The introduction explains the various critiques of the category of the human that have been forwarded by posthumanist scholars. It observes that these trenchant critiques have also deprived theorists of a category of the human to invoke in the effort to mobilize people to respond to global climate change and environmental degradation. The chapter suggests that implicit in the critiques of the category of the human are an idea of the human as animal, dependent, embodied, and material. It observes that these ideas are similar in kind to those found in developments in science studies and contemporary biology in which organisms and habitats are conceived as mutually constituting. It proposes that an exploration of the critiques in combination with the scientific developments could provide a basis for an alternative theory of humans as biocultural. The chapter addresses several concerns about using science in humanistic inquiry and recounts the author’s interdisciplinary training in molecular and cellular biology. It then lays out a detailed map of the book’s argument.