I have to admit the question of why humankind is so hairless compared to most other mammals has crossed my mind, but it never occurred to me that Science itself would not actually know:
For most of the past century it was assumed that the problem had been solved. Raymond Dart, the anthropologist who recognised the significance of the famous Taung baby’s skull in 1924, began promoting the idea that while the apes’ ancestors stayed in the trees, our ancestors moved onto the open plains. There the males became hunters, got overheated in the chase, and shed body hair to cool down.
The problem with that theory is that no other mammal has resorted to this method of cooling down. Hair insulates animals against the sun by day as well as against the cold by night. The hominid females are not thought to have become overheated hunters, so they would merely have suffered the downsides of hairlessness - being cold at night, more prone to abrasions, and having no fur to provide a handhold for infants to cling to. Yet they ended up even more hairless than the males.
Dart’s solution, while the front-runner for more than 50 years, failed to win everyone over. In 1970, Russell W. Newman from the US Army Research Institute of Environmental Medicine argued in Human Biology that hominids could never have evolved on the plains with their “unique trio of conditions: hypotrichosis corpus, hyperhydrosis, and polydipsia”. In other words, too little hair, too much sweat, and a need to drink little but often. Newman’s paper ran counter to contemporary beliefs and was largely ignored.
William Montagna, the most indefatigable student of primate skin of his generation and then at the Oregon Regional Primate Research Center, regretfully reported in 1972 that after years of research his investigations had “failed to explain the unique feature of man’s skin - his almost complete nakedness. We are left with the major objective… unattained.”
And it keeps going. Many intelligent people and big name scientists have come up with plausible sounding theories and they all get shot down, sooner of later. The fact is, they really don’t know!
Speaking of Naked Apes, here is zoologist Desmond Morris interviewing Kate Bush on his BBC talk show in 1980: