Humans assemble a specialized microbiome from a world teeming with diverse microorganisms. Comparison to the microbiomes of great apes provides a dimension that is indispensable to understanding how these microbial communities form, function and change. This evolutionary perspective exposes not only how human gut microbiomes have been shaped by our great-ape heritage but also the features that make humans unique, as exemplified by an expansive loss of bacterial and archaeal diversity and the identification of microbial lineages that have co-diversified with their hosts.