Published in The Hindu on February 24, 2010
In less than 10 years after the human genome from a living individual was first sequenced, scientists have successfully sequenced a complete ancient human genome (only partial ancient human genomes and mitochondrial DNA have been sequenced in the past). The study was published recently in Nature. The sample studied was one of the four excellently preserved human tufts from a male paleo-Eskimo obtained from about 4,000-year-old permafrost at Qeqertasussuk, Greenland. As against the gold standard of sequencing the genome 10 times, the ancient human genome was sequenced 20 times over nearly 80 per cent of its length. Repeated sequencing helps enhance the level of accuracy, ensuring that any differences seen between ancient and modern human genomes are true. There is great interest in studying ancient humans to understand the routes of human migration from Africa. The ancient human’s mitochondrial DNA sequenced in 2008 helped in identifying him as belonging to Saqqaq culture of East Siberia. Tracing the route of migration from East Siberia to Greenland through Alaska and Canada also became possible by comparing the ancient human genome with modern genome data; Saqqaqs had split from Chukchis, their closest relatives, some 5,500 years ago.
Sequencing the nuclear DNA and comparing the functional single-nucleotide polymorphisms (SNP) with modern human genome data helped in identifying his appearance and roots. The study revealed that he had brown eyes, dark thick hair, and brown skin, which are typical Asian characteristics. His shovel-graded front teeth and earwax of the dry type are typical of Asians and native Americans. His metabolism and body mass index indicates that he was adapted to living in a cold climate. Sequencing the complete ancient genome became possible as hair tufts were excellently preserved in permafrost. However, the samples recovered in 1986 were stored at room temperature in the National History Museum of Denmark, Copenhagen, until they were studied recently. Past studies in humans and animals have shown that unlike bones and teeth, hair shafts are able to preserve mitochondrial DNA in larger quantities and for longer periods of time even at room temperature; the shaft’s melanin material probably prevents DNA damage. But the latest study has shown that it is indeed possible to extract nuclear DNA from hair shafts, especially when hair is preserved in ice. The real challenge will be to extract nuclear DNA from samples recovered from temperate and tropical regions, where the majority of ancient human remains are found.