Distant DNA links have been discovered between Aboriginal Australians and tribes living deep within the Amazon rainforest.
Researchers in the US and Denmark have established that people sharing DNA links with Aboriginal people crossed the Bering land bridge between Siberia and the Americas thousands of years ago.
The revelations, published in Nature and Science Magazine, are the strongest signs yet that there could have been multiple migrations from Siberia to the Americas tens of thousands of years ago.
The findings have captivated the international science community, with Jennifer Raff, an anthropological geneticist at the University of Texas, telling Nature on Tuesday they were, "honestly one of the most exciting results we've seen in a while."
The authors of the Nature study concluded that a mysterious ancient people they dubbed "Population Y" migrated from Siberia to the Americas more than 15,000 years ago.
"Some Amazonian Native Americans descend partly from a North American founding population that carried ancestry more closely related to indigenous Australians, New Guineans and Andaman Islanders than any present-day Eurasians or North Americans," the authors found.
The authors of the Nature study stressed that they do not believe that an "unmixed population" related to Australasians migrated to the Americas.
Rather, they say, it's likely that a group they dubbed "Population Y" — which contributed "Australasian" genes to Amazonians from the Suruí and Karitiana tribes — was already mixed with a Native American lineage by the time it reached the Amazon.
They said up to 85 per cent of the ancestry in the Suruí, Karitiana and Xavante Amazonian tribes came from Population Y, while about 1-2 per cent came from Australasians.
Danish researchers, whose work was published separately in Science Magazine this week, also found links between Australasian DNA and DNA found in Amazonian tribes.
But the Danish researchers argue that the Australasian DNA made its way to the Americas 9000 years ago, instead of 15,000 years ago, and suggested it got there via Aleutian Islanders, who also carry some DNA characteristics of Indigenous Australians.
The authors of the Science Magazine research wrote: "These results suggest there may be a distant Old World signal related to Australo-Melanesians and East Asians in some Native Americans".
Jeremy Austin, deputy director of the Australian Centre for Ancient DNA at the University of Adelaide, described the findings as "perplexing".
Scientists had previously mapped human migrations using mitochondrial DNA, which is passed down along maternal lines, Dr Austin said.
But recent advances in human genome sequencing had complicated the picture, he said.
"The more we can sequence genomes from anything, and in this case humans, the more interesting these things become," he said.
"The microscope is getting more and more powerful, and so we get these results that have people scratching their heads and saying, 'how did we get DNA of Papua New Guineans or Indigenous Australians hiding in the Amazonian rainforest population?.'"
