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Analysis of DNA from early settlers of the Pacific overturns leading genetic model

More than 3,000 years ago, a group of people set out from the Solomon Island chain in the southwestern edge of the Pacific Ocean and steered their outrigger canoes toward the horizon, with no land as far as their eyes could see. These people and their descendants were the first to cross more than 350 kilometer stretches of open ocean into a region known as Remote Oceania. Now, DNA sequences are for the first time telling us more about the ancestral origins of these people, and their genetic legacy that lives on in Pacific Islanders today.

Analysis of DNA from early settlers of the Pacific overturns leading genetic model
DNA was extracted from this 3,000 year old skull and mandible from Vanuatu 
[Credit: Frederique Valentin]
A scientific team led by researchers at Harvard Medical School, University College Dublin, and the Max Planck institute for the Science of Human History, and including Binghamton University Associate Professor of Anthropology Andrew D. Merriwether, analyzed DNA from people who lived in Tonga and Vanuatu between 2,500 and 3,100 years ago, and were among the first people to live in these islands. The results overturn the leading genetic model for this last great movement of humans to unoccupied but habitable lands.

"The genetic data so far hasn't been able to prove it one way or another. Nobody ever looked directly in the past to test the question. Up until now, all of the hypotheses were based on the blood samples and cheek swabs of living people," said Merriwether, who spent a decade collecting and analyzing several thousand DNA samples in the Bismarck Archipelago with colleagues Jonathan Friedlaender of Temple University and George Koki of Goroka, Papua New Guinea.

Analysis of DNA from early settlers of the Pacific overturns leading genetic model
A loop motif displayed on a shard of Lapita pottery recovered from Nukuleka in Tonga 
[Credit: David Burley]
When the researchers examined the DNA sequences they found -- to their great surprise -- that the ancient individuals carried no trace of ancestry from people who settled Papua New Guinea more than 40,000 years ago, in contrast to all present-day Pacific islanders who derive at least one-quarter of their ancestry from Papuans. This means that the Remote Oceanian pioneers swept past the archipelago that surrounds New Guinea without much mating with local people.

"We had a hard time even getting this paper published, because it's controversial. But the results are very unambiguous," said Merriwether. "These early people don't show signs of Papuan DNA -- the people from New Guinea and Solomon Islands and the Bismarck Archipelago all have this much-older DNA. And we don't see signs of that, almost at all, in these ancient remains. So that implies very strongly that the people who went out there really sort of bypassed those islands, or they didn't interbreed with them. Because the modern populations have interbred with them, some over the last 3,000 years, and have some of those genes, people had assumed they must have settled and then moved on to the next island, settled and moved on. It doesn't look like they did that now. It looks like it really changes our view of history. So it's pretty significant."

Analysis of DNA from early settlers of the Pacific overturns leading genetic model
A 3,000 year old burial site in Vanuatu with bones arranged in a triangular pattern 
[Credit: Frederique Valentin]
"A major and not previously recognized migration must have spread the Papuan ancestry that is found everywhere in the Pacific today " said Dr. David Reich, a senior author at Harvard Medical School and at the Howard Hughes Medical Institute.

The researchers also documented how mixture between Papuans and the first pioneers of Remote Oceanian has shaped the genomes of present-day Pacific populations, from genetic diversity to ancestry proportions from archaic Denisovans.

Analysis of DNA from early settlers of the Pacific overturns leading genetic model
Wamu (L), a local elder, prepares bamboo poles to carry water from the river to the village while Daniel Watas 
carries his son and an already filled bamboo pole, on Vanuatu's southern island of Pentecost 
[Credit: Binghamton University]
"A particularly striking finding is the different ancestry observed on the X-chromosome, which is inherited mainly from females," said lead author Dr. Pontus Skoglund of Harvard Medical School and Stockholm University. "This reveals that the vast majority of the ancestry from these open water pioneers that survives today is derived from females, showing how DNA information can provide insights into cultural processes in ancient societies."

"This is the first genome-wide data on prehistoric humans from the hot tropics, and was made possible by improved methods for preparing skeletal remains" said Dr. Ron Pinhasi at University College Dublin, a senior author of the study. "The unexpected results about Oceanian history highlight the power of ancient DNA to overthrow established models of the human past."

The findings are published in Nature.

Source: Binghamton University [October 03, 2016]

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