Just to rip something from wikipedia
Possible reasons for the evolution of human bipedalism include freeing the hands for tool use and carrying, sexual dimorphism in food gathering, changes in climate and habitat (from jungle to savanna) and to reduce the amount of skin exposed to the tropical sun. The first two explanations have been criticized [by whom?] for projecting modern social concerns and prejudices onto ancestral species. The latter two have been criticized for not making sense in the context of the forest and woodland biomes occupied by human ancestors. An alternative explanation is the mixture of savanna and scattered forests forced proto-humans to travel between clusters of trees and bipedalism offered greater efficiency for slow, long-distance travel between these clusters than knuckle-walking quadrupedism.[32]
