengali peasants did assimilate the native tribal peoples of the region, whether it to be the Munda Santhals or Tibeto-Burman relatives of the Tripura. With the massive numbers of ethnic Bengalis in comparison to Tibeto-Burman groups it seems one would need a great deal of gene flow in any model which posited that exchange between these two groups over long periods of time explain the high fractions that one finds of non-South Asian ancestry. In all of India there are only 10 million speakers of Tibeto-Burman languages, vs. the 240 million speakers of Bengali alone in the Indian subcontinent.

Where does this leave us? From what I gather you’ll probably not make it into the first round of results for HAP, but if you have 23andMe results and haven’t it sent it to Zack, and want to learn more about the historical genetics of the Indian subcontinent, you can still get involved! With my parents Zack now has an N = 2 of Bengalis. It would be nice to get more. We still need samples from North-Central India. The number of Punjabis is in the 5-10 range, Tamils is around 5. Enough to make inferences, but certainly not robust enough to bet the house on. In the near future I’ll get results from my other siblings, and I’ve decided to “upgrade” to the V3 chip. Once that comes in I’ll phase some of the results, and probably start comparing myself to my siblings, “phase” the results, etc.

* Native Americans, descendants of pre-Columbian Americans, have the inverted results from South Asians, mostly Asian with a European minority. This is not just due to recent European admixture. Rather, though Amerindians have affinities to East Asians, the two groups have been distinct for at least 10,000 years, and probably considerably longer.

** Also, some have stated that the people of Noakhali are sly and cunning, adept at following the letter of the law, but not the spirit. I only know this because when I was young one of my father’s friends, also from Bangladesh, complained that a mutual acquaintance from Noakhali who made much of his piety (he put his wife in purdah when she arrived from Bangladesh) requested that someone else purchase a pornographic magazine for him. His reasoning was that he did not want to be seen purchasing the magazine. It was a sin to purchase such an item for a good Muslim. Later my father and his friend (who was from northern Bangladesh for what it’s worth) commiserated that such was the way of the people of Noakhali, amongst whom you have to have your wits about you lest they exploit some angle for their own self-interest. The pious-porn-non-purchaser was notorious for being a non or late payer of rent when he was a lodger with other Bangladeshis, always emphasizing his religious piety as surety of final payment of the debt. He also eventually finagled a loophole in the immigration law of the time, obtaining green card with relative ease and no necessity of sponsorship. The proper connotation of how people from Noakhali are is probably captured by the American English word slick.

(Republished from Discover/GNXP by permission of author or representative)
 
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The following passage is from the epilogue of The Real Eve: Modern Man’s Journey Out of Africa by Stephen Oppenheimer:

In this book I have offered a synthesis of genetic and other evidence. Everything points to a single southern exodus from Eritrea to the Yemen, and to all the non-African male and female gene lines having arisen from their respective single out-of-Africa founder lines in South Asian (or at least near the southern exit). I regard the genetic logic for this synthesis as a solid foundation, and I have based the rest of my reconstruction of the human diaspora upon it. Obviously, the ‘choice’ of starting point (mine or theirs) determined all the subsequent routes our ancestors and cousins took. Tracing the onward trails is only possible as a result of marked specificity in regional distribution of the genetic branches The geographic clarity of both male and female gene trees is a big departure from the fuzzy inter-regional picture shown by older genetic studies. The degree of segregation of lines into different countries and continents is in itself good evidence that once they got to their chosen new homes, the pioneers generally stayed put, at least until the Last Glacial maximum forced some of them to move. This conservative aspect of our genetic prehistory also provides a partial explanation for the fact that when we look at a person, we can usually tell, to the continent, where their immediate ancestors came from, and underlies differences that some of us still call ‘race.’

Oppenheimer wrote the above in the early aughts, as his book was published in 2003. Much of this is generally in line with the ‘orthodoxy’ of the day. I believe that Oppenheimer’s assertion that there was one southern migration out of Africa by anatomically modern humans has gained some advantage over the alternative model of two routes, northern and southern, over the past ten years (Spencer Wells’ The Journey of Man sketches out the two wave model). Other assertions and assumptions have not stood the test of time. In particular, I would contend that generally the ‘conservative aspect of our genetic prehistory’ can no longer be taken for granted. Specifically, it seems likely now that much occurred after the Ice Age and during the Neolithic.


