Humans have hair for several overlapping reasons rooted in evolution and biology. Here’s a concise overview.

Core purposes

• Thermoregulation and protection: hair helps insulate the body and can aid in evaporative cooling, especially on the scalp where it protects the skull from sun and heat. It also reduces friction and minor abrasions on skin surfaces.

• Sensory and signaling roles: hair, including fine vellus hair and thicker terminal hair, can enhance sensory input through the arrector pili system and hair follicle receptors, contributing to environmental awareness.

• Pheromonal and social/sexual cues: certain body hairs (e.g., underarm and pubic hair) may play a role in signaling maturity and sexual maturity, and can influence social/sexual interactions.

Why body hair is reduced compared to other mammals

• Evolution of sweating: humans developed a high density of eccrine sweat glands and relatively sparse dense body fur, enabling efficient cooling through sweating, which reduced the need for insulating fur. This shift likely aided long-distance endurance activities and thermoregulation in hot climates.

• Protective balance: while body hair decreased for cooling, scalp hair remained longer to protect the head from UV radiation and help preserve warmth in cooler conditions.

Genetics and variation

• Hair length, density, and growth cycles are influenced by genetics (e.g., distinctions in the duration of the anagen phase) and hormonal factors. Certain genetic variations can lead to unusually long or prominent hair in some individuals.

• Color and texture arise from melanin content and follicle biology, with aging affecting melanin and hair coloration.

Practical implications

• Hair care and identity: hair serves functional roles but also reflects identity and personal care choices, with styling, shaving, and grooming affecting appearance and comfort.

• Health indicators: changes in hair texture, density, or growth can reflect hormonal shifts, nutritional status, or dermatologic conditions, making hair a potential indicator of overall health.

If you’d like, I can tailor this to a specific aspect (evolutionary history, genetics, or functional biology) or provide a short, readable summary for a science presentation.