One of our oldest companions is a microbe called Helicobacter pylori. It has been colonising our stomachs, and co-evolving with us, for the past 100,000 years. It was with us when were still confined to Africa, and came along for the ride as we spread throughout the world. You can actually trace how humans spread through places like the Pacific islands by comparing the strains of H.pylori in the stomachs of their modern descendants.
H.pylori is the main cause of stomach cancer. But most people who carry the bacterium—that is, half the world’s population—suffer no ill effects, except for some low-grade inflammation. This may be because we have such a long history of co-evolution with this microbe. We have come to a peaceful understanding with one another.
But in some parts of the world, this relationship breaks down. When the European conquistadors came to Central and South America, they brought their own strains of H.pylori with them, and many of these replaced the strains carried by native Amerindians.
This historical mismatch still persists today, and could be seriously affecting the health of modern South Americans.
By studying people from two Colombian town, Tuquerres and Tumaco, a team of scientists led by Pelayo Correa has found that when H.pylori strains share a different ancestry to their hosts, they are more likely to cause cancerous stomach lesions.
According to their study, these incompatibilities between humans and H.pylori largely explains why the two towns have 25-fold differences in their stomach cancer rates, even though they are just 200 kilometres apart.
I’ve written about this fascinating story for Nature News, so head over there for the details.