Evolution: What's on the menu?
food substrates has evolved from interactions with the trillions of microorganisms that co-exist in our gut. But what evolutionary factors shape the diversity of gut microbial communities? Reporting in Science, Ley, Gordon and colleagues now provide us with a microbial view of mammalian evolution with their finding that mammalian diet, phylogeny and gut physiology influence the composition of the gut microbiota and that microbial communities have co-diversified with their hosts. Gordon et al. conducted a census of the faecal bacterial communities of 59 different mammalian species living in zoos in San Diego and St Louis, USA, and in the wild, including 17 non-human primates and animals that had an unusual diet for their taxonomic lineage. New network-based analyses together with more traditional tree-based analyses were then used to relate gut community ecology to the mammalian hosts that harbour them. In general, the gut microbial composition of members of a single mammalian species was similar regardless of its provenance, supporting the notion of inter-generational transfer of gut communities. The ability to use plants as a food source evolved repeatedly from ancestral carnivorous mammals. Moreover, most extant mammals are herbivores. Based on 16S ribosomal RNA, the gut microbiota of herbivores in this study was found to be more diverse than that of omnivores and carnivores, which suggests that diet is an important determinant of microbial community structure. The authors’ work indicates that promiscuous gut microorganisms allowed unrelated mammals with similar gut structures to assemble similar microbial communities and that the microbial solution to herbivory in different gut structures (for example, a simple gut, a hindgut or a foregut) was similar regardless of host lineage. Humans seem to be representative of typical omnivores and can be placed together with our omnivorous primate relatives. This bottom-up perspective of animal evolution implicates gut microorganisms as co-conspirators in the spectacular evolutionary success of mammals. According to Gordon and colleagues, ‘‘our next task is to characterize the microbial communities’ gene content (microbiomes) to better understand how these observations relate to the metabolic capacities of the microbiota and expand these studies to other members of the animal kingdom.’’ Gillian Young