This is a book about the natural history of oxygen-free environments and their microbial inhabitants. Life originated in the pre-oxic world, and anoxic conditions still persist in many places on Earth such as lake sediments, the guts of ruminants, and the deep waters of some marine basins. The authors draw on evidence from geology, biochemistry, ecology, and evolutionary systematics to describe the forces which shape the structure, function, heterogeneity, and evolution of anaerobic communities, and also to discuss the nature of early life on Earth.