FECAL METHANOGENS AND VERTEBRATE EVOLUTION

It has been assumed that the feeding habits of vertebrates predispose the variety of intestinal differentiations and the composition of the microbial biota living in their intestinal tracts. Consequently, the p resence of methanogenic bacteria in the various differentiations of th e large intestine and the foregut of herbivorous vertebrates had been attributed primarily to the existence of anaerobic habitats and the av ailability of carbon dioxide and hydrogen originating from the ferment ative microbial digestion of plant-based diets. However, Australian ra tites, many murids, and several New World primates lack methanogens, d espite their intestinal differentiations and their vegetarian feeding habits. Crocodiles, giant snakes, aardvarks, and ant-eaters on the oth er hand release significant amounts of methane. A determination of met hane emissions by 253 vertebrate species confirmed that competence for intestinal methanogenic bacteria is shared by related species and hig her taxa, irrespective of different feeding habits. In ''methaogenic'' branches of the evolutionary tree, a variety of differentiations of t he large intestine evolved and, in some cases, differentiations of the foregut. In contrast, the lack of competence for methanogens in chiro pterans/insectivores and carnivores apparently has precluded the evolu tion of specialized fermenting differentiations of the digestive tract . Our observations reveal that the presence of intestinal methanogenic bacteria is under phylogenetic rather than dietary control: competenc e for intestinal methanogenic bacteria is a plesiomorphic (primitive-s hared) character among reptiles, birds, and mammals. This competence f or methanogenic bacteria has been crucial for the evolution of the amn iotes.