Despite its importance in understanding the social relations of labour little attention has been paid by Western Marxists to evolutionary theory. Taking as a starting point an unfinished essay by Engels, the author argues that the human species must be seen as discontinuous with its nearest biological ancestors – that a qualitative distinction was brought about by social labour. It is argued that the most likely forms of human organization were co-operative and field studies are discussed which apparently provide evidence for tool use and linguistic ability among the higher primates. The relationship between hand and brain in terms of Marxist psychology is also elaborated.