..models that suggest humans started walking upright as a means to cool off aren’t looking at the whole picture because in addition to the metabolic heat generated, the cooling effect gained in losing body hair would be much more profound in a person walking upright, than would be the case for one who remained crouching.
Thus, they argue, it was only after hominins started walking upright that they began losing their fur, though they do agree that the latter change came about as a means of keeping cooler.
Read more at: http://phys.org/news/2011-12-early-humans-lost-fur-bipedalism.html#jCp