Human Migration Out of Africa, Tracked Down by an Unwelcome Follower

Fossil proofs show that modern human species emerged in eastern Africa and around 50,000 years ago spread out of this continent.

But these first humans did not carry with them only a culture of stone tooling but also a whole array of parasites.

Some parasites were loosely connected to humans, like fleas and lice, but others have a strong evolutionary bound with the human organism, like Helicobacter pylori, a bacteria that provokes ulcers and stomach cancer.

A recent research has proven the origin of these gut parasites in the early human populations and its unwittingly spread outside Africa to Eurasia. "The evidence is really convincing," says microbiologist Mark Achtman of the Max-Planck Institute for Infection Biology in Berlin, lead investigator of an international team that made the study.

The researchers have been collecting H. pylori from the guts of human belonging to 51 ethnic groups around the world since 1999.

By the time of the genetic analysis, they had gathered 532 strains. The scientists sequenced seven bacterial DNA fragments from the DNA of the bacteria and using population genetic models they separated clusters of strains pointing genetic and geographic patterns.

East African populations presented the highest genetic diversity of H. pylori, thus in this region the bacteria lives from a longer period of time. The further a population was from east Africa, the lower was the genetic diversity of the bacterium.

These data clearly show that the bacterium appeared in early humans in east Africa, having time to store so many variants.

Moreover, the root of all the clusters was located in east Africa. On the more recently inhabited regions, fewer bacterial strains could accumulate. "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.

A vivid debate is about when did early humans go out of Africa: 100,000 years ago or in the past 60,000 years?

Computer simulations using the genetic diversity of H. pylori bias to the hypothesis of a more recent exodus, about 58,000 years ago. "The real novelty here is the timing," says population geneticist Keith Crandall of Brigham Young University in Provo, Utah.