Saturday, September 24, 2016

No Population Continuity Between Pre Toba And Extant Humans In India

A few years ago stone tools were discovered in the Jurreru Valley region of Kurnool district, South India, in sediment stratigraphically below a volcanic ash layer dated to around seventy four thousand years ago. This was the deposit of the famous Toba eruption. Michael Petraglia, an archaeologist based at the University of Oxford, England, suggested that these tools were made by Homo sapiens. This would mean that our species had first migrated out of Africa and into India perhaps as early as hundred thousand years during the Marine Isotope Stage 5 interglacial phase when ecological corridors may have opened up between Africa, Arabia and the Indian subcontinent. This is much before the more commonly accepted dates of around fifty to sixty thousands years ago. Other scientists objected and argued that the tools were made by an earlier species of archaic Homo, perhaps descendants of Homo erectus who had migrated to India more than a million years ago. The various theories of the dispersal of Homo sapiens from Africa has been summarized well recently in an article by Huw. S Groucutt and colleagues.

The earliest unequivocal skeletal evidence of the presence of anatomically modern humans in the Indian subcontinent comes from Sri Lanka where these remains have been dated to be around thirty five thousand years old. They represent humans from the later wave of the out of Africa migrations.

A related question was left dangling. If these tools were made by people from an earlier migration of Homo sapiens, then is there population continuity between those older migrants and living Indians?  Did later migrants mix with the earlier inhabitants or did the earlier human populations go extinct without leaving a genetic legacy in us.

There were other hints of the presence of an older wave of Homo sapiens migration into India. The Indian Early to Mid Pleistocene hominin skeletal record is quite poor with examples only from the Narmada Valley at Hathnora and Nethankari . At the latter site, a humerus interpreted to represent a "short and stocky" early Homo sapiens has been found associated with delicate bone implements. The remains may be around seventy five thousand years old or even older. At Hathnora, two clavicles and a partial 9th rib was recovered from a layer of fluvial sediment. The materials are thought to be about one hundred and fifty thousand years old and have been interpreted to be an archaic Homo sapiens. What is the margin of error on these dates? Could they be a little younger and represent the early MIS 5 phase migration from Africa around one hundred to one hundred and twenty five thousand years ago? This population seems to have persisted for several tens of thousands of  years as is evidenced by the younger remains at Nethankari.

There is evidence in the form of tools  as well as skeletal material found in Israel, the Arabian Peninsula and China, that indicate that anatomically modern humans did migrate out of Africa as early as a hundred thousand years ago.  A series of DNA studies of global human populations published a few days ago seems to say that people from these earlier migrations died out without contributing ancestry to extant humans. The three studies say that all non-African humans have descended from a single wave of migration  that took place between fifty thousand and eighty thousand years ago.

The scientists, A.R. Sankhyan and colleagues, working on the remains of the short and stocky Narmada Valley hominin had suggested that this population may have contributed ancestry to later short bodied people of South Asia, for example the Andamanese tribes. This scenario now looks untenable. These older (putative) Homo sapiens  in India and elsewhere died out without leaving a genetic trace.

The exception to these findings seems to be in Papua New Guinea. One study finds that 2% of the genome of present day Papuans originated from an earlier expansion  of modern humans out of Africa.

Carl Zimmer has written a good summary of the results.

Here are the links to the papers -

1)  Genomic analyses inform on migration events during the peopling of Eurasia
2) The Simons Genome Diversity Project: 300 genomes from 142 diverse populations
3) A genomic history of Aboriginal Australia

Why didn't people from the two separate waves of modern human migrations mate? The answer likely is because they never met. These older Homo sapiens populations went extinct before the new settlers came. I say this because recent genetic work has shown that one almost inevitable outcome of the meeting of two peoples, however different they may be, is sexual intercourse. When modern humans left Africa fifty-sixty thousand years ago they met and interbreed with Neanderthals and Denisovans, two older hominin groups whose ancestors had left Africa about half a million years ago.

Consider also what happened much later in the Holocene. The end of last ice age and the advent of agriculture saw population growth and the migration and mixing of people. Many of these populations had diverged and remained relatively isolated for more than twenty thousand years, accumulating significant cultural, linguistic and physical difference between them. Yet, the result of the meeting of these people was mostly not the genetic disappearance of one group, but admixture and the formation of modern groups with multiple streams of ancestry.

Today's Europeans contain ancestry from three different groups. A small fraction from earlier resident hunter gatherers and the more substantial fraction from Near East farmers and from Central Asian steppe pastoralists. When Europeans began colonizing the Americas, the native populations suffered immensely from disease and subjugation. But there was also genetic admixture. Native Americans today, both from South and North Americas, contain a noticeable amount of  European and African ancestry.

In the Indian context multiple events of mixing in the Holocene took place between residents (the Ancestral South Indians) and migrants from the Eurasian regions (the Ancestral North Indians). Additions layers of ancestry to the Indian melting pot (but common more in the eastern parts of the country) were contributed by migration of the Tibeto-Burman and the Austroasiatic people from the north east.

Why did the older group of Homo sapiens go extinct? According to Dr Pagani, one of the scientists involved in the first study I listed above “They may have not been technologically advanced, living in small groups,”... “Maybe it was easy for a major later wave that was more successful to wipe them out.”

Or as I suggested, they went extinct before the new settlers arrived. Living in small isolated populations leaves people vulnerable to disease and environmental catastrophe. One such event could have been the Toba eruption which had considerable environmental impact in South Asia. Could that have played a role in the demise of older Homo sapiens groups in Asia?  It would be interesting to see if there is archaeological evidence of an overlap between the two groups of modern humans anywhere.

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Groucutt, H., Petraglia, M., Bailey, G., Scerri, E., Parton, A., Clark-Balzan, L., Jennings, R., Lewis, L., Blinkhorn, J., Drake, N., Breeze, P., Inglis, R., Devès, M., Meredith-Williams, M., Boivin, N., Thomas, M., & Scally, A. (2015).               Rethinking the dispersal of
              out of Africa
             Evolutionary Anthropology: Issues, News, and Reviews, 24 (4), 149-164 DOI: 10.1002/evan.21455

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