Ancient Human Relatives Crossed Formidable Wallace's Line

The most recently discovered ancient human relatives crossed one of the world's most prominent marine barriers in Indonesia, later interbreeding with modern humans as they made their way through the region and on to Australia and New Guinea, a new study argues.

Denisovans were first discovered when a genetic analysis three years ago from a Denisova cave in the Altai Mountains in northern Asia resulted in a complete genome sequence. Mysteriously, Denisovan DNA is largely absent in current populations in mainland Asia, however, while genetic evidence pointing to their hybridization with modern human populations has been detected in Indigenous populations in Australia, New Guinea and surrounding areas.

The study, published in the journal Science, suggests this is because the Denisovans were able to cross the Wallace Line, formed by a powerful marine current along the eastern coast of Borneo. As one of the world's biggest biogeographic barriers, the Wallace Line marks a dividing line between European and Asian mammals to the west and Australasia's marsupials 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," co-author Alan Cooper, the director of the University of Adelaide's Australian Centre for Ancient DNA, said in a statement. "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 findings are important, argues co-author Chris Stringer, a researcher from the Natural History Museum in the UK, for piecing together the story of human history.

"The conclusions we've drawn are very important for our knowledge of early human evolution and culture," he said. "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."