An extended Tsimane family prepares for the morning, while a large pot of cassava roots cooks in preparation to prepare shocka local fermented beer.
Michael Gurven
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Michael Gurven
The Hadza people are hunter-gatherers which have roamed northern Tanzania for thousands of years.
The Tsimane indigenous people feed, fish and hunt in the Bolivian Amazon, about 7,000 miles away.
These two rural communities, separated by an ocean, may seem to have little in common. But a new study shows they share remarkable similarities when it comes to the bacteria in their guts.
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Experts say the new document in the magazine Nature suggests that this microbial kinship may have taken shape long ago, before human migration separated the ancestors of these groups.
The study, which used stool samples from both groups, also showed that the Hadza and Tsimane microbiomes were markedly different from those of city dwellers, even those living nearby.
The findings suggest that certain strains of gut bacteria have long coexisted with humans, perhaps for their mutual benefit. It’s an idea that some scientists say raises questions about the impacts of losing these microbes to modern lifestyles.
“If we were inhabited by very similar microbes over a long period of time, this suggests that our human genomes might have come to expect a certain set of microbes and functions,” says Justin Sonnenburg, professor of microbiology and immunology at Stanford University and lead author of the study. “When you start to lose biodiversity in your gut, your human biology may have to adapt and adapt to big problems.”
Other researchers unaffiliated with the journal say the hypothesis is intriguing, but not yet proven — and that there may be a reason why these microbes tend not to show up in the guts of city dwellers.
“Another alternative is that the profile of the industrialized microbiome may actually be adapted to an industrialized lifestyle,” explains André Moellerassociate professor of evolutionary biology at Princeton University. “From a health perspective, is it good or bad that industrialized humans have lost many ancestral microbial taxa (groups)? Actually, we don’t know.”
Documenting what has been preserved and lost is a step toward understanding, he says.
Microbes hitchhiking on migrating humans
Sonnenburg has previously compared the microbiomes of the Hadza to those of industrialized people. He and his colleagues wanted to compare these two communities. “They lead a lifestyle much more similar to that of our ancestors and are therefore more hospitable to microbes passed down from generation to generation during human evolution,” says Sonnenburg.
Still, the major overlap was “surprising,” he said. By comparing the genetic material present in stool samples from dozens of individuals collected by the Tsimane Health and Life History ProjectWith a previous analysis of the Hadza community, Sonnenburg and colleagues found that about 1,200 identical bacterial species lived in their guts, a nearly 90% overlap.
In a typical gut microbiome, hundreds or even thousands of species interact with each other – primarily bacteria, as well as fungi and viruses. “Microbes help us digest food in our gut, by secreting molecules that are absorbed into the bloodstream,” explains Sonnenburg. “And our blood, of course, circulates throughout our body.”
Gut bacteria can influence mood, behavior and the body’s response to infections, affecting fundamental elements of biology and health, he adds.
Microbiomes are dynamic, which means they can also change rapidly depending on the foods we eat and the medications we take.
So it’s remarkable that the Hadza and the Tsimane – isolated communities living far apart, eating different vegetables, fish and meats – have gut bacteria that are roughly the same, Sonnenburg says.
He and his colleagues believe that the common microbial profile of the Hadza and Tsimane peoples dates back to their common ancestors in Africa, before some of these humans. left the continentcrossed Eurasia and crossed a land bridge that once connected Russia with Alaska, eventually settling in the Bolivian Amazon. Later, with rising sea levels, the land bridge disappeared.
“Once the land bridge was removed, we clearly knew that it would be very difficult for them to exchange gut bacteria through traditional means of human contact,” explains Well goodtheoretical biophysicist at Stanford and co-author of the study.
Using dating techniques based on the rate at which bacteria evolve, they determined that, on average, microbial strains likely shared a common ancestor about 17,000 years ago, with some dating back much further. “This is consistent with the idea that these populations in South America, Africa and probably other parts of the world are not only acquiring microbes from their environment that fit their lifestyle, but also transmitting microbes across generations,” says Sonnenburg.
Other researchers agree that the findings support the idea that large swaths of the gut microbiome migrated with humans — and likely evolved with them, too.
“If bacteria and humans had a common history over many, many generations, there would be more opportunities for selection,” says Taichi Suzukiassistant professor at Arizona State University not affiliated with this article, who conducts microbiome research. “We would expect more potential dependencies between (co-evolved) gut microbes and our own genome, basically.”
How the microbiome develops
Babies are born with a blank microbiome and immediately begin acquiring gut bacteria.
“We first get our microbes from our mother’s vaginal canal, from our mother’s skin,” says Arizona State’s Suzuki. “Then, as we grow, we obtain microbes primarily from other individuals in the home and community.”
This human-to-human transmission occurs through the fecal-oral route – by ingesting microscopic particles of poop in the air and dust – and serves the important function of transmitting specific microbes to humans, says Moeller at Princeton. “What might make this less disgusting is realizing how tiny these things are,” he says. “They’re a micron or less, and in principle you only need one cell to start a new population.”
The gut also picks up generalist microbes that exist in the environment by ingesting food, soil and water, Moeller explains. For example, E.coli — a bacteria found in many places and which grows in the bodies of humans and many animals.
Some researchers hypothesize that microbes that co-evolved with humans might help protect human health — or at least cause less harm. For example, in one case study, those who African ancestry were found to have a much lower risk of becoming seriously ill from an African strain of H. pyloriprobably because their genomes developed alongside bacteria, compared to those of other ancestors who were prone to developing deadly stomach cancer. It’s an example that shows the potential harm of a mismatch between gut microbes and the human genome, Suzuki says.
According to the paper, about 60 percent of the gut microbes that the Hadza and Tsimane share are rare or absent in the intestines of people who live modern, industrialized lives. “It is very unlikely that the microbes that inhabit my gut are those that were passed down from generation to generation from my great-great-grandparents,” says Sonnenburg, co-author of the study at Stanford. “They are much more likely to be a product of my current lifestyle and the people I interacted with locally.”
He believes the loss of these ancestral microbes could harm human health. Although it’s difficult to prove causation, “many arrows point in the same direction, which is that our modern microbiome has changed in ways that can spread inflammation, can be unhealthy for us, and lead to inflammatory diseases,” says Sonnenburg.
Much scientific work will be needed to prove or disprove the theory. “When it comes to evolutionary questions like: Which of (these) microbes have we co-evolved with? Which ones have we lost that would be beneficial to have back? It’s still early for this set of questions,” says Princeton’s Moeller.
However, he adds, determining which intestinal bacteria have been present in humans for a long time allows us to move things forward…
Gn Health
