In the prehistoric green Sahara of Holocene North Africa-in contrast to the Neolithic of Europe and Eurasia-a reliance on cattle, sheep and goats emerged as a stable and widespread way of life, long before the first evidence for domesticated plants or settled village farming communities(1-3). The remarkable rock art found widely across the region depicts cattle herding among early Saharan pastoral groups, and includes rare scenes of milking; however, these images can rarely be reliably dated(4). Although the faunal evidence provides further confirmation of the importance of cattle and other domesticates(5), the scarcity of cattle bones makes it impossible to ascertain herd structures via kill-off patterns, thereby precluding interpretations of whether dairying was practiced. Because pottery production begins early in northern Africa(6) the potential exists to investigate diet and subsistence practices using molecular and isotopic analyses of absorbed food residues(7). This approach has been successful in determining the chronology of dairying beginning in the 'Fertile Crescent' of the Near East and its spread across Europe(8-11). Here we report the first unequivocal chemical evidence, based on the delta C-13 and Delta C-13 values of the major alkanoic acids of milk fat, for the adoption of dairying practices by prehistoric Saharan African people in the fifth millennium BC. Interpretations are supported by a new database of modern ruminant animal fats collected from Africa. These findings confirm the importance of 'lifetime products', such as milk, in early Saharan pastoralism, and provide an evolutionary context for the emergence of lactase persistence in Africa.