Surveys of humans from around the world have revealed differences in gut microbiota composition among geographically separated populations. But because humans from the same regions often share common ancestry as well as dietary and cultural habits, most studies have not been able to differentiate among the effects of heritable factors and external factors on the composition of the gut microbiota. Here we discuss how the analysis of gut microbial communities of chimpanzees residing in Gombe Stream National Park has provided an unprecedented opportunity to measure the effects of external factors while controlling for heritable factors. The differences in gut microbiota composition between separated host populations of chimpanzees are due almost entirely to external factors, with the contribution of heritable factors to intraspecific variation in gut microbiota composition being too small to detect. The dominant influence of external factors in generating differences among the gut microbiota of our closest relatives lends promise to the possibility of manipulating the composition of the gut microbiome within human hosts. These results highlight the need for controlled studies that isolate the roles of specific external factors, such as diet, cultural practices and geography, in generating differences in the gut microbiota composition.