Ever try to run long distances in a fur coat?
Yup.
Dissipation of heat is the function that most conspicuously distinguishes human skin from that of all other animals. Removal of excess heat is greatly facilitated by the loss of body hair because it increases thermal conductance and permits additional heat loss through sweating.
A strong case can be made for the evolutionary loss of apocrine sweat glands in humans because these sweat glands are most common in heavily furred animals. The African apes exhibit a ratio of approximately 40% apocrine sweat glands to 60% eccrine; the great preponderance of eccrine sweat glands in modern humans probably evolved under the strong influence of natural selection, following the loss of the apocrine glands.
This process was probably propelled by increases in body size and activity levels associated with modern limb proportions and striding bipedalism.
And you very nearly nailed the title of a paper, dwise!
The Energetic Paradox of Human Running and Hominid Evolution
Current Anthropology, Vol. 25, No. 4 (Aug. - Oct., 1984), pp. 483-495
There is also this:
A naked ape would have fewer parasites
Proceedings of the Royal Society
Volume 270, Biology Letters Supplement 1 / August 07, 2003
According to this paper ...
Evolution of nakedness in Homo sapiens
Journal of Zoology 273 (1), 1-7.
... there are a dozen different hypotheses.
The cooling device hypothesis
The hunting hypothesis
The bipedality hypothesis
The allometry hypothesis
The clothing hypothesis
The vestiary hypothesis
Neoteny hypothesis
Carrion-eating hypothesis
Sex-related hypothesis
Aquatic ape hypothesis
Adaptation-against-ectoparasites hypothesis
The author kicks the snot out of all of them -- except parasites and ornamental.
As the ectoparasite burden on hominids increased, having fewer parasites may have become more important for survival than a warm fur coat. Natural selection may have started to favour shorter-haired and less parasite-ridden individuals, leading to the naked ape of today (Rantala, 1999). Selection pressure towards nudity may have been enforced by the many lethal diseases that are carried by blood-sucking ectoparasites. For example an outbreak of typhus, various forms of spotted fever, bubonic plague or any similar pandemic could have wiped out an entire fur-bearing segment of the human population (Olson, 1966).
This ”naked skin’ makes humans more vulnerable to UV radiation and to both high and low temperatures (Amaral, 1996). Furthermore, fur would protect humans from wounds, sores and insect bites, which may cause serious inflammations. A hairy individual also looks larger, which confers an advantage especially in sexual selection and in defence against predators. Thus, nakedness causes many clear costs for the naked ape.
In The Descent of Man, and Selection in Relation to Sex, Darwin argued that man, or rather primarily woman, became divested of hair for ornamental purposes and that women subsequently transmitted the sexual advantage of nakedness almost equally to their offspring of both sexes. Darwin (1871) had also collected evidence that many species had evolved features that were in themselves inconvenient or injurious, but that were retained because they were attractive to the opposite sex.
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