Our Stomach Bacteria Reveal the History of Migrations

Editor’s Note: This article previously appeared in a different format as part of The Atlantic’s Notes section, retired in 2021.

This week, my colleague Cari wrote a fascinating piece about how the genes of living people contain traces of migrations that happened millennia ago:

Using around 2,000 DNA samples—slightly more than half from 63 locations in Europe and the Middle East, and the rest taken from another 87 locations elsewhere in the world—the researchers looked at where certain shared genetic markers fell on the chromosome to determine the date of admixing, the term for when two previously separate groups begin to merge.

For example, among northern Europeans, highest rate of admixing took place “around the late first millennium C.E., a time known to have involved significant upheaval in Europe,” while admixture between north African and southern European populations was dated to a time span “consistent with migrations associated with the Arabic Conquest of the Iberian peninsula.” But there were surprises, too. Most notably, Capelli and his colleagues discovered evidence for an influx of Mongolians into Europe that predated the reign of Ghengis Khan.

That’s very cool, and part of a trend. Other studies have also used today’s genes to reconstruct yesterday’s migrations, especially in the United Kingdom and the rest of Europe. But I find it even more amazing that you also can do this same kind of analysis by looking at the genes of our gut bacteria.

In 2009, Yoshan Moodley and colleagues at the Max Planck Institute for Infection Biology analysed various strains of the stomach bacterium Helicobacter pylori from people across Austronesia. H.pylori can cause ulcers and stomach cancers, but in most people, it does no harm. It is also a longstanding part of our bodies, having infected humans for at least 58,000 years, since the time when our species was confined to Africa. As humans moved out of that continent and colonised the globe, H.pylori came with us, changing and diversifying along the way.

So, in many parts of the world, there are uncanny similarities between the family trees of modern H.pylori strains and the human populations that carry them. If you have the former, you can reconstruct the latter. For example, Moodley found that certain strains of H.pylori, which are unique to the Pacific, originated in Taiwan, before spreading to the Philippines, New Guinea and Fiji, and Polynesia. This perfectly mirrors the order in which people arrived in the islands thousands of years ago.

That’s incredible! The genes of these organisms, which live with us but are not fully part of us, still recapitulate our globe-trotting adventures as readily as our own genes might. If all of our records and archaeological artefacts disappeared, we could still glean parts of our history from the linings of our stomachs.