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Hypothesized scenario for the differentiation of H. pylori into two ecospecies and subsequent global spread (not to scale). Thick grey tree represents a simplified history of human population differentiation (based on ref. 49 ). Helicobacter evolution is represented by thinner lines, which are within the grey tree during periods of evolution in humans. Green line represents H. pylori before the evolution of the two ecospecies; blue line represents Ubiquitous ecospecies; red line represents Hardy ecospecies; dotted red lines indicate that Hardy strains have not yet been detected on the branch and therefore may have gone extinct.

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