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Scientists use intestinal bacterium to track human migration to Melanesia

DNA from the H. pylori in modern human stomachs indicates that prehistoric humans lived in Australia and New Guinea for about 25,000 years first.

January 24, 2009|Karen Kaplan

Scientists have found further evidence that prehistoric humans populated Australia and New Guinea roughly 25,000 years before they migrated to the neighboring islands of Melanesia. Call it a gut feeling.

The new evidence comes from the DNA of Helicobacter pylori, a parasite that makes its home in the human gastrointestinal tract. People who live in developing countries without access to modern medicines are most likely to harbor the bacterium, which can cause ulcers and stomach cancer.

Population geneticists have previously used human DNA to trace our ancestors' migration out of Africa. DNA acquires tiny changes over time that are passed from generation to generation, and each small band of pioneers that leaves the main group develops its own distinct pattern of genetic mutations. By comparing those patterns in people alive today, scientists can reconstruct the path taken by humans as they spread out around the world.

This time, researchers focused their attention on H. pylori in people's stomachs. The bacterium was well-established in the human gut when the first explorers left Africa about 60,000 years ago. Molecular biologist Mark Achtman and his colleagues figured its evolution should mirror that of its Homo sapiens hosts.

"It was obvious that this would work," said Achtman, who has appointments at the Max Planck Institute for Infection Biology in Germany and the Environmental Research Institute at University College Cork in Ireland.

The researchers collected 212 H. pylori specimens from gastric biopsies and mucus samples provided by indigenous people in Taiwan, Australia, New Guinea and the Melanesian islands of New Caledonia. Those samples yielded 196 distinct haplotypes -- blocks of DNA that are inherited together -- that were used to track the bacteria's ancestry.

Fifty of the haplotypes belong to a family called hpSahul, common among Australian Aborigines and New Guinea highlanders. By comparing those haplotypes to versions of H. pylori in Asia, the researchers determined that humans who carry hpSahul today branched off from their mainland Asian forebears between 31,000 and 37,000 years ago.

An additional 54 of the bacteria haplotypes were from the hspMaori family, which was common among native Taiwanese, Melanesians and Polynesians in New Caledonia. The researchers figured that if the islands were populated by people from Taiwan, the hspMaori versions of H. pylori would be pervasive among native Taiwanese as well as more diverse.

Both predictions turned out to be true.

The eastward migration from Taiwan to Melanesia dates to about 5,000 years ago, the researchers found. Their results were published Friday in the journal Science.

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karen.kaplan@latimes.com

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