A study of ancient DNA suggests that a mysterious sub-species of ancient human could have reached Australia after crossing Wallace’s Line in southeast Asia over 100,000 years ago.
The Denisovans are the newest addition to the human family tree following the discovery in 2008 of a 40,000 year-old finger bone in the Denisova cave in Siberia’s Altai Mountains. Scientists were stunned when analysis of its mitochondrial DNA revealed that it was genetically distinct from modern humans and Neanderthals. They were further amazed when they found that some modern human populations carried traces of Denisovan DNA, the relic of ancient interbreeding between the two subspecies.
Puzzlingly, the traces of Denisovan DNA in modern human genomes appear to be confined to indigenous populations in Australia, New Guinea and surrounding areas, but absent or at very low levels in populations on mainland Asia where the fossil was found.
Professor Alan Cooper of the University of Adelaide and Professor Chris Stringer of London’s Natural History Museum believe they now have the answer why. The pattern can be explained if the Denisovans had succeeded in crossing Wallace’s Line, the world’s most formidable biogeographic barrier. It is formed by powerful marine currents along the east coast of Borneo and divides Eurasian mammals and Australasian marsupials.
Professor Cooper said: “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. 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.”
Professor Stringer added: “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 behaviours 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 colonise New Guinea and Australia around 50,000 years ago, met and interacted with the Denisovans,” said Professor Cooper. “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.”
Professor Cooper believes that, having crossed Wallace’s Line over 100,000 years ago, it would be “amazing” if the Denisovans hadn’t then reached Australia – a crossing made simpler by much lower sea levels in the Paleolithic. “If you cross Wallace’s Line you’ve done all the hard work,” he said. While no Denisovan remains have so far been found outside of Siberia, he reckons such discoveries could just be waiting to be found.