Compared to most mammals, humans are relatively bald. We're certainly the most hairless of the primates. Only a handful of other mammals are as sparsely covered with body hair, including elephants and rhinoceroses. Since elephants and rhinos spend a lot of time submerged in water to stay cool, having little hair makes sense. In exchange, they have thicker skin to prevent too much heat from escaping their bodies.
Put a man next to a chimpanzee -- our closest genetic relative -- and the man's body appears much more exposed [source: National Human Genome Research Institute]. But rather than having fewer hairs, the distinguishing factor is the size of them. In fact, humans have around the same density of hair follicles across the body as other primates [source: Barlow]. The two species also share the same hairless body parts: the lips, palms and soles of the feet. Yet, human body hair is finer and shorter than chimpanzee hair.
So why, during the course of human evolution, did humans shed their thicker primate coats? At some point, thick hair must have become more of a burden than an asset, and scientists have determined a number of potential answers as to how that came about. One theory proposes that early man was a water-dwelling ape, and less hair was better suited to his aqueous environment. Another explains it as an adaptation to prevent the spread of parasites, since they thrive in thicker fur. Others point to the milestone of bipedalism that occurred around two million years ago. By standing erect instead of on all fours, humans expose only one-third of their bodies to direct sunlight [source: Barlow]. With that posture, a full fur covering wouldn't be necessary for protection from the sun.
Whatever the evolutionary trigger was, the sum of those adaptations has left humans with sporadic body hair patterns. Our heads, underarms and genitals have thicker patches, while places like the back generally have sparse coverings. Though it may seem random at first, there's a method to this mane madness.