Friday, May 18, 2018

When farmers migrated to southeast Asia, according to the DNA

By analyzing genome-wide DNA from the remains of ancient Southeast Asian individuals, scientists have shed new light on the past 4,000 years of genetic history from the region. Their analysis helps reveal potential migration events that may have led to the region's rich cultural and linguistic diversity observed today.

Southeast Asia has a complex history of human occupation, yet studying this history through genetics has remained a challenge because, for one, its humid and tropical environment presents unfavorable conditions for DNA preservation. As such, hypotheses based on archaeological findings - predicting the human origins of cultural shifts (like the use of tools and pottery) brought on by the introduction of farming, for example - remain unresolved by genetic studies.

Here, Mark Lipson, David Reich and colleagues obtained DNA data of 18 ancient Southeast Asian individuals from the Neolithic period to the Iron Age (4,100 to 1,700 years ago), found in modern-day Vietnam, Myanmar, Thailand and Cambodia.

With additional support from archaeological and linguistic data, the researchers identified two major waves of genetic mixture indicative of specific migration events. The first wave, occurring during the Neolithic period, consisted of early farmer migrants from South China who mixed with local Southeast Asian hunter-gatherers.

A couple thousand years afterwards, during the Bronze Age, an additional "pulse" of genes flowed from China to Southeast Asia, reflecting another influx of farmers, the researchers say. Interestingly, the pattern of human movement in Southeast Asia parallels that observed in Europe, where ancient DNA studies have also revealed genetic mixture associated with transitions in agricultural culture.


An international team led by researchers at HMS and the University of Vienna extracted and analyzed DNA from the remains of 18 people who lived between about 4,100 and 1,700 years ago in what are now Vietnam, Thailand, Myanmar and Cambodia.

The team found that the first migration took place about 45,000 years ago, bringing in people who became hunter-gatherers.

Then, during the Neolithic Period, around 4,500 years ago, there was a large-scale influx of people from China who introduced farming practices to Southeast Asia and mixed with the local hunter-gatherers.

People today with this ancestry mix tend to speak Austroasiatic languages, leading the researchers to propose that the farmers who came from the north were early Austroasiatic speakers.

"This study reveals a complex interplay between archaeology, genetics and language, which is critical for understanding the history of Southeast Asian populations," said co-senior author Ron Pinhasi of the University of Vienna.

The research revealed that subsequent waves of migration during the Bronze Age, again from China, arrived in Myanmar by about 3,000 years ago, in Vietnam by 2,000 years ago and in Thailand within the last 1,000 years. These movements introduced ancestry types that are today associated with speakers of different languages.

The identification of three ancestral populations--hunter-gatherers, first farmers and Bronze Age migrants--echoes a pattern first uncovered in ancient DNA studies of Europeans, but with at least one major difference: Much of the ancestral diversity in Europe has faded over time as populations mingled, while Southeast Asian populations have retained far more variation.

"People who are nearly direct descendants of each of the three source populations are still living in the region today, including people with significant hunter-gatherer ancestry who live in Thailand, Malaysia, the Philippines and the Andaman Islands," said Reich, professor of genetics at HMS and co-senior author of the study. "Whereas in Europe, no one living today has more than a small fraction of ancestry from the European hunter-gatherers."

Reich hypothesizes that the high diversity of Southeast Asia today can be partly explained by the fact that farmers arrived much more recently than in Europe--around 4,500 years ago compared with 8,000 years ago--leaving less time for populations to mix and genetic variation to even out.
The new findings make it clear that the multiple waves of migration, each of which occurred during a key transition period of Southeast Asian history, shaped the genetics of the region to a remarkable extent.

"The major population turnover that came with the arrival of farmers is unsurprising, but the magnitudes of replacement during the Bronze Age are much higher than many people would have guessed," said Reich.

Also unexpected were the linguistic implications raised by analyses of the ancestry of people in western Indonesia.

"The evidence suggests that the first farmers of western Indonesia spoke Austroasiatic languages rather than the Austronesian languages spoken there today," Reich added. "Thus, Austronesian languages were probably later arrivals."

Additional samples from western Indonesia before and after 4,000 years ago should settle the question, Reich said.

No comments: