The Genetics of Indo-Aryan Migration
According to the The Indian Genome Variation Consortium (2005), the population of the subcontinent can be divided into four morphological types: Caucasoids in the north, Mongoloids in the northeast, Australoids in the western, central and southern regions of the country and Negritos largely restricted to the Andaman Islands; however, these groups tend to overlap because of admixture. The majority of genetic differences among Indians appears to be distributed along caste lines, rather than along ethnic lines, although genetic differences do exist between predominantly Indo-European-speaking northern and predominantly Dravidian-speaking southern Indian populations, as was also observed by Reich in a recent 2009 study.
In 2008, The Indian Genome Variation Consortium produced another study, this time emphasizing the significant genetic differentiation which exists between Dravidian-speaking, Indo-European-speaking, Tibeto-Burman-speaking and Austro-Asiatic-speaking populations. The researchers write: "Thus, although there are no clear geographical grouping of populations, ethnicity (tribal/nontribal) and language seem to be the major determinants of genetic affinities between the populations of India. This is concordant with an earlier finding based on allele frequencies at blood group, serum protein and enzyme loci (Piazza et al. 1980)." The authors further observe that "it is contented that the Dravidian speakers, now geographically confined to southern India, were more widespread throughout India prior to the arrival of the Indo–European speakers (Thapar 1966). They, possibly after a period of social and genetic admixture with the Indo–Europeans, retreated to southern India, a hypothesis that has been supported by mitochondrial DNA analyses (Basu et al. 2003). Our results showing genetic heterogeneity among the Dravidian speakers further supports the above hypothesis. The Indo–European speakers also exhibit a similar or higher degree of genetic heterogeneity possibly because of different extents of admixture with the indigenous populations over different time periods after their entry into India. It is surprising that in spite of such a high levels of admixtures, the contemporary ethnic groups of India still exhibit high levels of genetic differentiation and substructuring."
In a major study (2009) using over 500,000 biallelic autosomal markers, Reich hypothesized that the modern Indian population was the result of admixture between two genetically divergent ancestral populations dating from the post-Holocene era. These two "reconstructed" ancient populations he termed "Ancestral South Indians" (ASI) and "Ancestral North Indians" (ANI). According to Reich: "ANI ancestry is significantly higher in Indo-European than Dravidian speakers, suggesting that the ancestral ASI may have spoken a Dravidian language before mixing with the ANI." Furthermore, Reich observes: "It is tempting to assume that the population ancestral to ANI and CEU spoke 'Proto-Indo-European', which has been reconstructed as ancestral to both Sanskrit and European languages, although we cannot be certain without a date for ANI–ASI mixture."
Similarly, an earlier study conducted by Watkins et al. (2008) states:
"The historical record documents an influx of Vedic Indo-European-speaking immigrants into northwest India starting at least 3500 years ago. These immigrants spread southward and eastward into an existing agrarian society dominated by Dravidian speakers. With time, a more highly-structured patriarchal caste system developed. India is now broadly characterized by Indo-European (e.g. Hindi, Urdu, and Punjabi) speaking populations found in the central and northern regions and by Dravidian (e.g. Tamil, Telugu, and Kannada) speaking populations in the southern and southeastern regions. ... Although other interpretations may be possible, our data are consistent with a model in which nomadic populations from northwest and central Eurasia intercalated over millennia into an already complex, genetically diverse set of subcontinental populations. As these populations grew, mixed, and expanded, a system of social stratification likely developed in situ, spreading to the Indo-Gangetic plain, and then southward over the Deccan plateau. A strong patrilineal social structure, accompanied by a developing practice of caste endogamy, may have contributed to an asymmetric apportioning of Y-chromosome, autosomal, and to a lesser extent, mtDNA lineages."
The geneticist PP Majumder (2010) has recently argued that the findings of Reich et al. (2009) concerning Indo-Aryan expansion into the Indian subcontinent are in remarkable concordance with previous research using mtDNA and Y-DNA:
"Central Asian populations are supposed to have been major contributors to the Indian gene pool, particularly to the northern Indian gene pool, and the migrants had supposedly moved into India through what is now Afghanistan and Pakistan. Using mitochondrial DNA variation data collated from various studies, we have shown that populations of Central Asia and Pakistan show the lowest coefficient of genetic differentiation with the north Indian populations, a higher differentiation with the south Indian populations, and the highest with the northeast Indian populations. Northern Indian populations are genetically closer to Central Asians than populations of other geographical regions of India... . Consistent with the above findings, a recent study using over 500,000 biallelic autosomal markers has found a north to south gradient of genetic proximity of Indian populations to western Eurasians. This feature is likely related to the proportions of ancestry derived from the western Eurasian gene pool, which, as this study has shown, is greater in populations inhabiting northern India than those inhabiting southern India. In general, the Central Asian populations are genetically closer to the higher-ranking caste populations than to the middle- or lower-ranking caste populations... . Among the higher-ranking caste populations, those of northern India are, however, genetically much closer than those of southern India. Phylogenetic analysis of Y-chromosomal data collated from various sources yielded a similar picture. Higher-ranking caste populations have been the torch-bearers of the Hindu caste system that was formalized by the Indo-European immigrants. It is likely, therefore, that there was a greater proportion of admixture between higher-ranking caste populations and Indo-Europeans. The fact that high-ranked caste populations inhabiting southern India do not exhibit as much affinity with central Asian populations as those of northern India may be explained by the recent finding that the south Indian, Dravidian speaking, populations may have admixed with north Indian populations bearing ancestral signatures of the western Eurasian gene pool more recently."
The author summarizes his findings by stating that:
"Within India, consistent with social history, extant populations inhabiting northern regions show closer affinities with Indo-European speaking populations of central Asia that those inhabiting southern regions. Extant southern Indian populations may have been derived from early colonizers arriving from Africa along the southern exit route. The higher-ranked caste populations, who were the torch-bearers of Hindu rituals, show closer affinities with central Asian, Indo-European speaking, populations. ..."
Further building on Reich et al.'s characterization of the South Asian population as historically based on admixture of ANI (Ancestral North Indian) and ASI (Ancestral South Indian) populations, a 2011 session paper by Moorjani et al. states that a "major ANI-ASI mixture occurred in the ancestors of both northern and southern Indians 1,200-3,500 years ago, overlapping the time when Indo-European languages first began to be spoken in the subcontinent."
A 2011 study published in the American Journal of Human Genetics indicates that Indian ancestral components are the result of a more complex demographic history than was previously thought. According to the researchers, South Asia harbours two major ancestral components, one of which is spread at comparable frequency and genetic diversity in populations of South and West Asia, the Middle East, the Near East and the Caucasus; the other component is more restricted to South Asia. However, rather than ruling out the possibility of Indo-Aryan migration, these findings suggest that the genetic affinities of both Indian ancestral components are the result of multiple gene flows over the course of thousands of years, with Indo-Aryan expansion into the subcontinent but one of many complex demographic episodes. The study authors write:
"Summing up, our results confirm both ancestry and temporal complexity shaping the still on-going process of genetic structuring of South Asian populations. This intricacy cannot be readily explained by the putative recent influx of Indo-Aryans alone but suggests multiple gene flows to the South Asian gene pool, both from the west and east, over a much longer time span."
Read more about this topic: Genetics And Archaeogenetics Of South Asia