What determined the evolution of different faces in humans?
Given the prevalance of myopia in humans I would have thought that facial recognition was secondary to other indicators such as gait recognition which is also unique to individuals though can be copied to an extent. Voice and shape are also used to identify people. 

Being able to recognise individuals would allow the formation of groups, allow for structures of trust, reciprocal altruism to develop as well as punishing transgressors. Repeat interactions with other people require some kind of identification, but I dunno if facial recognition is the only/primary one.