In 2004, a Chinese woman named Xie Qiuping won the Guinness World Record for the longest human scalp hair at 5.627 meters—the length of an adult male giraffe! While this is an anomaly, humans are the only mammals that have negligible body hair, but extremely long hair on their heads. “It's such an important part of our identity as a species and yet, we understand so little about it,” said Maksim Plikus, a developmental biologist at University of California, Irvine, who studies the regeneration of hair, according to the Scientist. Now, in a paper published today in the British Journal of Dermatology, Plikus and his colleagues—Nina Jablonski, an anthropologist at The Pennsylvania State University and Sung-Jan Lin, a dermatologist at the National Taiwan University—discuss why and how humans evolved this unique feature and speculate on the possible genetic mechanisms that drive the exuberant growth of these long and luscious locks. One theory dates back to life millions of years ago, when human ancestors roamed across Africa, exposed to the blazing sun. As they spent a lot of time outdoors searching for food and water, they needed efficient mechanisms to keep their body cool. “In human evolution access to fresh water was always something that individuals and groups had to think about and plan over,” Jablonski said. “So having a thermoregulatory mechanism that would help to conserve precious water would be advantageous.” The absence of fur and abundance of sweat glands helped keep the body cool even after bouts of exercise, such as hunting animals.2 However, the head received the most solar radiation and probably needed other ways to keep it from overheating. Prehistoric cave paintings from Europe and Australia and Egyptian mummies with preserved hair suggest that long hair was a universal feature across different populations of humans.3,4
Tightly curled and long scalp hair of ancient hominins could have evolved to shield the scalp from the sun, reduce heat transmission due to the air pockets between the curls and minimize water loss through sweat.5 While it is not known when curly hair emerged, it is likely that long hair growth evolved later to compensate for curly hair’s tendency for wear and tear.
Hair length depends on the hair growth cycle. A hair follicle can either be in the active growth phase called anagen or the rest phase called catagen.6 Stimulatory signals trigger the proliferation of matrix cells in the hair follicle during anagen, lengthening the hair strand. A continuous supply of these cells is essential for the sustained growth of hair. A typical human scalp hair follicle stays in anagen for five to seven years, producing hair that is 50–110 centimeters (19–43 inches) long. For exceptionally long hair, such as that of Qiuping, the matrix cells would have to undergo over 11,000 divisions over more than 30 years!
“Hair follicle is like a biological 3D printer. Based on the molecular program you feed it, it'll print something tiny or something super long,” said Plikus. “You don't need to add any new components. You just have to instruct the various cells to work for longer and print for longer.”
Though no other mammal can boast of the exceptional hair lengths seen on human heads, the potential for long hair growth is conserved across the class, from the wispy “mustaches" of emperor tamarin monkeys to the meter-long body hair skirt of extinct Woolly mammoths. Studies on human dermatological conditions that alter hair length could provide insights on the molecular basis and genetic regulators of long scalp hair in humans. Mutations in fibroblast growth factor 5 (FGF5), a protein that promotes catagen entry, cause familial trichomegaly, a condition where body hair and eyelashes grow unnaturally long.7 Since the mutations prolong anagen throughout the body, it is likely that the gene is differentially controlled through modifications in its regulators in the scalp cells. Contrastingly, variants of the protein Wnt Family Member 10A shorten anagen, possibly causing conditions like androgenetic alopecia, or male pattern baldness.8
Scientists have started to dig deeper in the enigma of human scalp hair. Comparative studies of body and head hair using RNA sequencing and next-generation spatial sequencing, transplantation of human hair to rodent models, constructing computational models of hair growth and examining the intricacies of hair growth in individuals could unravel the molecular drivers of human hair growth. A clearer picture of how human scalp hair reaches considerable lengths could then lead to new therapeutic leads for conditions like hair loss or genetically stunted hair.
