The australopithecines were the first primate to be bi-pedal, which walks on two feet. So many theories and explanations have come up with different theories regarding the evolution of primates. Dr. Lovejoy proposed one such theory in which he found a correlation between dipedalsim and pair-bonding.
Their being pair-bonded implied that males and females were involved in mating for life. Bidpealism allowed the male to go on lookout for food because they were able to do it and share the gathered food with his mate and offspring. They were also able to make tools.
A different theory explores the reason why the australopithecines adapted this trait of an erect posture and whether it has something do with the heat lost they bore due to the direct tropical sun and exposure to overheating and heat stroke. By standing and walking in such a stance the area of the body that absorb the vertical rays of the overhead sun, is reduced to only the upper parts such as head and the shoulders providing shelter to the rest of the body from the hazardous UV rays.
Most of the body is also elevated high off the ground and, hence, benefits from the cooling affects from the wind. The adaptation of bidpedalism enhances heat loss in an animal that actively moves around on the tropical savanna. It does this by not only reducing the area of the body that absorbs the heat, but by also raising the body high above the ground level where higher wind speeds increase the cooling by the evaporation of sweat.(Hewes, 1961)
Hewes, G.W.,(1961) "Food Transport and the Origin of Hominid Bipedalism" American Anthropologist, 63, 687-710.