One school of thought, summarized in this NYT article, is that we humans are hairless bipeds because it allows us to dissipate heat better. In hot weather we can run farther (though not faster) than other animals because we don’t overheat. This allows us to do “persistence hunting,” where we pursue prey until it collapses from exhaustion. (This technique is sometimes also used in dating, also with less than 100% effectiveness.) Note that this is a specialized technique that is expected to work best at high temperatures, perhaps explaining why bipeds evolved in Africa. It is not likely to work for amateurs: I can’t find it any more, but I’ve read an account of a pair of runners in the West who tried to run down a horse on foot. After half a day of pursuit they thought they were starting to wear down their quarry, until it slipped into a herd of horses and they lost track of which horse they had been following and had to give up.
This article applies similar arguments about heat dissipation to show that Dumbo’s ears are necessary for body temperature management during flying.