A new genetic analysis led by researchers at Harvard University has found a common ancestor shared by some Amazonians and the indigenous peoples of Australia and New Guinea, suggesting that the first Americans did not arrive in a single wave as believed.

According to Smithsonian.com, the prevailing theory is that a lone group of humans travelled over a land bridge connecting Eurasia and modern-day Alaska some 15,000 years ago. This new research, however, suggests that it is unlikely that all Native Americans descended from just one group, and that two different groups might have migrated from one continent to the other.

“Our results suggest this working model that we had is not correct,” study co-author and Harvard geneticist David Reich explained to the website. The findings, which were published in the latest edition of the journal Nature, suggest that “there’s another early population that founded modern Native American populations” – a group originating from Australasia, he added.

Findings cast doubt on existing single-migration theories

The origins of the original Americans have long been a topic of debate, Smithsonian.com noted. Previous research has linked ancient humans to ancestral populations from Eurasia, and even Reich’s team had previously found evidence supporting the single-migration theory. In their new study, however, the genetic evidence appears to tell a different story.

Reich and colleague Pontus Skoglund found that the Suruí and Karitiana peoples of the Amazon were more closely related to the indigenous people of Australia, New Guinea and the Andaman Islanders than they were to Eurasians. Along with a team of Brazilian scientists, they analyzed the genomes of 30 Native American groups in Central and South America, using four different techniques to compare them to those of nearly 200 other global populations.

They found that three Amazonian groups (the Suruí, the Karitiana and the Xavante) were each more closely related with Australasians than any group in Siberia. Since these groups have as much in common with Australians as they do with New Guineans, the study authors believe that each of them share a common ancestry that dates back tens of thousands of years ago.

“We spent a lot of time being sceptical and incredulous about the finding and trying to make it go away, but it just got stronger,” Reich told Nature. He, Skoglund, and their colleagues believe that these ancestors had also crossed the Bering land bridge, but were ultimately replaced by first Americans over most of North and South America. “We think this is an ancestry that… crossed Beringia at some point, but has been overwritten by later events,” he added.

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