What is ancient DNA (aDNA) and what has it been used to study?
Ancient DNA can be carefully extracted from archaeologically recovered bones, teeth or fossil plant remains. Small fragments are processed to sequence the genome of those ancient organisms.

Techniques developed over the past three decades have led to a revolution in how we understand the evolution and genetic history of a range of animals and plants, including species that are extinct today. Palaeogeneticists have been able to establish, for example, how genetic variation might relate to the independent evolution of species on different continents, or how different subspecies of horses emerged after their domestication, or how populations that today appear distinct and in different geographical areas were once related and likely existed together in one region.

What are challenges of studying human DNA?
Special challenges are attached to the study of ancient human DNA, especially as contamination from modern human DNA is a real hazard and requires special techniques at every stage of the recovery and extraction process.

What can be done with aDNA from human samples?
Modern human DNA databases are built on samples from people like us, alive today. They have been used for several applications, prominently including attempts to understand the genetic predisposition towards certain diseases and responses to medicines in different social groups in South Asia.

The comparison of aDNA samples with other aDNA and modern DNA databases can reveal otherwise unsuspected genetic histories. Scientists can trace the deep ancestry of ancient individuals and assess how their genetic makeup is distinct on account of specific variant genes (alleles), mutations and other markers (99% of all human DNA is common) and see how this compares with that of modern groups.

What are the recent results? What do they establish?
The first paper is based on 523 aDNA samples ranging over 8,000 years across Eurasia. The paper is titled, ‘The Formation of Human Populations in South and Central Asia’.  The authors demonstrate clearly that over the last 10,000 years, the present-day distinctive mix of South Asian genetic variability was formed through the mixture of populations then resident in the region with successive groups who moved into the region. This is a process that happened not just once but several times.

They make clear that these mixing of populations were far from “invasions”, and the trends in their data show slow long-term processes of migration, co-mingling and integration.

What does the woman from Rakhigarhi teach us?
The second paper presents the results of the first successful aDNA extraction from prehistoric South Asia. Individual 6113 was an elite woman buried between 2300 and 2800 BCE (estimated) in a cemetery on the outskirts of the Harappan town of Rakhigarhi, located near the present day city of Hissar in Haryana. This is a scientific achievement, especially as efforts to extract archaeological DNA have hitherto been few in South Asia and several attempts resulted in DNA that was too degraded or was contaminated.

The DNA of this person from Rakhigarhi, it turns out, is a mixture with contributions coming from very ancient ancestry shared with Iranian populations and that from what the authors term Andamanese or South-East Asians in the deep past of her ancestry. Of all the ancient samples, contemporary to her that we can compare her to, she turns out to be genetically closest to another group who were buried in Khorasan (principally at the site of Shahr-i-Sokhta in Iran). These individuals — some of whose graves had objects that were previously known to have connections to the Indus Valley Civilisation — share a similar mixture of ancestry and are also outliers in the larger comparative database.

Source: The Hindu