Helicobacter pylori, a Gram-negative bacterium resident in the human gut that can cause ulcers, occurs in several forms that differ genetically and geographically. In the 7 March Science, Mark Achtman and colleagues at the Max Planck Institute for Infectious Biology, Berlin, Germany, show that sequence differences among extant H. pylori populations correlate with large-scale human migrations — both ancient and recent — and may help resolve questions about human movements through time (Science 299:1582-1585, March 6, 2003).

Achtman et al. sequenced eight genes from 370 strains of H. pylori, isolated from 27 geographical, ethnic, or linguistic groups throughout the world, and using a computer program, derived the most likely ancestral tree. Their model identified seven modern populations and subpopulations: three in East Asia, one in Europe, and three in Africa. A West African strain was also found at high frequency among blacks in Louisiana...

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