Study suggests gut microbes followed migrating humans across continents

Stool samples from both groups showed around 1,200 of the same bacterial species living in their guts, a near-90% overlap, according to the study. The finding suggests that this microbial kinship may have formed long ago, before the ancestors of these groups split during human migration. Both the Hadza and Tsimane microbiomes were also markedly different from those of city dwellers, including people living nearby, the study showed.

Justin Sonnenburg, a professor of microbiology and immunology at Stanford University and the study's senior author, said the major overlap was "startling." He said the two communities live a lifestyle much more similar to that of human ancestors and therefore more hospitable to microbes handed down over human evolution. "If we were inhabited by very similar microbes over a long period of time, it suggests that our human genomes may have come to expect a certain set of microbes and functions," Sonnenburg said. "When you start to lose biodiversity in your gut, there may be big issues for your human biology to adjust to and accommodate."

The gut microbiome typically contains hundreds to thousands of species, mostly bacteria along with some fungi and viruses, according to the article. Sonnenburg said the microbes help digest food in the gut and secrete molecules that are absorbed into the bloodstream. Gut bacteria can influence moods, behavior and the body's response to infections, affecting fundamental parts of biology and health, he added. Microbiomes are dynamic and can change quickly depending on foods and medicines, making the similarity between the far-apart Hadza and Tsimane especially remarkable, Sonnenburg said.

The researchers think the shared microbial profile goes back to common ancestors in Africa before some of those humans left the continent, passed through Eurasia and crossed a land bridge that once connected Russia with Alaska before eventually settling in the Bolivian Amazon. Later, as sea levels rose, the land bridge disappeared. Ben Good, a theoretical biophysicist at Stanford and a study co-author, said that once the land bridge went away, it was going to be very hard for the groups to exchange gut bacteria through traditional human contact means.

Using dating techniques based on the rates at which bacteria change, the team determined that on average the microbial strains likely shared a common ancestor about 17,000 years ago, with some dating much further back, according to the study. Sonnenburg said that is consistent with the idea that populations in South America, Africa and probably other parts of the world are not just acquiring microbes from their environment that match their lifestyle, but transmitting microbes over generations.

Other researchers not affiliated with the paper said the hypothesis is intriguing but not yet proven, NPR News reported. Andrew Moeller, an associate professor of evolutionary biology at Princeton University, said another possibility is that the industrialized microbiome profile may actually be adapted to an industrialized lifestyle. "From a health perspective, is it good or deleterious that industrialized humans have lost a lot of ancestral microbial taxa? We actually don't know," Moeller said. He said documenting what has been kept and lost is a step toward figuring that out.

The study relied on stool samples from dozens of Tsimane individuals collected by the Tsimane Health and Life History Project and previous analysis from the Hadza community, according to NPR News. The findings suggest that some strains of gut bacteria have long coexisted with humans, possibly to their mutual benefit, and some scientists say they raise questions about the impacts of losing those microbes to modern lifestyles.

Editor's Summary The study in Nature found that the Hadza of Tanzania and the Tsimane of Bolivia share nearly 90% of the same gut bacterial species, suggesting gut microbes traveled with migrating humans and were transmitted over generations. The research raises questions about what industrialized lifestyles may have changed in the gut microbiome, though outside researchers say the health consequences remain unproven.