Saturday, 25 February 2017

Pacific islanders may carry the DNA of an unknown human species: Genetic study reveals ancient Melanesians interbred with a mysterious hominid


Islanders in the Pacific Ocean may be may be carrying traces of a long lost human species locked up in their DNA. Today, modern humans inherit a small chunk of our genes from Neanderthals, with evidence that some of us carry the genetic remnants of a lesser known sister group, called the Denisovans. But genetic analysis of people living in modern Melanesia suggests they carry traces of a third, as yet unidentified prehistoric relative distinct from the others.

The island groups of Melanesia – which includes Papua New Guinea, Fiji and the Solomon Islands and others – are geographically cut off by the Pacific Ocean, with their DNA providing a unique window into how human ancestors spread across the region. The latest research, presented at a meeting of the American Society for Human Genetics in Vancouver, bolsters previous findings that there may be another strand to the story of modern humans, with multiple groups of prehistoric human interbreeding.

Genetic analysis of Europeans, Asians and others with non-African descent hints that ancient humans interbred with Neanderthals. Some groups inherited as much as four per cent of their DNA from these extinct human cousins. A single finger bone and a few teeth found in a cave in Russia revealed another branch of the family tree, the Denisovans, also left their genetic calling card in modern humans, accounting for as much as four percent of people's DNA in Melanesia.
Native people from Papua New Guinea in Melanesia are believed to owe between two and four per cent of their DNA to Denisovans and carry less Neanderthal DNA than other Asians. But genetic data reveal their ancestors may have bred with a third species of ancient human

Now, the latest number crunching has revealed another genetic twist in the tale of modern humans. Ryan Bohlender, a geneticist at the University of Texas, and colleagues looked at the rate of genetic mixing which would account for what;s seen in modern Melanesians and found that something didn't add up. As expected, their analysis found the genetic calling cards of Denisovans and Neanderthals, but it also revealed a high proportion of other extinct ancestry unaccounted for. To explain this mystery DNA, the team believe that ancient Melanesians must have bred with a third group of hominids. Presenting their findings in Vancouver, the team explained: 'We suggest that a third archaic population related more closely to Neanderthal and Denisova than to modern humans introgressed into the San genomes studied here'. 

By working out the amount of DNA shared by Neanderthals and Denisovans, they calculated that this third extinct human species likely branched off from their common ancestor 440,000 years ago. 'Overall, our findings confirm the human family tree is more complicated than we think it is,' said Dr Bohlender. He exlained: 'Other archaic populations are likely to have existed, like the Denisovans, who we didn't know about except through genetics.' Previous studies have shown that ancient Melanesians’ trysts with Denisovans may have helped them to adapt to new environments and spread across the Pacific and into Australia. Among the Denisovan genes are those which boost resilience to viruses and provide metabolic benefits, including increasing blood glucose levels and breaking down fats. Tracing the genetic lineage has revealed that modern humans interbred with Neanderthals a number of times and Denisovans at least once, before these two human cousins died out.

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