420px-AGMA_HérodoteThe false inferences of the early aughts were due to two primary problems. First, they relied heavily on the powerful new techniques of extraction and analysis of uniparental ineages; the male and female direct line of descent. Concretely, mtDNA and the nonrecombintant portion of the Y chromosome. The lack of recombination allows for relatively easy reconstruction of phylogenies assuming a coalescent model. Second, the inferences attempt to make connections between the patterns of variation in modern populations, and what one may infer about the past from those patterns. Obviously constructing a phylogeny, or plotting haplogroup frequencies as a function of geography, is rather straightforward science. But using these results to generate inferences of the past is often more of an art than a science, and implicit assumptions lurk behind the causal chains. Consider for example the utilization of modern Anatolian (i.e., Turkish) genetic variation as a reference for the expansion into Europe of Neolithic farmers from the Near East. This of course presumes that modern Anatolians are a good proxy for ancient Anatolians. There are various suggestive reasons for why this is a plausible assumption, but assemble enough plausible assumptions, and rely on their joint likelihood, and you construct a very rickety machinery of possibility.

In early 2007 I began to have serious doubts about the orthodoxy of genetic conservatism. The primary trigger was the story of the Etruscans. Here is the crux of the issue: there are two models for the origins of the Etruscans, first, that they were the pre-Indo-European autochthons of Italy, or, that they were the migrants from the eastern Mediterranean, in particular Anatolia. The second may seem an outlandish hypothesis, but there were several tendrils of evidence to support it. But perhaps the ‘support’ which weighed most against it is that the fact that the Anatolian model has an ancient source, the Greek historian Herodotus. I should perhaps put historian in quotes as well, because Herodotus is often viewed more as a repeater of myths, and derided by some as the ‘father of lies’ (in this he stands in sharp contrast to contemporary perceptions of the ‘modern’ Thucydides, though revisionists have begun to challenge this narrative). In contrast, the model that Etruscans are indigenous to Italy, and that their ‘exotic’ foreign traits were simply acquired through trade and cultural diffusion, dovetailed well with the post-World War II ‘pots not peoples’ paradigm. That cultural change was ubiquitous, while at the same time populations were immobile. It was boring, prosaic, and conservative, and so an ideal null hypothesis.

But here it turns out that Herodotus was right, and archaeologists were wrong. Genetic analysis of modern Tuscans from isolated villages shows that some are surprisingly closely related to extant eastern Mediterranean lineages. Genetic analysis of Tuscan cattle showed that they were surprisingly closely related to extant eastern Mediterranean lineages of cattle. Finally, extraction of ancient Etruscan DNA showed that they were closely related to extant eastern Mediterranean lineages. The overlap was often with Anatolia, and combined with fragmentary linguistic and archaeological data, the evidence clearly points to an exogenous origin for the Etruscans. The boring null hypothesis was wrong. After these genetic stories gained prominence I went and reread recent archaeological texts on the Etruscans, and there were many models which showed exactly how Etruscan cultural uniqueness derived back to prehistoric Italy. It seems in hindsight that the prior assumption served as an interpretative filter, and people saw patterns that they were primed to see based on what they ‘knew’ to be the history of prehistoric and early Iron Age Tuscany.

Of course to refute the primacy of Oppenheimer’s conservative model of genetics one has to offer more examples than that of the Etruscans, and in particular, examples which are of greater scope and weight. I believe those examples exist. In the early aughts based on the mtDNA evidence the likelihood was that South Asian genetic variation is by and large a product of changes wrought upon the basic elements extant in the region around the end of the last Ice Age. The Y chromosomal data was more confused, though it did imply a closer relationship to groups in western Eurasia. But based on the mtDNA Oppenheimer posited a model whereby India was the mother of all non-Africans, that is, all non-African lineages derived from roots within the Indian subcontinent before the Last Glacial Maximum. This is at sharp variance with colonialist narratives of an Aryan invasion of the subcontinent, and the subjugation of the natives by quasi-European overlords, who are the ancestors of the moder upper castes. The charged ideological import of this model is transparently obvious.

Unfortunately the reality is likely more complex. I suspect that some form of Oppenheimer’s model is correct, insofar as South Asia was likely an important way station for modern humans as they left Africa, and pushed into other regions of Eurasia, on to Australasia and the New World. This interpretation does gain support from mtDNA, the direct maternal lineage. But a new analysis of South Asian genetic variation using a substantial proportion of the autosomal genome implies in fact that South Asians are possibly descendants of an ancient hybridization event between a native population with deep roots in the subcontinent, and a quasi-European population which was exogenous to the subcontinent.* Genetically the quasi-European population is quite close to northern Europeans, similar to the genetic distance between modern Finns and Italians, not trivial, but far closer than that between modern South Asians and Europeans. Was this the ancient Aryan invasion? I remain skeptical of this particular detail for various reasons, as I suspect that the history of the Indian subcontinent is in fact even more complex than has been assumed before (I think it is more likely that the quasi-Europeans came before the Indo-Aryans, who arrived late, and had a stronger cultural than genetic influence).

