In Tanzania, not far from the Serengeti, live the Hadza, a community of about 1,300 people. For such a small group, they attract a lot of scientific attention.

Many of the Hadza live solely on the animals they kill, along with honey, berries and a few other wild foods. For the first 95 percent of our species’ history, there was no other way to live.

So the Hadza have been closely scrutinized for clues about the hunter-gatherer way of life: how they find their food, how much energy they use — even how much sleep they get.

On Thursday, scientists described another way in which the Hadza are exceptional. Their gut microbiome — the bacteria that live in their intestines — swings through a predictable annual cycle.