Jesse Bering explains:
Although increasingly glaborous (hairless) skin evolved for adaptive purposes—it may have enabled our ancestors to keep cool, for example, while traveling across the hot savanna—the sure-footed pace at which genes for depilated flesh were selected posed some cosmetic problems. Kellett and Gilbert observe that the evolution of our sebaceous glands, which were accustomed to dealing with hair-covered flesh, lagged behind this change in our appearance. As a consequence, all that oily and waxy sebum, normally committed to lubricating fur, hadn't much fur to lubricate. So the sebum started to build up and clog our pores instead.
The Dish 