A bug's-eye view of human history

Helicobacter pylori, which is associated with stomach ulcers and cancer, has been linked by genetic analysis with the major migrations, from the introduction of farming in Europe to the European colonisation of America and a thriving slave trade

A bacterium colonising the stomachs of half the world's population shows how humans colonised the world.

Helicobacter pylori, which is associated with stomach ulcers and cancer, has been linked by genetic analysis with the major migrations, from the introduction of farming in Europe to the European colonisation of America and a thriving slave trade 500 years ago.

It is generally passed down families rather than spread between people. As a result, genetic analysis of H. pylori populations shows how five distinct ancestral bacterial populations came to infect people in isolated human populations in Africa, the Near East, Central Asia and East Asia.

Subsequent migrations resulted in seven modern bug populations that are now merging due to globalisation, according to a study published in the current issue of journal Science by Dr Daniel Falush, a British scientist who works at the Max Planck Institute, Berlin, and colleagues in Germany, France and America.

The bacterium contains 50 times more variation in its genetic code than the humans who carry it, providing more insight into the evolution of mankind than the human genetic code. By comparing the bug's code and classifying the populations they infect by language, the team could reconstruct the five ancestral populations of today's bacteria present at least 10,000 years ago.

Then they could mark the traces of migrations of humans, including the arrival of Europe's first farmers 8,500 years ago, though subsequent migrations have scrambled the bug's European genome so they are "mongrels", said Dr Falush.

The analysis also shows the colonisation of Polynesia, between 4,000 and 5,500 years ago, the expansion of the Bantu population across Africa from 5,000 years ago, and that the bug was introduced to the Americas some 12,000 years ago, rather than by Europeans 500 years ago.

Distinct bacterial subpopulations evolved during the isolation that followed the Polynesian expansion to the Pacific, the Siberian migrations to the Americas and the Bantu expansion.

The European invasion of North America and the transatlantic slave trade between 470 and 150 years ago, are also revealed. In the former, native bacteria now compete with imported foreign bacteria, and in the latter similar bugs now populate people in West Africa as those in Louisiana and Tennessee.

Genetic differences in the different H. pylori populations may partly explain the substantial variation in bug virulence in different human populations and could influence the efficacy of drugs and vaccines.