The genetics of migration

Genes carry history. It is a history that is characterised by the secrets lying within the DNA. One such fascinating history is the story of human migration to the Indian subcontinent over thousands of years ago that gave rise to the existing genetic diversity.

India is a patchwork of tribal and non-tribal population that speak many different languages and have distinct cultural identities. The Indo-European languages, spoken across northern and central India, and also in Pakistan and Bangladesh have been frequently connected to the so-called ‘Indo-Aryan invasions’ from central Asia around 3,500 years ago. South India on the other hand is dominated by Dravidian languages.

Retracing the routes taken
Where did all of them originate from and how did they migrate? This is an area of a big controversy among scholars and scientists, and the field is replete with multiple theories. While some of the facts are universally accepted, there are intense debates on the migration routes that the ancient men and women took and the timings of their arduous journeys. Analysing the archaeological evidence with modern-day genetics, a team of European scientists have now retraced several waves of migration to the subcontinent, opening up a new window to look at Indian history. They provided a refined portrait of people’s movement to South Asia, based on the molecular clock hidden inside the genes. Following the migration out of Africa, the Indian subcontinent was probably one of the earliest corridors of dispersals taken by anatomically modern humans. A remarkable genetic diversity, probably the second highest after sub-Saharan population supports this view.

Although the oldest modern fossil in the subcontinent (found in Sri Lanka) dated to about 36,000 years ago, genetic and archaeological evidence suggest that the migration of modern humans began around 50,000 years ago, after the eruption of Mount Toba volcano in Sumatra around 74,000 years ago. Those who came early from Africa were hunter-gatherers. But further waves of settlement came from the direction of Iran, after the last Ice Age ended around 20,000 years ago, and with the spread of early farming. These ancient signatures are clearly seen in the mitochondrial DNA, which tracks the female line of descent. But what the European group of scientists found is the variation in the Y-chromosome that tracks the male line. Here, the major signatures are much more recent. Most controversially, there is a strong signal of immigration from central Asia, less than 5,000 years ago.

This looks like a sign of the arrival of the first Indo-European speakers, who arose amongst the Bronze Age people of the grasslands north of the Caucasus, between the Black and Caspian seas.

They were male-dominated, mobile pastoralists who had domesticated horse and spoke what ultimately became Sanskrit. The archaeological evidence too support the theories on the movement of these horse-based, pastoralist and chariot-using men, located in the grassland and river valleys east of Ural mountains, towards east and south around 3,800 years ago. “The spread of Indo-European languages is one of the most widely discussed topics in prehistory, and one of the most controversial – and nowhere more so than in the Indian subcontinent,” said Pedro Soares of the Institute of Molecular Pathology and Immunology of the University of Porto, Portugal and one of the members of the team that studied the migration genetics.

Old notions
In India, many historians and linguists deny that immigration of Indo-European speakers ever took place. They have been supported by researchers using both mitochondrial DNA and the Y-chromosome – in the former case, because the impact of the new arrivals on the maternal gene pool was quite small, as the newcomers were male-dominated, and in the latter because of problems with Y-chromosome resolution and dating that have only been resolved by whole-genome studies in the last year or so. The new findings are likely to set aside the old notions. “It is clear from our analysis that both the spread of agriculture from the Near East and the dispersal of Indo-Europeans from Central Asia had a huge impact on the genetic structure and history of the region,” Pedro said.

Also, the analysis demonstrates a very strong sex bias in the ancestry of South Asians. The female line of descent is mostly autochthonous (indigenous) and traces back to the first settlement around 55,000 years ago. However, the male line of descent emphasises more recent ancestry since the Last Glacial Maximum from  Asia, the team reported in the journal BMC Evolutionary Biology.

“The Indian subcontinent is not only very culturally diverse, but also harbours a very rich genetic diversity. It was one of the first regions to have been settled by modern humans soon after our species left Africa. Trying to disentangle all these diversities is an interesting challenge,” said Marina Silva, the study’s first author from the University of Hudderfield, UK. “In this study, the authors have dealt with a larger geographical region in examining human dispersals and have provided more fine-grained timing of sex-unbiased and sex-biased dispersals,” commented Partha Majumder, the director of the National Institute of Biomedical Genomics, West Bengal, who was not associated with the study. “Another interesting finding of this study is that sex-biased dispersals seem to be more prominent in the context of peopling of South Asia, as compared to other geographical regions such as East Asia. The significance of their finding is that the nature of peopling of even contiguous geographical regions may be vastly different.”

A problem confronting archaeogenetic research on the origins of Indian populations is the dearth of sources, such as preserved skeletal remains that can provide ancient DNA samples. To find a way out, the researchers used samples from the current population of South Asia and neighbouring regions and used a sophisticated molecular tool to go back into the history.

The study not only showed that the arrival of Indo-Aryans’ was a male-biased process, but also suggested that this division for South Asian populations is far too simplistic, since there were several movements of people both into and within the subcontinent that shuffled some lineages across populations, pointed out Pedro.