Charles Darwin’s Descent of Man (1871) explained human evolution in terms of kinship between human and animal species, and its publication sparked a period of intensified interest in the “human animal.” As critics and historians have noted, many saw this kinship as a marker of degeneration, a new fall inaugurating civilization’s decline. My project, in contrast, considers the potential this new formulation offered for reimagining what it meant to be human, particularly at a time marked by rapid social and technological change. Curiosity about the human animal expressed itself in the development of new sciences and in new literatures, from the literary naturalism of Jack London to the proto-science fiction of Mark Twain. Reading US novels, newspaper articles, and scientific texts together, I uncover the unstable, transgressive, and ultimately creative connections being made between the human and the animal at the turn of the century.