© 2017 Informa UK Limited, trading as Taylor & Francis Group. Most archaeological discussions of surplus production tend to focus either on its role in the emergence and maintenance of social complexity (whether among hunter-gatherers, farming communities or incipient states) or on the enabling properties of surplus as a basis for technological advances and aesthetic elaboration. Here, we offer a rather different perspective on surplus as an initiator of communitas and driver of ethnogenesis following a period of intense socio-ecological stress, environmental degradation and localized demographic decline during the nineteenth century. The particular case study concerns the Maa-language-speaking Ilchamus community who currently occupy areas around the southern end of Lake Baringo in the Central Rift Valley, Kenya. Drawing on a combination of new archaeological evidence, oral accounts and archival sources, the paper details the processes whereby destitute groups were drawn together into acts of surplus food production, initially of grain via the implementation of a system of irrigated agriculture and subsequently of cattle through the mobilization of kinship and related ties. In so doing, disparate older identities were abandoned or transformed and a different, unifying ethnicity–Ilchamus–emerged based on a new moral economy of shared prosperity.