Long Haired Girl

Most species evolved as the climate in Africa changed, to adjust their thermoregulation to the intense UV and sunlight at the equator, mostly by panting. Early hominids likely possessed fur similar to other large apes, but about 2.5 million years ago they developed a greater distribution of sweat glands that enabled them to perspire over most of the body. It is not clear whether the change in body hair appearance occurred before or after the development of sweat glands. Humans have eccrine sweat glands all over our body. Aside from the mammary glands that produce a specialized sweat called, milk, most mammals just have apocrine sweat glands on their armpits and loin. The rest of their body is covered in eccrine glands. There is a trend in primates to have increased eccrine sweat glands over the general surface of the body. It is unclear to what degree other primates sweat in response to heat, however.
The sweat glands in humans could have evolved to spread from the hands and feet as the body hair changed, or the hair change could have occurred to facilitate sweating. Horses and humans are two of the few animals capable of sweating on most of their body, yet horses are larger and still have fully developed fur. In humans, the skin hairs lie flat in hot conditions, as the erector pili muscles relax, preventing heat from being trapped by a layer of still air between the hairs, and increasing heat loss by convection.
Historically, some ideas have been advanced to explain the apparent hairlessness of humans, as compared to other species.
The thermoregulatory hypothesis (developed by Dr. Peter Wheeler, 1984, 1985) suggests that when human ancestors started living on the savanna, humans began sweating more to stay cool. It is posited that thick hair got in the way of the sweat evaporating, so humans evolved a sparser coat of fur. Although hair provides protection against harmful UV radiation, since our hominin ancestors were bipedal, only our heads were exposed to the noonday sun. Humans kept the hair on our head which reflects harmful UV rays, but our body hair was reduced. Opponents of the thermoregulatory hypothesis would say that losing hair added an extreme weakness to cold, but, seeing as how humans figured out cutlery around 2.6 million years ago, our ancestors easily could have found clothing within the 1.4 million years between cutting up their kills and losing their hair.