Unraveling the Evolution of Humans' Long Scalp Hair: Insights from the Biomedical Engineering Research

2025/4/2

"What makes humans human?" This timeless question continues to inspire scientific inquiry and is yet to be answered in full. With the advancement of biological anthropology and evolutionary biology, a valuable research window has opened for studying human-specific apomorphies, enabling researchers to analyze the differences between humans and their primate relatives. Such traits include bipedalism, expanded brain capacity, and the ability to use complex tools. Among these, one notable feature has often been overlooked: long scalp hair. Lo-Yu CHANG , a medical student under the guidance of Professor Sung-Jan LIN of NTU's Dept. of Biomedical Engineering, conducted a multinational collaborative study titled "Evolution of Long Scalp Hair in Humans" and the results were published in the British Journal of Dermatology in January 2025. The study analyzed the characteristics of humans' long scalp hair and proposes that this trait likely evolved with the emergence of anatomically modern humans (AMHs). At that time, human hair should have been long and tightly curled, as commonly seen in modern African populations. Such hair is advantageous for thermoregulation as it prevents body temperature from rising sharply under the sun. Later, straight hair evolved only after AMHs migrated out of Africa. This article also proposes potential regulatory mechanisms for controlling hair length in different body regions and long scalp hair, offering directions for future research.

Compared to long hair on the scalp, human body hair has undergone miniaturization in the opposite direction. Scientists speculate that the reason scalp hair was preserved through natural selection is its advantage in protecting the head from strong direct sunlight. Additionally, experiments have shown that tightly curled hair can effectively shield the scalp from direct sunlight, thereby preventing excessive increases in skin temperature. This extremely curly form represents the earliest pattern of human hair growth. It is preserved in African populations but disappeared after humans migrated out of Africa. Unlike most mammals, whose hair growth is tightly regulated in length—except for animals selectively bred by humans—human scalp hair can grow to extraordinary lengths. Hair length is primarily determined by the duration of the anagen (growth) phase of the hair follicle. In humans, this phase typically lasts 5 to 7 years, which is much longer than in other animals. Although hair growth characteristics differ among various modern human populations, the significantly prolonged anagen phase is a synapomorphy, or shared evolutionary trait. This suggests that long scalp hair was already present when AMHs appeared 300,000 years ago. Over time, it became an important medium for expressing health and social status, which serves as an example of evolutionary exaptation.

The hair growth cycle consists of the anagen (growth) phase, catagen (regression) phase, and telogen (resting) phase. The molecular mechanism behind the trait of human long hair stems from changes in how the anagen phase is regulated—specifically, a weakening of the mechanism that terminates the growth phase. This allows hair follicles to avoid entering the catagen phase and to prolong the anagen phase nearly indefinitely. Human hair grows about 1.5 cm per month. According to current records, a person who has not had a haircut for 30 years can grow hair exceeding 5.7 meters in length. However, the specific genes and molecular regulatory mechanisms involved are not yet fully understood. It is speculated that the cause is not a single gene mutation, but rather a change in the gene regulatory network related to body-region specificity, enabling scalp hair to grow without limit. In recent years, research into rare hereditary hair disorders and paleogenomics has progressed significantly, offering opportunities to enhance our understanding of the mechanisms behind human hair growth. Studying the biology of long scalp hair not only helps illuminate the mystery of human evolution, but also provides new clues for treating hair-related conditions such as alopecia (baldness).

To access the full text, please visit: https://doi.org/10.1093/bjd/ljae456.