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Ancestral Population in India

October 18, 2009

Prabir Purkayastha

A RECENT study by scientists from the Centre for Cellular and Molecular Biology (CCMB) and Harvard has come out with a set of findings about the nature of the ancestral Indian population. The study -- Reconstructing Indian population history (Nature, 24 September 2009 | doi:10.1038/nature08365) -- is not a sharp break with the past as some of the newspaper reports have reported but very much in line with past studies. Roughly, the study shows that the Indian population is an admixture of an Ancient South Indian (ASI) and a slightly younger Ancient North Indian (ANI) population. The proportion of the two varies, roughly from south to north, with the North Indian population being closer to the Euro-Asian population outside India. They have also found, similar to the results of an earlier study that the Euro-Asian component is higher in the higher castes.

Another interesting find in the study is that the Onge group in Andaman, who number today in only a few hundreds, is much more closely related to the ASI population and must have broken off before the ANI population appears in India. The other striking result is that there is a scheduled caste-scheduled tribe continuum, something that DD Kosambi had proposed in his seminal work on Indian history.

The major difference of this study with the earlier ones is the amount of data they used in the study. While the other studies had looked at only a few genetic markers in the samples of the people they had taken, this study uses a much higher number of markers.

How do we study genetic variations in a population? Some of the genes in our DNA sequence have multiple forms that they exist in and the alternate forms are called alleles. This means that within the human genetic sequence, there are different expressions of the genes that produce differences within the population. For example, one form of this gene (or allele) produces brown eyes, the other green. A gene is a DNA sequence within the genome and each Nucleotide is a specific location within this DNA sequence. If the DNA sequence is looked on as a ladder, each location is like a rung coded from its basic building blocks in this DNA ladder. If a nucleotide has only two forms -- there is a single mutation in a nucleotide -- we call these single nucleotide polymorphisms (SNPs). The variation of the SNPs in a population is a measure of the genetic diversity of the population.

Most of these different versions of the nucleotides do not lead to any difference in the people these differences are neutral. The SNPs that display functional differences are only about 1per cent of the total 10 million SNPs that exist in the human population. Although the DNA sequences of any two unrelated people differ by only a small amount less than 0.1per cent -- this small amount of genetic material can provide insights into ancient migrations and origins of the current populations.

The current study differs from the earlier ones in the amount of data that they have used. They have looked at 560,000 SNPs in the Indian population. Though their SNP numbers were very large, the sample size of groups and people chosen were not high. They took only 132 individuals from 25 groups in 13 states. The earlier studies had looked at much smaller number of SNPs and therefore the fact that even with a much larger number of SNPs, the same results have been reached is a vindication of the power of genetics in unravelling some of these questions.

Though they have been careful not to suggest that the genetic studies tell us anything about language Indo European or Dravidian -- the data seems to suggest that there is more of an influx from the North West of India to North India. This is of course a trivial result, as geographically this is but natural. The more interesting result is that there is no evidence of genetic outflows from India to the Euro-Asian population outside: the gene flows have been inwards. This makes the RSS thesis of India Europeans being descended from an Aryan Vedic population in India that spoke Sanskrit (in their view, the mother of all Indo-European languages) a non starter. However, when has facts ever interrupted their thinking processes?

The press reports of this study have been quite mixed. Some have claimed that the genetic studies show there is no difference between South Indians and North Indians. Some have also claimed that the Aryan Dravidian divide is a myth. The study clearly shows that while the ANI and ASI population are present in almost all the Indian population groups, the proportions are different. The more north we go, the proportion of ANI rises, while the more south we go, the more it falls. The Indian Palaeolithic population consisted of first the ASI, which settled at least 65,000-70,000 years ago. The second group, the ANI came around 40,000-50,000 years ago. The Onge is a part of the ASI population and branched off from the ASI about 40,000 to 50,000 years ago and show no ANI markers.

Different groups in India seem to have a small founding population. This would signify that groups have tended to interbreed from its small initial numbers and this inbreeding goes back for a very large number of generations. Therefore, the thesis that caste is either a freezing of occupation based groups or a later invention of the British is shown to be quite bogus. That gene flows across groups are limited show that caste is descent-based and formed quite early in India.

The small founding population and inbreeding populations have some medical implications. Each of the groups have a larger share of what are called recessive genes and this would show up in a higher prevalence of genetic diseases based on recessive genes.

As we have written earlier, no serious historian today posits a huge influx of Indo Aryan speaking people replacing the original population in the north. The spreading of language can take place through dominance of a group, conquering the rest and becoming the new elite. This squares well with what the genetic record now tells us and is very much in line with what historians such as Iravati Karve, DD Kosambi and Romila Thapar have maintained. It is also in line with the linguistic evidence that we have.

There has been a relative scarcity of such studies for the Indian population. India has a much greater diversity in not only its cultural matrix but also at the genetic level. The other such diverse population is found in Africa. Given that Africa has the oldest ancestral population this is not surprising. To give depth to such studies, we need to sample across a much larger set of groups both cultural and geographical. The study is therefore of great significance and will pave the way for a better understanding of the ancient migration patterns in South Asia.

Last Updated on Friday, 23 October 2009 05:18