Humans have hair on their heads primarily as a thermoregulatory adaptation and for protection. The hair on the scalp helps shield the head and brain from the sun's intense rays, preventing overheating and excess water loss, particularly important when early humans evolved under the hot sun in Africa. Hair also helps preserve body heat at night and acts as a minor cushion protecting the head from impacts. Additionally, hair on the head carries important social and sexual signals related to age, health, and status. This long scalp hair is unique compared to other mammals, as humans lost most body hair to allow for better cooling through sweating while retaining hair on the head to protect the brain and regulate temperature.
Thermoregulation
The scalp receives the most solar radiation, so hair serves as a natural protection against overheating and sunburns, helping conserve water. The hair also insulates the head, retaining heat produced by the metabolically active brain during colder temperatures.
Protection
Hair lessens the impact of minor blows to the head, offering a small physical barrier, reducing injury risks.
Social and Evolutionary Significance
Long scalp hair is a distinctive human trait selected not only for its protective function but also because it conveys key information about an individual's age, sexual maturity, and general health, playing a role in social and mating dynamics.
Biological Mechanisms
The peculiar ability to grow long hair results from a longer active growth phase of hair follicles regulated by genetic factors, unlike other mammals whose hair growth cycles are shorter.
Thus, hair on our heads is an evolutionary adaptation balancing protection, temperature regulation, and social signaling.
