November 1, 2011
Humans Share Genes With Earlier Human Species
Humans not only mated with the ancient Neanderthals, but according to a new study from Uppsala University researchers, the East Asian population of the modern species may have also mated with a hominin species known as Denisovans that lived in Siberia 40,000 years ago.
Denisovans are only known from a few bone fragments, including a finger bone, a tooth and a possible toe bone, which is still undergoing analysis. Denisovans likely split from the Neanderthal tree around 300,000 years ago, but little is known about them. But just as researchers uncovered clues that ancient humans and Neanderthals mated, they have also found genetic evidence of the Denisovans in modern residents of the Pacific islands of New Guinea and the Philippines, et al.
Denisovans, which got their name from the cave in Siberia from which their remains were discovered, have shown that the links between humans and earlier cultures are not as simple as previously believed.
Professor Mattias Jakobsson, of Uppsala University in Sweden who conducted the study with graduate student Pontus Skoglund, said hybridization took place several different times throughout evolution and the genetic traces can still be found in several regions around the world.
“We´ll probably be uncovering more events like these,” said Jakobsson. “Previous studies have found two separate hybridization events between so-called archaic humans - different from modern humans in both genetics and morphology - and the ancestors of modern humans after their emergence from Africa.”
“There was hybridization between Neanderthals and the ancestors of modern humans outside of Africa and hybridization between Denisovans and the ancestors of indigenous Oceanians,” he added. “The genetic difference between Neanderthals and Denisovans is roughly as great as the maximal level of variation among us modern humans.”
While the research expands the Denisovan genetic influence, with Denisovan genes discovered in modern East Asian populations, the genetic signal is less strong in this population than it is in the Oceanic islands such as the Philippines, Jakobsson said.
On the Asian mainland, the genetic influences of Denisovans are strongest in southern China and Southeast Asia. “We are actually finding gene flow in Southeast Asia,” Jakobsson told LiveScience. “So it's not restricted to the Oceanian parts of the world.”
Jakobsson first ran complex computer simulations of genetic data to understand how the limited information collected could be biased. He and his colleagues then examined genetic data from more than 1,500 modern humans from around the world.
Comparing the modern data to the Denisovan genome revealed that Southeast Asians have a much higher proportion of Denisovan-related DNA than other world populations except for the Oceanic islanders. While Oceanic populations have around a 5 percent fraction of Denisovan DNA, Southeast Asians have 1 percent.
Jakobsson´s research is published in Monday´s issue of the journal Proceedings of the National Academy of Sciences.
It´s hard to tell when the Denisovan and human interbreeding occurred, Jakobsson said, but since Europeans don´t have Denisovan ancestry, it´s likely the mating occurred between 23,000 and 45,000 years ago, after Southeast Asians and European populations diverged.
“While we can see that genetic material of archaic humans lives on to a greater extent than what was previously thought, we still know very little about the history of these groups and when their contacts with modern humans occurred,” said Skoglund.
“With more complete genomes from modern humans and more analyses of fossil material, it will be possible to describe our prehistory with considerably greater accuracy and richer detail,” said Jakobsson.
Jakobsson and his colleagues are now working on further studies on early human genetics and the steps that led to the modern human genome. The more scientists delve into the primordial soup, the more complex the genetic picture becomes, he said.
Only small bits of genes are likely left behind of some ancient populations, including the Denisovans. “We don´t really know what they looked like, how they behaved or anything like that,” Jakobsson told LiveScience. “It´s really genetics that gives us an edge here.”
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