Bacteria Crossed Continents With Us. Industrialization Derailed the Journey.

More than 1,200 species of gut bacteria live in both Hadza foragers in Tanzania and Tsimane farmers in Bolivia's Amazon. Their ancestral populations parted ways tens of millennia ago. The microbes, it appears, went with both.

The finding comes from a Stanford-led study that appeared in Nature on Oct. 7. Researchers ran the first deep genetic census of Tsimane stool samples and set it beside earlier work on the Hadza, among the last hunter-gatherer peoples on Earth. The Tsimane guts held roughly 1,400 species in total. About 90 percent of them also turned up in the Hadza.

That overlap is odd on its own. The Hadza hunt impala and porcupine and dig for tubers. The Tsimane grow plantains, manioc, rice and corn. Different menus, different continents, nearly the same residents.

To date the arrangement, the team read mutations in DNA that bacteria pass straight from parent cell to daughter cell, which pile up at a steady, clocklike rate. They also screened out the gene swapping bacteria do with one another, so recent trades would not pass for shared ancestry. For many species, the estimated split dates match the great human migrations from Africa and, later, into the Americas.

Then the uncomfortable part. Roughly 60 percent of the shared species are scarce or missing in people from industrialized countries. Earlier work by the same group counted some 750 species in a typical Hadza gut and 250 in a typical Californian one. Whether that loss feeds obesity, Type 2 diabetes and autoimmune disease, all rare in non-industrialized groups, remains unanswered.

The microbes survived the walk from Africa to South America. In industrialized guts, many seem to have dropped out within a few generations of antibiotics and processed food.

Read the full story at Stanford Medicine, October 7, 2026

Hot Take: A forest holding a third of its neighbor's species would get a formal conservation assessment. The average Californian gut gets a yogurt commercial.