The North-South ancestral divide
doi:10.1038/nindia.2009.294 Published online 23 September 2009
Nearly all Indian groups descend from mixtures of two ancestral populations, though Andaman tribes are an exception, a new study has found1. The study comes as a relief for population geneticists worried about India's under representation in the global genome-wide surveys of human variation.
Scientists at the Centre for Cellular and Molecular Biology (CCMB) in Hyderabad and US researchers at Harvard Medical School, the Harvard School of Public Health and the Broad Institute of Harvard and MIT have used modern genomic technology to explore the ancient history of India.
The research reveals that nearly all Indians carry genomic contributions from two distinct populations — ancestral north Indians who are related to western Eurasians and ancestral south Indians, who are not related to any group outside India. This is where the Andaman population comes in — it appears to be related exclusively to the ancestral south Indian lineage and therefore lacks ancestral north Indian ancestry.
CCMB senior researcher Kumarasamy Thangaraj says following an ancient mixture, these groups experienced periods of genetic isolation from each other for thousands of years. The study has immediate medical implications for people of Indian descent.
The team comprising Thangaraj, Singh and US researchers David Reich, Nick Patterson and Alkes Price analysed more than 500,000 genetic markers across the genomes of 132 volunteers who donated DNA. These research subjects were drawn from 25 diverse Indian groups representing 13 states, six language families, so called 'upper' and 'lower' castes and tribal groups.
Genetically speaking, there's a difference of only 0.1% between the genome sequences of any two unrelated persons. However, this seemingly small difference in genetic material provides clues that can help reconstruct the historical origins of modern populations. It also points to genetic variations that increase the risk of certain diseases in populations, Reich, an associate professor of genetics at Harvard Medical School and an associate member of the Broad Institute of Harvard and MIT, says.
The Indian groups surveyed have inherited 40-80% of their ancestry from the ancestral North Indians and the rest from the ancestral South Indians, Reich says.
However, it is impossible to distinguish castes from tribes using the data. The genetics proves that they are not systematically different. This supports the view that castes grew directly out of tribal-like organisations during the formation of Indian society, he says.
Broad Institute mathematician Nick Patterson, who developed the mathematical theory to analyse this data calls the Andamanese 'unique'. "Understanding their origins provides a window onto the history of the ancestral south Indians, and the period tens of thousands of years ago when they diverged from other Eurasians", he says.
"Our project to sample the disappearing tribes of the Andaman Islands has been more successful than we could have hoped, as the Andamanese are the only surviving remnant of the ancient colonisers of South Asia," Singh, former director of CCMB and co-author, adds.
The discovery that many groups in modern India descend from a small number of founding individuals, means that India is genetically not a single large population, but instead is best described as many smaller isolated populations. The widespread history of founding individuals might explain why the incidence of genetic diseases among Indians is different from the rest of the world, according to Thangaraj.
Among Finns and Ashkenazi Jews, the incidence of recessive genetic diseases is attributed to similar ancestral patterns. Further studies of Indian groups and their founders should lead to discovery of genes that cause devastating diseases.
- Reich, D. et al. Reconstructing Indian population history. Nature 461, 489-494 (2009) | Article