Three years ago the genetic analysis of a little finger bone from Denisova cave in the Altai Mountains in Siberia led to a complete genome sequence of a new line of the human family tree - the Denisovans. Now scientists believe that the Denisovans, who lived about 41,000 years ago, somehow managed to cross one of the world’s most prominent marine barriers in Indonesia and later bred with modern humans on their way to Australia and New Guinea.

Wallace’s Line is a significant sea barrier 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. The fauna on either side of the barrier are so different from one another because the marine straight is very hard to cross.

“On one side you have all your tigers and rhinos and monkeys and on the other you have all your marsupials, giant lizards and Australia," said Professor Alan Cooper of the University of Adelaide in Australia. "This is the probably one of the world's most famous biogeographic lines."

Until now it was thought that ancient humans were unable to cross Wallace’s Line, however research conducted by Professor Cooper and Professor Chris Stringer of the Natural History Museum in the UK suggests that genetic patterns can only be explained if the Denisovans succeeded in crossing the barrier.

The researchers found that Denisovan DNA is virtually absent in current populations on mainland Asia, even though this is where the original fossil was found, but was present in indigenous populations in Australia, New Guinea and surrounding areas.

"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," said Professor Cooper. "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 have implications for our understanding of the technological ability of Denisovans.

"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."

Now that researchers have found what the Denisovans achieved, the next logical question should be how?

By April Holloway