Despite clothing themselves in animal fur, our ancestors would never have survived unless simultaneously they had acquired a mechanism to maintain their core temperature constant (at around 96 degrees Fahrenheit) irrespective of the fluctuating temperatures — from blazing heat to freezing cold — of the external environment. That system involves a vast network of blood vessels beneath the skin whose dilation or contraction directly affects the loss (or conservation) of heat within the body — supplemented by the powerful heat dissipative effects of the copious secretions produced by the sweat glands. “Those examining the skin of other animals will look in vain for a comparable labyrinthine system of blood vessels,” observes the biologist William Montagna.
James Le Fanu in The Oldie, February 2013
The article concludes that human nakedness remains an enigma.
Notional advantages are: (a) the visible display of absence of skin parasites that fur might hide from view; (b) the streamlining effect (think dolphins, seals) in the event that we took a million-year diversion as aquatic apes during our evolution from forest-swingers to bipedal savannah-rangers.
Disadvantages include: (a) less flexible thermal regulation compared with fur, which deflects the heat of the sun in the day and retains heat by insulation for the cold of the night; (b) oddity of the grubby tangles of hair still sprouting from various landmarks on the human body which would seem to favour the opponent or predator in a physical struggle.
Darwin suggested sexual selection pure and simple, a sort of quirk of fashion by which our ancestors came to favour increasing sparseness of fur, perhaps as a marker of distinctness from the hairy host of other animals they shared their environment with.