Many of the bacteria that are present in our intestines are already with us even before we became humans. Findings of a new study have revealed that our gut bacteria have existed for at least 15 million years, or when we were still pre-human apes.

The new research, which involved the comparison of gut microbiomes of humans and primates, have revealed that our gut bacteria are partly determined by our evolutionary history and not just external factors such as medicine, geography and diet.

Andrew Moeller, from the University of California, Berkeley, and colleagues used fecal samples from Tanzanian chimps, Congolese bonobos, Cameroonian gorillas and some people from Connecticut to understand the evolutionary relationship between gut microbes in different ape species.

Moeller and colleagues then ran genetic tests on three different groups of bacteria that make up about 20 percent of the microbes found in the human gut: Bacteroidaceae, Bifidobacteriaceae and Lachnospiraceae.

By looking at a gene known as gyrase B, the researchers discovered that when the lineage of a common ancestor of humans and apes split into two new species, at least two of the groups of gut microbes did the same.

This suggests that our gut bacteria, which are known to influence likelihood for obesity and a range of diseases such as cancer and diabetes, have evolved with us. Just as humans share common ancestors with all other apes, the bacteria found in our intestines also share common ancestors with the microbes that the apes carry.

The researchers said this provides evidence that some human gut bacteria are the direct descendants of gut bacteria that thrived within our common ancestors with ape.

"In a way, host speciation is like continental drift: When two continents drift apart, whole biotas begin to diverge. Here, as the hosts are splitting, a good chunk of their microbiota is also splitting and diversifying," said Moeller.

The researchers were also able to date when the human and chimp bacteria split happened, which is around 5.3 million years ago. The human-gorilla gut bacteria split, on the other hand, occurred much earlier at about 15.6 million years ago.

"Divergence times of these cospeciating gut bacteria are congruent with those of hominids, indicating that nuclear, mitochondrial, and gut bacterial genomes diversified in concert during hominid evolution," the researchers wrote on their study, which was published in the journal Science on July 22.

"This study identifies human gut bacteria descended from ancient symbionts that speciated simultaneously with humans and the African apes."

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