You’re Missing Microbes. But Is ‘Rewilding’ the Way to Get Them Back?

The science behind the idea of restoring the intestinal microbiome to an ancestral state is shaky, skeptics say, and in some cases unethical.

As the sun set in Tanzania on a September evening in 2014, Jeff Leach inserted a turkey baster filled with another man’s feces into his rectum and squeezed the bulb. The feces, he said, came from a hunter-gatherer who was a member of the Hadza people and lived nearby. Mr. Leach said he was trying to “rewild” his microbiome, giving himself microbes that can protect against chronic and autoimmune diseases that plague people in Western societies — including obesity, diabetes and irritable bowel syndrome. The theory relies on the idea that people like the Hadza have diets and lifestyles that are more like those of ancient populations, and harbor such microbes. Channeling tropes that could have come from colonial era literature, Mr. Leach said the man he got the feces from “had only recently dined on zebra and monkey.” Rewilding the microbiome is now a rising area of study — combining microbiology, epidemiology and anthropology — with big money at stake. Finch Therapeutics, a microbiome start-up founded by scientists at M.I.T., recently raised $128 million in an I.P.O., even though it has no product on the market.