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Steppe migration to India was between 3500-4000 years ago: David Reich

Two recent papers-
The Formation of Human Populations in South and Central Asia ( Vageesh Narasimhan et al) and
An Ancient Harappan Genome lacks Ancestry from Steppe Pastoralists or Iranian Farmers (Vasant Shinde et al) – have sparked different interpretations on what they reveal about the genetics of ancient Indians. The papers were authored by a team of geneticists from Harvard Medical School working with Indian scientists. They studied ancient DNA from sites in Europe, Central Asia and South Asia, including a sample from the Indus Valley Civilisation site of Rakhigarhi, before drawing their conclusions. One of which was the contested claim that descendants of pastoralists from Eurasian steppes migrated to the Indian subcontinent in the first half of the second millenium BCE, “almost certainly” bringing Indo-European languages. Their studies also claim the Steppe migrants eventually contributed 0-30% of the genes of groups living in India today. In an email interview, Harvard geneticist Prof David Reich breaks down the findings. Excerpts.

1) What are the big takeaways from the 2 recent studies you co-authored – Vasant Shinde et al 2019, and Narasimhan et al 2019?

1. At least some of the people of the ancient people of the Indus Valley Civilization were a mixture of south/southeast Asian-related hunter gatherers and Iranian-related hunter-gatherers. I say Iranian-related because their ancestors may actually have lived in South Asia rather than the Iranian plateau for many thousands of years before the time of the IVC. We just don’t know yet where they lived because of lack of ancient DNA from the relevant times and places.

2. A population like the people we call the “Indus Valley Cline” – consisting of a Harappan individual from the site of Rakhigarhi, plus 11 individuals who were buried at the sites of Gonur in Turkmenistan and Shahr-i-Sokhta in Iran and as likely migrants from the Indus Valley Civilization — is the primary source population of both North and South India.

3. Some time in the first half of the second millennium BCE, descendants of Steppe pastoralists entered South Asia from the north, eventually contributing 0-30% of the genes of groups living today (varying depending on the present-day group), and also almost certainly bringing Indo-European languages. There is no evidence that the actual people who brought these genes to South Asia were pastoralists by occupation – their ancestors were pastoralists.

2) What more can you tell us from your studies about this Steppe migration? Mostly male? Was it a significant number – so as to make such drastic changes in the gene pool of such a large area?

It is entirely plausible, and in my opinion even likely, that the movement of people bringing this ancestry to the Indian subcontinent was not sex-biased, and involved both males and females. However, the process by which people carrying this ancestry mixed with people with ancestry like the individual from Rakhigarhi, was a sex-biased one, whereby most of the Steppe ancestry to mixed population was contributed by males. Note that according to our paper, in the Swat Valley, Steppe ancestry mixes into South Asia in a sex-biased way but in the REVERSE pattern, that is, most of the Steppe ancestry is coming from females.

“In the Late Bronze Age and Iron Age individuals of the Swat Valley, we detect a significantly lower proportion of Steppe admixture on the Y chromosome (only 5% of the 44 Y chromosomes of the R1a-Z93 subtype that occurs at 100% frequency in the Central_Steppe_MLBA males) compared with ~20% on the autosomes (Z = −3.9 for a deficiency
from males under the simplifying assumption that all the Y chromosomes are unrelated to each other since admixture and thus are statistically independent), documenting how Steppe ancestry was incorporated into these groups largely through females (Fig. 4). However, sex bias varied in different parts of South Asia, as in present-day South Asians we observe a reverse pattern of excess Central_Steppe_MLBA–related ancestry on the Y chromosome compared with the autosomes (Z = 2.7 for an excess from males).”

These differences could be explained by a non-sex-biased migration from Central Asia into South Asia of people carrying Steppe ancestry, followed (at some point later) by preferential incorporation of females from this population into the Swat Valley peoples, and preferential incorporation of males from this population into the ancestors of most present-day South Asians.

3) When did they reach the Indian subcontinent?

We know this rather precisely from our analysis: the first half of the second millennium BCE.

4) Was this the ‘collision’ that formed present day Indian populations?