Finally, there is another region of the world where it seems likely that the old orthodoxies of genetic conservatism will be overthrown. That region is Europe. The scientific orthodoxy of deep time continuity is strong enough that it has percolated into the public consciousness, the leader of the British National Party even referred to the deep roots of white British in demarcating who he believed ‘indigenous people’ of the Isles were. But newer data is more supportive of the hypothesis that in fact Neolithic farmers who arrived from elsewhere are the likely ancestors of most Europeans, not the hunter-gatherers who remained after the Ice Age. Extraction of ancient DNA has yielded a set of results which simply are not explicable assuming the older models of genetic continuity, which were based on inferences made from modern population variation. If I had to hazard a guess, I would have some, though not high, confidence in the following story. First, the indigenous hunter-gatherers are assimilated or marginalized by waves of Neolithic farmers pushing out from the eastern Mediterranean. The demographic expansion does not necessarily sweep outward along a southeast-northwest axis, rather, it follows the Mediterranean and Atlantic fringes, as well as along river systems in the interior. Its impact is weakest in the northeast of Europe, where Middle Eastern crops are least suitable, and the natives have the most time to absorb the cultural toolkit of the newcomers so as to resist their advance. Second, and far later, there was another wave pushing out from the region of the Ukraine to the Volga, likely the ancestors of the Indo-Europeans. Tentatively I would contend that these were the carriers of the Kurgan culture, and also brought the allele for lactase persistence. Again, for ecological reasons the populations of the northeast Baltic and into the forests of northern Russia were most insulated from this push (and non-Indo-European languages persisted in Iberia down to Roman times, and specifically in the Basque-country down to modern times, though I suspect this is a function of distance). So modern European populations may be assumed to be tri-hybrid, first a synthesis of Middle Eastern farmers overlain upon the Paleolithic substrate, and second a synthesis of Indo-Europeans from the east overlain upon pre-Indo-European substrate. Unlike the case of India I suspect teasing out these patterns in modern populations is more difficult because the genetic distance between the three ancestral populations is far smaller than between the indigenous peoples of India before the quasi-Europeans arrived.

This leaves much of the world untouched by my speculations, but I believe showing that the genetically conservative null hypothesis is now in serious doubt in South Asia and Europe is sufficient to knock it from being a necessarily default assumption through which we must filter our interpretations. I do not believe that the reordering of human variation and the welter of population movement after the Ice Age was equivalent in effect to the Out of Africa migration, but I do believe that it was important enough to make the world of 2000 BCE very different from that of 15000 BCE in regards to genetic variation. In some cases, such as Central Asia from the Caspian to the Taklamakan the world of 2000 CE is fundamentally different from the world of 0 CE.

I will then end with a prediction, one in which I do not have much confidence, but which may no longer be wrong on the face of it with these new data in mind. Here is a passage from page 7 of Jared Diamond’s Guns, Germs, and Steel:

Initially, archaeologists considered the possibility that the colonization of Australia/New Guinea was achieved accidentally by just a few people swept to sea while fishing on a raft near an Indonesian island. In an extreme scenario the first settlers are pictured as having consisted of a single pregnant young woman carrying a male fetus…..

Let me stipulate that Diamond seems skeptical of the extreme model, but it illustrates the consensus that Australian Aboriginal populations are descended from the first settlers. That is, the modern populations of indigenous Australians are the direct descendants of those who swept Out of Africa along the fringe of the Indian ocean, through Southeast Asia, and arrived in Australia (more specifically, Sahul), on the order of 40 to 60 thousand years ago. From what genetic data I have seen this may be true. But I do not know of any extractions of ancient DNA, and it seems to me that the analysis of the phylogenetics of Australian Aboriginals is relatively sketchy. Therefore, I will suggest that within the last 10,000 years there has been a major new migration of people into Australia, and the modern range of genetic variation of Australian Aboriginals is significantly different from that of the populations of the Ice Age. I suggest this primarily because the dingo arrived within the last 10,000 years, more likely as recently as 4,000 years ago. With the expansion of the utility of ancient DNA extraction and analysis this question may be answered in the near future. I would still bet I’m wrong with the hypothesis I just offered, but I’m far less sure than I would have been 2 years ago.

Note: This post emerged from a conversation I had with Kevin Zelnio and Dave Munger.

* I say ‘quasi-European’ because the population may have origins outside of the boundaries of modern Europe at the Urals. Perhaps in western Siberia. Additionally, the idea of ‘Europe’ is relatively new, and exhibits little ancient cultural coherency.

Image source: Wikipedia

(Republished from Discover/GNXP by permission of author or representative)
 
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Razib Khan
About Razib Khan

"I have degrees in biology and biochemistry, a passion for genetics, history, and philosophy, and shrimp is my favorite food. If you want to know more, see the links at http://www.razib.com"