Getting ‘back to nature’ is the premise of a range of physical cultural practices that have become popular and profitable in the past decade. Mud running, CrossFit, and the Paleo Diet each call back to premodern ideals about nature, the body, and society, championing the naturality of functional fitness, the primacy of pre-agricultural diets, and the primordiality of mud as antidotes to the ills of modern life. This essay names ‘back to nature’ fitness and argues that it emerges as part of a proliferate social discourse concerning what it means to be human, at a moment when the status of the human is being called into question by environmental uncertainty. ‘Back to nature fitness’ evokes a nostalgia for a state of premodern innocence, mounts a critique of modern sedentariness, ‘abominable’ foods and other ‘sins’ of civilisation, and suggests that purity can be rekindled through ascetic, embodied practice. I argue that this shared philosophy is underpinned by a modernistic understanding of human history, one beset by troublesome dualisms yet strengthened by the symbolic power of the human body to act as a proxy for ‘nature’ itself. More broadly, I show why sociologists should beware this physical cultural primitivism in attempts to analyse the complexity of nature in physical culture, and suggest impurity to be a more accurate and ethical mode of understanding body-environment relations.