Some ancient human relatives somehow managed to cross one of the world's widest marine barriers in Indonesia to interbreed with modern humans, scientists say.

Genetic analysis suggests the Denisovans -- named for the Denisova cave in the Altai Mountains in northern Asia where the first fossil evidence was found -- interbred with modern humans moving through the area on the way to Australia and New Guinea.

However, Denisovan DNA appears to be rare or even absent in current populations on mainland Asia, even though this is where the fossil was found, researchers 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," said Alan Cooper of the University of Adelaide's Australian Center for Ancient DNA.

This pattern can be explained if the Denisovans had succeeded in crossing the so-called Wallace's Line, one of the world's largest biogeographic barriers formed by a powerful marine current along the east coast of Borneo.

Wallace's Line, marking the division between European and Asian mammals to the west from marsupial-dominated Australasia to the east, is named for British naturalist Alfred Wallace, who along with Charles Darwin conceived the theory of evolution through natural selection.

"The only place where such a [Denisovan] 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 finding Denisovans spread beyond this significant sea barrier opens all sorts of questions about the behaviors and capabilities of this group, and how far they could have spread, the researchers said.

"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," Chris Stringer of the Natural History Museum in London said.

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