Migration and miscegenation

The ancient history of India's population

In his 1994 magnum opus, The History and Geography of Human Genes, the great Italian geneticist Luigi Luca Cavalli-Sforza synthesised what was known about humanity’s deep past, and argued that the genetic differences amongst present day populations would allow a reconstruction of the great historical migrations. But beginning in 2009, the study of ancient DNA from hundreds of thousands of year old skeletons, using techniques pioneered by Svante Paabo at the Max Planck Institute in Germany, and extended by David Reich at Harvard, led to what has been labelled the “ancient DNA genetic revolution”. This has overturned many of Cavalli-Sforza’s conclusions. 

Instead of studying tiny slices of the past contained in our maternal mitochondrial DNA and Y chromosomes from the paternal line, the new ancient DNA methods study the entire genome, including the mutations that have occurred in the genes of unrelated genomes. The higher the density of differences in the mutations separating two genomes on any segment, “the longer it has been since the segments shared a common ancestor as the mutations accumulate at a more or less constant rate over time.” (Reich, p. 4) 

In his important book, Who We Are And How We Got Here, Reich lucidly summarises this revolution. In an earlier column (“Caste, gene and history wars”, November 2009) I had used Reich et al’s paper (“Reconstructing Indian Population history”, Nature, 24 September 2009) to argue against the fashionable view of various historians and anthropologists that caste in India was invented by the British Raj rather than originating in ancient India, as I had argued in The Hindu Equilibrium. But today, the ‘gene wars’ have shifted to the question of whether Indian population’s genetic origins are purely indigenous (as Hindu nationalists claim) or contain a foreign element (the Aryan hypothesis). 

Two studies using the older methods of examining only the paternal Y chromosomes or the maternal mitochondrial DNA, came to contradictory conclusions (S Wells et al, Proceedings of the National Academy of Sciences (PNAS), vol.98, no.18,2001: and Reich (2018),p.128). The former showing a close connection to West Eurasians, the later showing the maternal line had been isolated within India without mixing with foreign neighbouring populations. Whilst a more recent study by Sanghamitra Sahoo et al (PNAS, vol. 103, no.4, 2006) finds that “the Y chromosome data consistently suggest a largely South Asian origin for Indian caste communities and therefore argues against any major influx from regions north and west of India”.   

How can these contradictory findings from the older genetic methods be reconciled? From the fascinating detective work of Reich and his associates, using extensions of their new ancient DNA methodology on the vast collection of DNA of 300 groups and 1,800 individual samples collected by students of  Lalji Singh and Thangaraj Kumaraswamy for the Centre for Cellular and Molecular Biology in Hyderabad, Reich and associates have established a genetic history of the origins of the current Indian population. They find that today’s Indians are a mixture of Ancestral North Indians (ANI) and Ancestral South Asians (ASI) who before their mixture were as different from each other as Europeans and East Asians today. The ANIs are related to Europeans, Central Asians, Near Easterners and the Caucasus. The ASIs are the descendants from people who have no relatives in any present day populations outside India. They show that the ANI and ASI mixed in India so that “everyone in mainland India today is a mix, albeit in different proportions, of ancestry related to West Eurasians, and ancestry more closely related to diverse East Asian and South Asian populations” (p.135). They find no support for the Sahoo et al position of a purely South Asian origin for Indian caste communities. 

The explanation for the discrepancy between the Y-chromosome and mitochondrial DNA patterns from the older methodology is that most of the ANI genetic input came from males. They find that the ANI were a mixture of about 50 per cent steppe ancestry and 50 per cent Iranian farmer ancestry from the groups the steppe people displaced in their southern migration. The ASI were a mixed fusion of populations with 25 per cent due to Iranian farmers expanding eastwards and 75 per cent from the former indigenous hunter gatherers. This points to a remarkable parallel between the prehistory of the two similar sized Eurasian sub continents —Europe and India .

Reich finds that with the collapse of the Indus Valley civilisation (for which there is no genetic data) and the composition of the Rig Veda about four and three thousand years ago, there was a mixture of previously segregated populations. The Indian population came to have different proportions of ANI ancestry, with ANI ancestry deriving more from males than females. This is to be expected from “an Indo-European-speaking people taking the reins of political and social power after four thousand years ago and mixing with the local peoples in a stratified society, with males from the groups in power having more success in finding mates than those from the disenfranchised groups.” (p.140)

Reich and associates also find evidence that there were six jatis of ANI ancestry who had a higher ratio of steppe relative to Iranian farmer ancestry than expected, and they were all in the Brahmin varna. This steppe substructure in the ANI is still present in Brahmin genes. They conclude that “not just Indo-European language but also Indo-European culture as reflected in the religion preserved over thousand of years by Brahmin priests, was likely spread by peoples whose ancestors originated in the steppes”.

An essential part of this culture was the caste system with its myriad of endogamous jatis. In an interesting byway, Reich and his associates found that the degree of genetic differentiation among jatis was many times larger than genetic groups in Europe. They found evidence that like the Ashkenazi Jews, Hutterites and the Amish of today, these jatis are the products of ‘population bottlenecks’, where a relatively small number of individuals and their descendants have many offspring and remain genetically isolated from the surrounding peoples. These population bottlenecks lead to mutations being carried through generations, and when these mutations increase in frequency then there is a high chance of individuals in the group inheriting the same mutation from both parents leading to rare but group specific diseases like Tay-Sachs amongst Ashkenazi Jews. With India’s huge population and an estimated one-third of jati groups descending from bottlenecks stronger than those in Ashkenazi Jews, searching “for genes responsible for disorders in these Indian groups have the potential to identify risk factors for thousands of diseases.” (p.147)

Finally, this new genetic research indirectly confirms many hypotheses in The Hindu Equilibrium. First, the caste system is an ancient system not one invented by the British Raj. Second, the endogamous stratified system brought in by the Indo-European language speakers was adopted and adapted in India as it provided a decentralised method of social control to tie scarce labour to abundant land required for intensive plough agriculture in the vast Indo-Gangetic plain. Third, this successful social institution was transferred to the alluvial river valleys in the South by its custodians, the Brahmins, in return for land grants which allowed them to live in isolated cantonments maintaining their endogamous identity. Finally, that India’s cultural “unity amongst diversity” was provided by the steppe religion brought in by the Indo-European language speakers which became Brahminical Hinduism.