Beginning with an engagement with Kay Anderson’s recent post-humanist approach, I propose an alternative explanation for the rise of an innatist discourse of race around the mid-nineteenth century. I argue that the shift to innatist ideas of racial difference has to be seen as a matter of specific geographies of political contestation and trans-imperial mobilisation emerging in the spatial assemblages of colonial frontier zones during the early nineteenth century. I suggest that, aside from their violent struggles with indigenous peoples in these dispersed but interconnected locales, what prompted the refinement, dispersal and repetition of biologically determinist racial thought was emigrant British settlers’ mobilisation against the critique of fellow Britons inspired by a religious humanist doctrine of universality. This explanation shares Anderson’s focus on the affective realm of a colonial frontier, but it differs from her interpretation with its emphasis on the contested politics of colonial conquest and above all on the lingering political possibilities of humanism. In engaging with a specific post-humanist interpretation of innatism, this article seeks to address some of the implications of post-humanism more broadly for histories of race.