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A gut check of human migration

January 22, 2009 | 11:10 am

    Scientists have found further proof 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 bacteria, which can cause ulcers and stomach cancer.
    Population geneticists have previously used human DNA to trace our ancestors’ migration out of Africa. They rely on the fact that DNA acquires tiny changes over time that are passed from generation to generation. Each time a small band of pioneers leaves the main group and ventures into new territory, it 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. The bacteria was already well established in the human gut when the first explorers left Africa roughly 60,000 years ago. Molecular biologist Mark Achtman and his colleagues figured its evolution should mirror that of its Homo sapien hosts.
    “It was obvious that this would work,” said Achtman, who has appointments at the Max Planck Institute for Infection Biology in Berlin and the Environmental Research Institute at University College Cork in Ireland.
    The research team gathered 212 H. pylori specimens from gastric biopsies or mucus samples provided by indigenous people in Taiwan, Australia, New Guinea and the Melanesian islands of New Caledonia in the southwest Pacific. Those samples yielded 196 distinct haplotypes –- blocks of DNA that are inherited together –- which the scientists used to track the bacteria’s ancestry.
    Fifty of the haplotypes belong to a family called hpSahul, which is common among Australian aborigines and the highlanders of New Guinea. By comparing those haplotypes to versions of H. pylori in Asia, the researchers were able to determine that the 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 part of a family called hspMaori, which was found in native Taiwanese, Melanesians and Polynesians in New Caledonia.
    The researchers surmised that if the islands were populated by people from Taiwan, the hspMaori versions of H. pylori would be pervasive and most diverse among native Taiwanese. They studied samples from members of five of the six aboriginal tribes in Taiwan and found that both of their predictions were true.
    The eastward migration from Taiwan to Melanesia occurred about 5,000 years ago, the researchers found. Their results will be published in Friday’s edition of the journal Science.

-- Karen Kaplan


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