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World traveler. The movements of Helicobacter pylori (inset) through human history.

Every Stomach Tells a Story

There's a map in your stomach. Scientists have reconstructed the ancestry of Helicobacter pylori, bacteria that live in half the human guts on Earth, and it corresponds to patterns of human migration. The discovery, published in the 7 March issue of Science, could help archaeologists confirm or revise what they know about prehistory.

Humans who want to know where they came from have, quite naturally, focused on themselves. They have unearthed their ancestors' bones and old, broken pots, deciphered the relationships of their languages, and sequenced their chromosomes in search of an answer. But organisms living inside our bodies may also hold important clues, molecular biologist Mark Achtman of the Max Planck Institute for Infection Biology in Berlin and colleagues reasoned. The team, co-led by Sebastian Suerbaum, chose to study H. pylori, because it is not easily transmitted, has 50 times the genetic diversity of humans, and exchanges genes frequently with other members of its species. Those traits mean that strains of H. pylori might be very similar in the bellies of people who have lived together for generations but very different from strains in other groups.

The researchers drew on H. pylori sequence data from 370 people in 27 populations, some from previous samples, others they collected themselves from Africans and American Indians. They plugged the sequences into an algorithm used for human population modeling, which co-author Daniel Falush modified to allow for hybridization among strains of bacteria. The model identified seven populations that closely match human migrations previously deduced from human artifacts, genes, and languages. For example, H. pylori, like humans, originally came from Africa. It went through a bottleneck during its spread to Polynesia and Micronesia, but not during the population of the Americas via the Bering land bridge. And H. pylori colonized Europe in two waves, one of them corresponding to the arrival of farmers from the Middle East. Finally, the bacteria show the traces of more recent migrations, such as European colonization and the African slave trade.

"It is great to see that what we call the standard model of human evolution gets confirmation from entirely separate evidence," says population geneticist Luca Cavalli-Sforza of Stanford University in California, the first researcher to trace human migrations with DNA. As for future studies, "one cannot exclude that in time bacteria may add completely new evidence."

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Mark Achtman's lab