https://phys.org/news/2018-07-diet-evolutionary-relationships-gut-microbiome.html

A common theme in the microbiome field is that host diet has large effects on the gut microbiome—both across lifespans (week-to-week changes in host diet change the gut microbiome) and across evolution (mammals with similar diets have similar gut microbes regardless of their evolutionary histories), said Katherine Amato, lead author of the study and assistant professor of anthropology in the Weinberg College of Arts and Sciences at Northwestern.

Therefore, they expected to see many similarities between leaf-eating primates, regardless of how closely related they were to each other. Rather, the researchers discovered that diet mattered much less than host evolutionary relationships in shaping the gut microbiome.