Different Human Species Crossed Wallace's Line to Interbreed with Modern Population

Were humans all one species? A recent finding seems to think so. But the jury may still be out when it comes to our ancient ancestors in some areas. Scientists have proposed that the most recently discovered ancient human relatives, the Denisovans, somehow managed to cross one of the world's most prominent marine barriers in Indonesia and then interbred with modern humans. This would help explain the surprising differences and similarities between this population of humans and modern humans.

Three years ago, researchers conducted a genetic analysis of a little finger bone from Denisova cave in the Altai Mountains in northern Asia. This revealed a new line of the human family tree, the Denisovans. Since then, genetic analysis has pointed to their hybridization with modern human populations. Yet this pattern of hybridization has only been detected in Indigenous populations in Australia, New Guinea and the surrounding areas. Now, scientists may have a theory as to why that is.

"In mainland Asia, neither ancient human specimens, no 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, one of the researchers, in a news release. "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."

Wallace's Line marks the division between European and Asian mammals to the west from marsupial-dominated Australasia to the east. It's actually one of the world's biggest biogeographic barriers which is formed by a powerful marine current along the east coast of Borneo. Yet the researchers believe that it's distinctly possible this population of humans could have crossed this barrier and bred with modern humans. In fact, they believe there are also other human populations in the area.

"The recent discovery of another enigmatic ancient human species Homo floresiensis, the so-called Hobbits, in Flores, Indonesia, confirms that the diversity of archaic human relatives in this area was much higher than we'd thought," said Chris Stringer, one of the researchers, in a news release. "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 findings not only reveal that there were distinct human populations, but also show that it's possible that Denisovans made an extraordinary journey across Wallace's Line. The findings reveal a little bit more about human evolution and how modern humans came to be.

The findings are published in the journal Science.