Long scalp hair likely evolved to help early humans regulate body temperature and communicate social signals such as age, health, and social status, according to a recent study.

In the study, published in the British Journal of Dermatology, researchers noted that this distinct human trait emerged approximately 300,000 years ago with anatomically modern humans (AMHs) and has remained common across all modern populations despite genetic variations.

The researchers, led by Lo-Yu Chang, MD, of the College of Medicine at the National Taiwan University in Taipei, Taiwan, and his colleagues, highlighted how the biological mechanism for long hair growth in humans stemmed from the prolonged duration of the hair follicle's growth phase, known as the anagen phase. In most mammals, this phase is tightly regulated, limiting hair length.

However, in humans, hair follicles appear capable of continuously recruiting progenitor cells, allowing theoretically unlimited hair growth.

"We propose a model wherein hair cycle progression into and out of anagen is regulated by evolutionary malleable molecular checkpoints," Dr. Chang and his colleagues noted, suggesting that attenuation of an existing out-of-anagen checkpoint rather than the evolution of a novel program contributed to this distinct human feature.

The study emphasized that although genetic factors determining hair length remain largely unknown, congenital conditions that affect hair length may provide insights into the molecular mechanisms responsible for this trait. The researchers indicated parallels with certain animal species and domesticated breeds that exhibit long body hair as a result of similar evolutionary mechanisms.

The findings opened potential avenues for future research, including studies on congenital disorders and somatic mosaicism conditions that feature altered hair growth. Understanding these factors may shed light on the molecular processes underpinning hair length regulation in humans.

The researchers underscored how this seemingly cosmetic trait played a potentially vital role in the evolutionary fitness of early humans, influencing not only survival but social interactions and communication.

The authors declared no conflicts of interest.