Reposted from ScienceNOW Daily News:
http://sciencenow.sciencemag.org/cgi/content/full/2007/207/2?etoc

Talk about following your gut. When humans first trekked out of Africa about 58,000 years ago, they carried with them stone tools, animal skins--and bacteria that cause ulcers and stomach cancer. These anatomically modern humans unwittingly had the guts to spread Helicobacter pylori into Eurasia, where they passed on the bacteria to their descendants, according to a report published online 7 February in Nature. "The evidence is really convincing," says microbiologist Mark Achtman of the Max-Planck Institute for Infection Biology in Berlin, principal investigator of an international team that traced the origins of the bacteria in modern humans.

The team has been using endoscopes to collect bacteria from the guts of humans from around the world since 1999. At last count, they had gathered 532 strains of H. pylori from people from 51 ethnic groups. After growing the bacteria in their labs, the researchers sequenced seven gene fragments from the DNA of the bacteria. Then, they used population genetics models to sort the strains into clusters that showed their genetic and geographic patterns. People from east Africa had the most kinds of H. pylori. The further people lived from east Africa, the fewer strains the researchers found. This suggests that the bacteria arose in humans in east Africa, because it would take more time to accumulate so much diversity--and because the root of all the clusters was in east Africa. Conversely, fewer people--and, thus, fewer bacterial strains--reached the hinterlands where the bacteria had less time to accumulate genetic mutations.

This finding is consistent with much other genetic evidence that modern humans originated in Africa, says Alan Templeton a population geneticist at Washington University in St. Louis, Missouri. But researchers disagree about the timing of that exodus--whether the ancestral stock of modern humans left Africa earlier than 100,000 years ago or in the past 60,000 years. In the new report, the researchers use computational simulations to date the spread of H. pylori out of Africa to about 58,000 years ago, which supports a more recent migration out of Africa. "The real novelty here is the timing," says population geneticist Keith Crandall of Brigham Young University in Provo, Utah.

The report does not address two other controversial questions--whether modern humans swept out of Africa in one or multiple migrations, and whether modern humans completely replaced the archaic people they encountered in Europe and Asia.