Walking to work on frigid winter mornings, I often ponder the loss of body hair that occurred as humans evolved from apes. Like every other feature of every organism, this change must have offered an evolutionary advantage. While many theories have emerged, I am inclined to believe that natural selection favored the capacity to disipate heat, leading to regression of dense hair from most of our body.
Moving from the shaded jungles to the open grasslands, early humans had to adapt to the intense sunshine while also becoming more active in their pursuit of game. A coat of dense hair would have complicated both of these lifestyle changes, increasing the risk of heat-related deaths and diminishing our ability to move swiftly over long distances. Some argue that loss of hair would increase solar-induced skin injury but early humans were dark-skinned, protected by high concentrations of melanin; on the other hand, the retention of scalp, facial and pubic hair may have served to protect areas where the skin is especially thin and sensitive.
Today, human populations that inhabit tropical regions tend to have less body hair (and darker skin) than those that occupy temperate regions of the globe, suggesting that reverse adaptation occurred as we emigrated from Africa. Humans that inhabit polar regions have also adapted to their cold environment by acquiring an endomorphic body habitus, with relatively short limbs and increased body fat. Of course, such anatomic changes develop gradually, over many generations, as natural selection acts on the human gene pool.
