Fundamentally different view: Gut bacteria evolved with humans

The composition of bacteria in the gut is linked to many health factors.

Fundamentally different view: Gut bacteria evolved with humans

The composition of bacteria in the gut is linked to many health factors. Researchers take a closer look at the microorganisms and divide them into family trees. When comparing them, they find surprising parallels to the development of humans in some species.

Dozens of gut bacteria have evolved in parallel with human evolution - and many of these are now highly dependent on it. This is the result of a genetic analysis in which the genome of people from Africa, Asia and Europe was compared with that of their respective intestinal flora. In the case of 36 of the 59 species of bacteria and so-called archaea examined, the evolutionary history matched that of humans surprisingly well. This is reported by an international research team led by Ruth Ley from the Max Planck Institute for Biology in Tübingen in the journal Science.

"We didn't know until now that our gut microbes have followed our evolutionary history so closely," Ley is quoted as saying in a statement from her institute. The biologist emphasizes that the results fundamentally changed the way we look at human intestinal bacteria.

It is true that individual microbes such as the stomach germ and colon cancer pathogen Helicobacter pylori were known to have been with humans since their time in Africa tens of thousands of years ago. However, the fact that so many bacteria share their developmental history with that of humans surprised the scientists. Because people's diets have changed significantly over time, populations have spread across different climatic zones around the world, and modern lifestyles could also have disrupted the composition of the intestinal flora.

The team - including researchers from the Institute for Tropical Medicine at the University of Tübingen and partner organizations in Vietnam and Gabon - first collected stool and saliva samples, including those from many mothers with their small children. In addition, there was already existing data from people in Cameroon, South Korea and Great Britain. Overall, the researchers analyzed genetic data from 1,225 people from Africa, Asia and Europe.

The researchers drew up family trees based on more than 20,000 variants in the genome. As expected, people from one continent had similar genomes indicating a common ancestry. Likewise, Ley and colleagues created family trees of 59 species of bacteria and archaea. They in turn compared this with the history of human origins.

36 species showed parallels to human evolution in their development. "It is also noteworthy that the strains that have followed our evolutionary history most closely are now most dependent on the gut environment," reports Ley. These bacteria have a reduced genome and are very sensitive to changes in temperature and oxygen levels, making it difficult to survive outside the human gut. They are also more sensitive to antibiotics than other intestinal bacteria.

"Many features characteristic of the parallel diversified species have likely adapted to the niche of an animal gut - not necessarily human," the team writes. "Whether humans have inversely adapted to these microbial species or strains remains to be investigated." The team advises that the differences in the intestinal flora of different people should be given greater consideration in forms of therapy such as stool transplantation.

In a Science comment, Andrew Moeller of Cornell University in Ithaca, New York, writes: "The results underscore that gut bacterial communities are not random collections of bacteria, but rather reflect the diverse ancestry of human populations."

Moeller sees parallels in bacteria that live in symbiosis with insects. As with the intestinal bacteria, which are heavily dependent on humans, these insect bacteria also show a greatly reduced genome, which is reduced to core sets of essential functions.

(This article was first published on Sunday, September 18, 2022.)

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