According to a study published in the journal Science, Denisovans – relatives to both Neanderthals and humans – somehow managed to cross Wallace’s Line after 600,000 years ago, and later interbred with anatomically modern Homo sapiens moving through the area on the way to Australia and New Guinea.

In 2010, a genetic study of a little finger bone found in Denisova cave in the Russian Altai Mountains led to a complete genome sequence of a new line of the human family tree – the Denisovans.

Since then, genetic evidence pointing to their hybridization with modern human populations has been detected, but only in indigenous populations in 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 was found.

“This pattern can be explained if the Denisovans had succeeded in crossing the famous Wallace’s Line, one of the world’s biggest biogeographic barriers which is formed by a powerful marine current along the east coast of Borneo,” said lead author Prof Alan Cooper of the University of Adelaide.

Wallace’s Line 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 that there has never been a genetic signal of Denisovan interbreeding in the area,” Prof Cooper added.

“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 that the Denisovans must have somehow made that marine crossing.”

“The recent discovery of another enigmatic ancient human species Homo floresiensis, the so-called Hobbits, in Flores, Indonesia, confirms that the diversity of archaic human relatives in this area was much higher than we’d thought,” added Prof Chris Stringer from the Natural History Museum in London, UK.

“The morphology of the Hobbits shows they are different from the Denisovans, meaning we now have 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. Knowing that the Denisovans spread beyond this significant sea barrier opens up all sorts of questions about the behaviors and capabilities of this group, and how far they could have spread.”

“The key questions now are where and when the ancestors of current humans, who were on their way to colonize New Guinea and Australia around 50,000 years ago, met and interacted with the Denisovans,” Prof Cooper said.

“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.”

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Bibliographic information: A. Cooper and C. B. Stringer. 2013. Did the Denisovans Cross Wallace’s Line? Science, vol. 342, no. 6156, pp. 321-323; doi: 10.1126/science.1244869