This research will explore microbial communities residing in human and nonhuman primate gastrointestinal tracts in order to understand the roles microbes have played in the evolution of humans, human ancestors, and related species. This project represents the first large-scale comparative research project centered on the roles of microbes in human evolution. With data collected as part of this project, the researchers will be able to assess whether changes in microbial communities influenced human evolution or whether humans evolved largely independently of microbial metabolism, where cultural or other innovations have played a larger role. The research will use two complementary data sets to investigate the significance of microbiomes in human and nonhuman primate diets. First, the taxonomic composition of microbes residing in twenty-four nonhuman primate species, including humans, will be described. Second, the metabolic processes undertaken by microbes in ten primate species, including humans, will be analyzed. These lines of evidence will be evaluated in light of dietary information for each species, with all major primate dietary categories represented. A broad taxonomic range of primates will be analyzed, and information about dietary changes during human evolution will be utilized. The genetic analyses of microbial samples will involve advanced and emerging sequencing and metagenomic techniques focused on two types of microbe: bacteria and archaea, a group of single-celled microorganisms that were once grouped with bacteria, but are now considered to be an evolutionarily distinct group. The main intellectual contribution of this study will be to understand the evolutionary dynamics of primate hosts and commensal microbes, particularly to determine if microbes provide benefits that hosts could not have evolved on their own. With respect to broader impacts, this research will establish an evolutionary baseline for assessing human microbial diseases. This will complement ongoing research through the Human Microbiome Project. The findings are expected to provide insights into a wide range of human gastrointestinal diseases. In addition, the project will train and develop the research capabilities of students and researchers, including those from underrepresented groups.