Rap... "This provided not only protection from the rain, but also from the sun in the hot African veldt."

I suppose... if you were walkin around with yer dick hangin out. I suggest that humans had the (big) brains to cover up. Perhaps the male human intelligence leap was brought on by cold rain and sunburn.

Or, perhaps by females saying "Put somethin over that. It's disgusting... lazin around watchin the game with yer dick hangin out you lazy bastard."