Genetic Ancestry of Indians. A New Paper Is Creating a Ruckus

A new paper about the genetic history of Indian populations in Nature is making waves in press releases. A reader asked me if I could explain the finding in plain english. Since I am not an expert in population genetics I will instead point to two posts that have dissected the paper. John Hawks writes about the topic here. Gene Expression details it here.

The salient points as I understand:

1) This is not some pioneering work as some of the Indian press reports suggest it is. The broad finding of this study have antecedents and are not that surprising or shocking.This study though does expand on earlier work by using a larger number of genetic data points and so is significant in its scope.

2) Modern Indian populations are derived from two ancient populations referred to as the Ancestral North Indians (ANI) and the Ancestral South Indians (ASI). Ancestral North Indians were Caucasoids, genetically closer to Eurasians, Europeans and central Asians while the Ancestral South Indians  were distinct from Caucasoids and Mongoloids. The Onge of the Andamans are a good model for the original Ancestral South Indians. The modern Indians are admixtures of these two populations (the Onge are not ancestral but an early branch of the ASI)

3) The findings indicate that there is a larger amount of genetic variation between Indian groups than there is between say European groups. This the authors suggest is a result of a small number of individuals founding different ethnic groups that then remained endogamous and therefore genetically divergent. This has important medical value as recessive diseases may correlate with ethnic groups.

4) Despite these inter group differences on average Indians from various groups and across castes are more closely related to each other than they are to outgroups like Europeans or East Asians. This points to the deep residence time of people in the subcontinent and continued gene flow across groups. This has led to reporting in the press that there is no north south divide and the Aryan-Dravidian divide is a myth. Again this finding is not new. There is earlier work that suggest similar genetic relationships among Indians.

5) The study also clearly shows that for some genetic lineages there is a gradient in relationship to ANI (west Eurasian) that is a function of geography and caste. For some genes north Indians (Indo European speakers) and upper castes are more closely related to ANI than are south Indians and lower castes. Here is a table that summarizes this result. The first few samples in the table are from south India (Dravidian and Tribal) and the lower portion of the table represents north Indians (Indo-European speaking people).

6) The paper says little about when this admixture with west Eurasian genes occurred but hints it may coincide with the arrival of Indo-European speakers which has generally been timed post collapse of the Harrappan city states around 1800 - 1600 B.C. This is off course is a controversial topic. The amounts of admixture with ANI is high in some samples. This may be taken by some as a validation of the Aryan Invasion scenario in which there was a massive migration and population replacement of indigenous people in northern India by Indo-European tribes. I don't see it that way. The northwestern region has always been a conduit into India. There would have been people movements from the Central and West Asia into this region related to the spread of agriculture (6000 - 8000 B.C ?). City states like the Harrapan complex had extensive trading ties (2600 - 2000 B.C) with the Bactria Margiana Complex in the Turkmenistan - northern Afghanistan - Tajikistan - Uzbekistan area and with the Elamite civilizations in western Iran. The people involved in trading  with these city states included those from the Pontic -Caspian Eurasian steppes.

So it is unlikely that any one historical event shaped these genetic relationships. Migration and population movements of Caucasoid people into India have been taking place longer than the advent of the Aryans although it does seem that the arrival of Indo-European speakers did leave a recognizable genetic imprint on older Indian populations. 

7) The Indian Press has made a hash of the finding. For example they have only reported those parts of the study that deal with the kinship among Indians and have stressed that castes and tribes cannot be differentiated or that there is no divide between the Aryans (roughly north Indians) and Dravidians (south Indians). That is all true for average relatedness. But the study also clearly points out that there are genetic differences between north and south Indians and between upper and lower caste in terms of the degree of relatedness to Eurasians. North Indians and upper castes are more closely related to  Eurasians. North Indian upper castes have even more Eurasian ancestry. This part was ignored by the press.

But I can't blame the press entirely. The scientists who gave interviews to the press didn't mention this. They wimped out on reporting this potential inflammatory and politically incorrect finding. This is just poor and irresponsible science outreach on part of the scientists. How can you ignore a finding that is staring out at you from the very paper you are talking about? The press may be guilty of not digging in but it was just reporting what the scientists told them.