Why Does Human Scalp Hair Grow so Long?
Xie Qiuping - Guiness World Record holder for longest hair - 18’ 5.54".
First published March 14, 2025
Every creature has body covering to protect it from its environment. This covering can aid in defense, camouflage, locomotion, and sensory perception. An animal’s skin may produce structures such as hair, nails, horns, feathers, scales, and shells. Hair has four phases: growth, resting, transition, and shedding. Then, the cycle repeats itself. Certain chemotherapy inhibits the hair follicle cells and prevents the growth phase of new hair leading to scalp hair loss. Several months after chemotherapy after recovery, new growth replaces scalp hair. Sheep, angora goats, alpaca and llama grow long body hair, allowing us to harvest mohair and wool repeatedly for many uses, notably making clothing. Humans are the only species in which scalp hair continues growing to unusual lengths, requiring trimming periodically. It is estimated that there are 950,000 women’s hair salons and 150,000 men’s barbershops in the U.S. to meet the needs to trim human scalp hair. So human scalp hair growth has a significant economic impact!
Humans are the only mammals that have negligible body hair that stops growing at a short length but scalp hair that grows on average three or so feet long but with a wide variation. (Figure 1) The record for hair length is that of a Chinese woman who won the Guinness World Record in 2004 with scalp hair length of 18 feet. Cave paintings from Europe and examination of Egyptian mummies with hair indicate that long scalp hair is a feature of homosapiens. After becoming bipedal and walking erectly, human ancestors roamed across Africa where heads were exposed to the blazing sun during the daylight hours outdoors searching for food and water. One hypothesis is that dense scalp hair had air pockets that aided in cooling and protected the scalp from the effects of prolonged exposure to UV rays. They needed efficient mechanisms to keep their body cool but that would not require scalp hair growth of such length. No other mammal has the exceptional hair lengths seen on human heads although unusually long body hair growth is present in several mammals, for example the mustaches of emperor tamarin monkeys.(Figure 2)
Studies of human dermatological conditions that alter hair growth could provide insights into the molecular basis and genetic regulators of hair growth in humans. Types of alopecia (absence of scalp hair growth), male pattern baldness and trichomegaly, a condition in which body hair and eyelashes grow unusually long, are disorders that allow basic scientists and research dermatologists to explore the genetic, biochemical and physiological basis of scalp hair growth in anticipation of treating these aberrancies. Evolutionary biologists are trying to decipher the factors that led to the unique behavior of human scalp hair.
Fig. 1. Julie Manet by Renoir, 1887
Fig. 2. Tamarin monkey