Environmental pressures that advantaged certain mutations.
What kind of environmental pressures??? Pollution? Food shortages? Ivy-league college springing up?
Living on the Savannah, there was an evolutionary advantage to walking upright (you could see further) so apes with that mutation had an advantage. Living in such a warm, sunny climate gave an advantage to apes with mutations for less body hair. The presence of many predators meant that apes who clustered together and co-operated had an advantage.
You see where this is going? Bit by bit, little things like that would add up and push a certain group of apes into the human "direction."