This is one of at least four major collisions we now know about:

a. The mixture of Iranian-related ancestry and South/Southeast Asian hunter-gatherer-related ancestry that formed the Indus Valley Cline on average 7400-5700 years ago.

b. The mixture of people on the Indus Valley Cline with people from the southeast carrying relatively more South/Southeast Asian hunter-gatherer-related ancestry after the decline of the mature Indus Valley Civilization around 4000-3000 years ago

c. The mixture of people on the Indus Valley Cline with people from the north carrying Steppe ancestry after the decline of the mature Indus Valley Civilization around 4000-2000 years ago

d. The mixture of these two mixed populations (b and c)

There are surely more collisions yet to be discovered!

5) And what was the route to the Indian subcontinent? From the Yamnaya culture in Eastern Europe to the Central steppes (BMAC) and then to South Asia?

The exact routes are currently unknown. Almost certainly it started in far eastern Europe more than 5000 years ago (with the Yamnaya or their close relatives), then 4500-4000 years ago moved possibly west to east-central Europe (but this westward-before-eastward deviation is not certain), and then moved far to the east across the Urals to the central Steppe (Kazakhstan) and Central Asia (places like Turkmenistan) before moving into South Asia 4000-3500 years ago.

It is likely, based on our analysis, that the population that contributed genetic material to South Asia was (roughly) ~60% Yamnaya, ~30% European farmer-like ancestry, and ~10% Central Steppe hunter-gatherer ancestry.

6) What difference, according to your study, did it make to the Indus Valley Civilisation gene pool ?

This ancestry admixed with people like those we sequenced from the Indus Valley Civilization, eventually contributing 0-30% Steppe-derived ancestry to present-day populations.

7) Was it the contrast in the genetic profiles of later Indian (South Asian) people and that of the 2600 BC Rakhigarhi woman and the 11 other Indus Valley people (discovered at sites related to IVC) that helped you get this picture?

Yes. With these individuals, we for the first time found ancient people who could serve as a statistically fitting genetic source for the largest component of ancestry in South Asian (the Iranian-related ancestry)

8) Does your Rakhigarhi study ‘An Ancient Harappan Genome lacks Ancestry from Steppe Pastoralists or Iranian Farmers’ in any way contrast the findings of your other ( Narasimhan et al) study ‘The Formation of Human Populations in South and Central Asia’.

The two studies are fully consistent. I am confident that there are no contradictions.

9) Digs at the Indus Valley site Rakhigarhi, from where the woman’s skeletal remains were discovered show an archaeological continuity. No signs of destruction. Is it possible to have a shift in a population and even possibly a change in civilisation without a disruption in material culture? Have you see that happen elsewhere?

This is entirely possible. We discuss this explicitly in our paper by making an analogy to a major and slightly earlier cultural and genetic transformation in western Europe, where we know more accurately what happened because of a richer ancient DNA record

“If the spread of people from the Steppe in this period was a conduit for the spread of South Asian Indo-European languages, then it is striking that there are so few material culture similarities between the Central Steppe and South Asia in the Middle to Late Bronze Age (i.e., after the middle of the second millennium BCE). Indeed, the material culture differences are so substantial that some archaeologists report no evidence of a connection. However, lack of material culture connections does not provide evidence against spread of genes, as has been demonstrated in the case of the Beaker Complex, which originated largely in western Europe but in Central Europe was associated with skeletons that harbored ~50% ancestry related to Yamnaya Steppe pastoralists (18). Thus, in Europe we have an unambiguous example of people with ancestry from the Steppe making profound demographic impacts on the regions into which they spread while adopting important aspects of local material culture. Our findings document a similar phenomenon in South Asia, with the locally acculturated population harboring up to ~20% Western_Steppe_EMBA–derived ancestry according to our modeling (via the up to ~30% ancestry contributed by Central_Steppe_MLBA groups)”

10) You’ve said that people who formed the population of the IVC is the single largest genetic contributor to people living in South Asia today? Can you elaborate?

The great majority of present-day South Asians are a mixture of two source populations that formed after the decline of the Indus Valley Civilization: the Ancestral South Indians (ASI) and the Ancestral North Indians (ANI). In our paper (Figure 4D), we show that the ASI and ANI both have high proportions of Indus Valley Cline ancestry (similar to that of the Rakhigarhi individual). Depending on the particular model used, this number could range from 30-60% for the ASI, and very roughly around 70% for the ANI. Since present-day South Asians are largely a mixture of ANI and ASI who in turn both have major proportions of Indus Valley Cline-type ancestry, this is the largest source of ancestry in present-day South Asians.

Source: Economic Times