Asa general rule, the larger a social group of primates, the more time its members spend grooming one another  The main purpose of this social grooming is the maintenance of social cohesion  Furthermore, group size among primates tends to increase proportionally with the size of the neocortex, the seat of higher thought in the brain  Extrapolating upon the relationship between group size and neocortex size, we can infer that early human groups were quite large
But unexpectedly, there is strong evidence that, apart from parents grooming their children, these humans spent virtually no time grooming one another
Many types of primates other than humans have fairly large neocortex regions and display frequent social grooming.