A mysterious ancient relative of modern humans somehow managed to find a way to cross Wallace's Line, one of the world's most prominent marine barriers in Indonesia.
Wallace's Line is one of the biggest biogeographic barriers that is formed by a powerful marine current along the east coast of Borneo.
It acts as a dividing line between the Asian mammals of the west and the marsupial-dominated countries to the east.
Three years ago, a new line of the human family tree was discovered after a finger bone was found inside a cave in northern Asia. The species was identified as the Denisovans and the finger bone was from a juvenile female who lived about 41,000 years ago.
Through the bone, scientists were able to genetically analyse the Denisovans and map their DNA.
However, genetic links have now been identified between the Denisovans and indigenous populations in Australia, New Guinea and surrounding areas, but their DNA is almost completely absent in mainland Asia, even though this is where the finger bone was found.
How the Denisovans got to Australia has remained a mystery and scientists now believe they must have somehow managed to get across Wallace's Line and then interbred with modern humans.
Alan Cooper of the University of Adelaide in Australia and Chris Stringer of the Natural History Museum in the UK have suggested the Denisovans crossed Wallace's Line in an opinion article published in Science.
"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," Cooper said.
"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."
Stringer said that the recent discovery of another "enigmatic" ancient human species, the Homo floresiensis, or Hobbits, shows the diversity of our early human relatives: "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 behaviours and capabilities of this group, and how far they could have spread."
Cooper said questions have now been raised about how modern humans met and interacted in Denisovans 50,000 years ago on their way to New Guinea and Australia, noting that modern humans also crossed paths with the Denisovans on their way west.
"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."