Archie, Tung, and their colleagues used a DNA sequencing technique on fecal samples from 48 baboons in two different social groups to identify the microbes found in the animals’ guts. They discovered that an individual’s social group and social network are strong indicators of which microbes will live in that individual’s gut microbiome—even when considering the roles of diet and kin. “What was surprising was how clear of a predictor it was,” Tung says.

The Amboseli baboons make a good proxy for humans for several reasons. One is that they live on the same East African savannas where humans likely evolved. “They may provide us with a snapshot that may be as close as we can get to a very early human,” Archie says.

And because the baboons’ environments are less complex than those of humans, the researchers are able to more precisely determine the way similar microbiomes reflect levels of interaction between grooming partners. In other words, they can take a lot of confounding variables that bedevil research on humans out of the equation.