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Gut Bugs Divulge Human Migration Patterns in Pacific

NEW YORK (GenomeWeb News) – The Pacific islands were populated during two migrations from Asia — one that occurred more than 30,000 years ago and another about 5,000 years ago — according to a new analysis of bacteria carried in the human gut.

In a paper appearing online today in Science, an international research team looked at the genetics of Helicobacter pylori strains carried by hundreds of individuals from Taiwan, Australia, New Guinea, Melanesia, and Polynesia. They found that two H. pylori strains — dubbed hpSahul and hspMaori — correspond with distinct migrations from Asia to the Pacific. The first peopled Australia and New Guinea roughly 31,000 to 37,000 years ago. The second populated Melanesia and Polynesia from Taiwan much later, about 5,000 years ago.

"Our results lend support for two distinct waves of migrations into the Pacific," senior author Mark Achtman, a researcher affiliated with the Max Planck Institute and the University College Cork, and his colleagues wrote. "First, early migrations to New Guinea and Australia accompanied by hpSahul and second, a much later dispersal of hspMaori from Taiwan through the Pacific."

Previous studies suggest humans migrated out of Africa to Asia roughly 60,000 years ago. From there, humans moved across a region called Sundaland — an area once connected to mainland Asia that now makes up the Malay peninsula, Sumatra, Java, Borneo, and Bali. But the precise migration patterns and timing for the population of the Pacific remains murky.

To get a better understanding of this migration, Achtman and his team took advantage of the longstanding relationship between humans and the pathogenic bacteria H. pylori, a bug best known for its link to stomach problems such as gastritis and peptic ulcers.

Although many people carry H. pylori without obvious symptoms, long-term infection may also increase an individual's risk of developing stomach cancer. Even so, H. pylori colonization is relatively common: the bug inhabits the guts of roughly half of individuals around the world and even more people in places lacking widespread H. pylori treatments.

Since the bug has evolved along with human populations, the researchers speculated that they would be able to tease out human migration patterns by comparing H. pylori strains from different human populations.

"H. pylori accompanied modern humans during their migrations out of Africa," the authors explained. "Subsequent founder effects, plus geographic separation, have resulted in populations of bacterial strains specific for large continental areas."

The researchers collected gastric biopsy or mucus samples from 212 Taiwanese and Australian aboriginals, New Guinea highlanders, and Melanesians and Polynesians from New Caledonia and sequenced seven H. pylori gene fragments from these samples.

When they compared the sequences with thousands of other H. pylori haplotypes from different parts of the world, the researchers found two main biogeographic H. pylori groups in native Pacific populations: hpSahul, named for the continent Sahul that once joined Australia, New Guinea, and Tasmania, and a younger strain, hspMaori, detected in Maoris, Tongans, and Samoans in New Zealand and a few individuals in the Philippines and Japan.

Their results suggest that the hpSahul H. pylori group split off from Asian H. pylori groups between 31,000 and 37,000 years ago. This likely coincided with migrations to New Guinea and Australia. A bit later — between 23,000 and 32,000 years ago — the population split again, creating separate hpSahul sub-populations in New Guinea and Australia.

The hspMaori strain, on the other hand, appears to have split off from Asian H. pylori groups more recently. The researchers' results suggest that the humans who carried these bugs moved from Taiwan, through the Philippines and Melanesia, to Polynesia about 5,000 years ago.

These migrations are consistent with those proposed by researchers at the University of Auckland who used computational linguistic analysis to look at the population histories of Pacific islanders based on language patterns.

In a commentary in the same issue of Science, University of Cambridge archeological researcher Colin Renfrew said that the genetic work by Achtman's team and the linguistic work by the New Zealand team "mark a substantial advance in our understanding of human population history in the Pacific area."

While Renfrew noted that the timelines associated with H. pylori genetic analyses have relatively large error rates and rely on archaeological calibration points, he expressed enthusiasm about the prospect of combining genetic and linguistic data to get a better handle on human population history.

"The convergence between the approaches suggests that a synthesis between linguistic and genetic interpretations of human history may soon be possible on a worldwide basis," Renfrew wrote.

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