The evolution of pattern hair loss – androgenetic alopecia in humans

Call it what you will, androgenetic alopecia, androgenic alopecia, pattern hair loss, or pattern baldness, it has been a part of the human race for as long as we have historical records. Evolutionary evidence suggests androgenetic alopecia (AGA) has been around longer than the modern human race. Our non-human primate relatives, chimpanzees, macaques, and gorillas, can also develop pattern hair loss caused by androgen hormones.

Pattern Hair Loss Development Mechanism: Androgenetic alopecia develops as a gradual reduction of scalp hair follicle size, and reduced time in the anagen active growth phase, leading to more hair follicles in the telogen resting stage of the hair cycle. In men, the hair loss is limited to the top of the head and can involve thinning and/or receding hair lines. In women the presentation is different with just diffuse thinning over the top of the head and sometimes thinning over the entire scalp. For both men and women, these patterns of hair loss are due to the effect of androgen hormones. Hormonal activity, and the response of hair follicles to the hormones, is coded for in our genes. While the environment has a big part to play in the development of pattern hair loss, we now know that genetics, and the genes we inherit from our parents, codes for the susceptibility to pattern hair loss development.

Evolutionary Speculation: Androgenetic alopecia is a very common form of hair loss and could be described as part of our general genetic phenotype, but the obvious question is how did it become embedded in our species? From an evolutionary standpoint, the reasons for the high prevalence of androgenetic alopecia in humans remain a subject of debate, and we are unlikely to find an answer with any great certainty. However, there is some evidence, and a lot of speculation, that AGA might reflect certain advantages for the affected individual.

Observations in Non-Human Primates: In several primate species, characteristics associated with aging, such as graying or thinning of hair, can be indicators of social rank, experience, and authority. Older, more senior males in primate groups often exhibit physical signs of aging, which may include pattern hair loss, and these signs can correlate with their status and dominance in the group. It’s not clear whether developing pattern hair loss then allows the affected individual to gain a more senior ranking, or rather, those males that develop a more senior ranking tend to be older and consequently are more likely to have developed pattern hair loss.

Pattern Hair Loss and Androgen Function: In non-human primates, those individuals that develop pattern hair loss tend to be larger and more dominant. Potentially, greater androgen activity leads to an increase in size, including bone and muscle mass, as well as pattern hair loss development. Physical size and strength does confer certain advantages in finding food, or indeed stealing it from weaker individuals, as well as defending against competing groups of primates. Consequently, these physical attributes may make males with pattern hair loss more desirable as mates. For females too, any presence of pattern hair loss tends to be associated with a larger size, and strength is an important attribute in survival, both for the individual and for their social group.

Social Signaling in Non-Human Primates: While the loss of hair on the head would provide a permanent form of visible “signal”, some research articles go one step further and suggest the loss of scalp hair in an angry, older dominant male would flush red and look very aggressive during a confrontation with another male. As such, pattern hair loss might actually be a way to accentuate the aggressiveness of dominant males and help to intimidate any challengers in a social group to back down. It could also provide an additional advantage during fighting in that there would be less hair for any opponent to get hold of.

Social Signaling in Ancestral Human Societies: Some theories suggest that androgenetic alopecia might also have had similar social signaling functions in ancestral human societies. They suggest that pattern hair loss could have been a signal of maturity, experience, or social status. In many cultures, signs of aging are associated with wisdom and respect, which might have had an advantageous social or reproductive value in the past. Considering that in ancestral human societies, the majority of people would die “young” by today’s standards, most males did not live long enough to develop significant pattern hair loss. Those that did would have stood out among the generally younger population, noted and respected for their longevity and the experience that comes with it. Potentially their other visible attributes, including hair loss, might have been seen as indicators of long term survival abilities.

Sexual Selection for Pattern Hair Loss: One hypothesis suggests that AGA may have developed as a result of sexual selection in humans. Baldness could have been a trait that signaled maturity and social status which might have been attractive to potential mates. This theory posits that, in ancestral human societies, signs of aging could have been associated with social dominance and wisdom. In turn, as the alopecia increased their desirability as mates, males would confer their longevity – and pattern hair loss – on their offspring. Over time and many generations, the genes for pattern hair loss may have become embedded, and even dominant, in some ancestral human populations.

Thermoregulatory Adaptation: Another theory proposes that hair loss in humans evolved as a thermoregulatory adaptation. The reduction of hair on the scalp, particularly in males who were more exposed to the sun in hot climates, could have helped in heat dissipation. In turn, this may have made those affected individuals more effective hunters, better able to persist in chasing prey over longer distances for greater periods of time. This improved ability to provide food might allow them to live longer and make more desirable as mates. However, this idea seems more questionable given that hair loss does not generally begin to be significant in many men until they reach their 30s and older – usually the time at which most males would die in ancestral human societies. Similarly in non-human primates, extensive hair loss is not really apparent until the indivdual is relatively old.

Genetic Mutations and Drift: Some genetic traits become prevalent not because they offer a survival advantage, but rather due to “random genetic drift”. AGA might have become widespread in ancestral human populations through such a process, without necessarily providing any distinct evolutionary advantage. Alternatively, the emergence of AGA in humans could also be attributed to random mutations in genes related to hair growth and androgen metabolism. Once these mutations occurred, they may have been passed down through generations, neither conferring any great advantage or disadvantage.

Caveats and Context: It’s important to note that while these observations and theories may be intriguing and compelling, they are not universally accepted or proven. The interpretation of pattern hair loss in evolutionary and social contexts is complex and influenced by many factors, including environment, genetics, and social structures of specific ancestral societies. Additionally, the manifestation of pattern hair loss in non-human primates and its exact parallels to human AGA need to be studied more extensively to draw firm conclusions. The genetic and hormonal mechanisms underlying androgen induced hair loss in different species may have similarities, but they also have distinct differences.

Conclusion: In summary, the genetic history of AGA in humans likely involves a combination of several factors, including sexual selection, thermoregulatory adaptations, random genetic drift, and the evolution of androgenic functions in our ancestors. The precise reasons for its prevalence and maintenance in human populations remain a subject of ongoing research and speculation. Understanding this evolutionary history of pattern hair loss not only sheds light on the condition itself but also offers broader insights into human biology.

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