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Mysterious ancient humans crossed Wallace’s Line

Recently discovered ancient relatives of today’s humans, dubbed the Denisovans, may have crossed one of the world’s most prominent marine barriers in Indonesia and later interbred with modern humans on their way to Australia and New Guinea.

Scientists at the British Natural History Museum and the University of Adelaide in Australia say there is genetic evidence pointing to hybridisation between the Denisovans and modern human populations. But only with indigenous people in Australia, New Guinea and surrounding islands.

In a report in the journal Science, available on Adelaide University’s website, the museum’s Professor Chris Stringer and Adelaide’s Professor Alan Cooper note that genetic analysis three years ago of a little finger bone from Denisova cave in the Altai Mountains in northern Asia led to a complete genome sequence of a new line of the human family tree.

Since then, genetic evidence has pointed to interbreeding among Denisovans and modern humans. Because of where this occurred suggests the Denisovans must have crossed the Wallace Line in Indonesia: evidence of the interbreeding has only been detected with indigenous populations in the Pacific countries of Australia, New Guinea and surrounding areas.

In contrast, Denisovan DNA appears to be absent or at very low levels in current populations on mainland Asia, even though this is where the fossil finger was found. Cooper and Stringer say this pattern can be explained if the Denisovans had succeeded in crossing the famous Wallace’s Line.

This is one of the world’s biggest bio-geographic barriers, which is formed by a powerful marine current along the east coast of Borneo and marks the division between European and Asian mammals to the west from marsupial-dominated Australasia to the east.

“In mainland Asia, neither ancient human specimens nor geographically isolated modern indigenous populations have Denisovan DNA of any note, indicating there has never been a genetic signal of Denisovan interbreeding in the area,” says Cooper, director of Adelaide’s Australian Centre for Ancient DNA.

“The only place where such a genetic signal exists appears to be in areas east of Wallace’s Line and that is where we think interbreeding took place – even though it means the Denisovans must have somehow made that marine crossing.”

Stringer says the recent discovery of another enigmatic ancient human species Homo floresiensis, the so-called Hobbits in Flores in Indonesia, confirms that the diversity of archaic human relatives in this area was much higher than scientists had thought.

A research leader in human origins at the Natural History Museum in London, Stringer says the morphology of the Hobbits shows they are different from the Denisovans, meaning there are at least two, and potentially more, unexpected groups in the area.

“The conclusions we’ve drawn are very important for our knowledge of early human evolution and culture,” he says. “Knowing that the Denisovans spread beyond this significant sea barrier opens up all sorts of questions about the behaviours and capabilities of this group, and how far they could have spread.”

The two scientists say the key questions now are where and when the ancestors of current humans, who were on their way to colonise New Guinea and Australia around 50,000 years ago, met and interacted with the Denisovans.

“Intriguingly, the genetic data suggest that male Denisovans interbred with modern human females, indicating the potential nature of the interactions as small numbers of modern humans first crossed Wallace’s Line and entered Denisovan territory.”

* Picture credit: John Foster.

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