Hair exclusively on the top of the head has become one of the defining characteristics of being human. As such a visible trait, it raises questions as to how and why it evolved this way. It is a long story, with questions of convergence and unique population stressors, but there is not a lot of research into the specifics, especially compared to other traits such as skin pigmentation. What we do know, so far, is that modern hair can tell a story of adaptation, gene flow and sexual selection.
Homo erectus, 2 million years ago, had roughly the same body size and shape as modern humans, except for the brain. At some point during the following one million years, something happened that meant cranial capacity could expand hugely, paving the way for the larger brained specimens we are today.
Many arguments have been put forward as to why; nutrition, fire, all may have played a part in the expansion of brain size, but one which is not covered as frequently is hair on the scalp.
Running a large brain requires a lot of energy and produces extra heat in the head. Add to this the heat of the sun in Africa, and early humans were beginning to overheat. Sweat glands offered a way to cool down the body, but it came at the high price of water and precious electrolytes. Natural selection’s options were to restrict the size of the brain, which is what it had been doing so far, or find a new solution.
The solution it found was curly hair. Scientists have examined the functionality of hair, especially curly hair, and found that it helps a great deal with thermoregulation. Having tight curls protects from the sun’s radiation and will keep the head cooler than if the head were bald. Straight and moderately curly hair protects as well, but not as efficiently as tight curls. They found that it was the layer of air between the coiled hair and scalp which created an extra layer of protection.
And these coils not only protect from radiation, but minimise the amount of sweating an individual must do to cool to the same degree. In fact, bald men are found to sweat twice as much on their head to make up for the lack of hairy protection. In an environment where the sun is strong and water a precious resource, tight curls offer the perfect solution to overheating.
With the evolution of tight curls of hair on the top of the head, the brain was protected from overheating and could continue to create the extra heat as the brain became larger. It also allowed humans to forage in the midday heat, unlike the predators of the time.
We don’t know for sure the order of evolution here. Did curly hair occur after the loss of body hair, or did it evolve at the same time? Did it appear once, or convergently in multiple populations? Hopefully one day, more research into the evolution of hair can give us these answers.
A few irregularities do crop up in the history of humans though. If having hair on our scalps is so important to thermoregulation, why is male baldness so prevalent? It is genetic and hair loss occurs in half of the male population by the time they are 50 years old. The answer may be more hair. The presence of beards may account for the loss of scalp hair. With both in place, thermoregulation becomes harder and overheating is a risk. By removing one section of hair, the head is back to its steady cooling. According to a study which compared balding rates in different ethnicities, white (British and Irish) men had the highest rates of complete balding, which makes sense in a land where radiation from the sun is not as strong and overheating due to the sun is less of an everyday issue.
Across the globe, hair now exists in more than just tight curls, it ranges in shape, length and colour, and can be loosely tracked through populations. Gene flow contributes heavily to this distribution, but mutations have led to a few very interesting instances.
In the Solomon Islands, a single random mutation created blonde hair in the island’s community. This single mutation was easily carried through the small population and now 10% of the population now has blonde hair, and 26% carry the recessive gene. This blonde hair occurred separately to the blonde hair found in Northern Europe and provides evidence that blonde colouration occurred at least twice in the evolution in human hair.
But Europe is where we see a big range of colour and where most of the research into the evolution of hair has taken place.
The theory of why there is so much variation here, and why it has been retained in the North and East, goes back to when humans first entered ige-age Europe. 35,000 years ago, hunter-gatherers were expanding into new territories in Eurasia. 20,000-15,000 years ago, the glacial maximum split the population in two, creating two very different environments. In the East, life continued as normal, and the risks involved in hunting remained unchanged.
In the West, hunting was now risky and many males in a society with sexual division of labour did not make it back from long, dangerous hunting trips. This created an unequal balance of sex and while this often leads to a polygynous society, gathering in the tundra was scarce and males were unable to provide for more than one female, so monogamy became the only sustainable lifestyle for these groups. This meant that there was intense selection for the best mates from females. The theory goes that because of the restricted options, sexual selection was strong, and in this case, leant toward the “rarer” traits as they stood out more. In these populations, that meant lighter hair began to be specifically chosen and consequently spread throughout the continent.
Based on research into the DNA which codes for hair, only red hair has been linked to lighter skin pigmentation, other colours seem to have no genetic benefit, suggesting it really was only sexual selection which made it so prolific in certain areas.
Hair itself has become a powerful tool for many, for expressing identity, for concealing or displaying their power and an identifier to those around them. In fact 72% of women surveyed in North America say that they feel empowered when they feel good about their hair. However, there has been limited research into the evolution of hair, both the spread of certain colours and textures, and any genetic benefits to having diverse hair across the globe. It might be that the answer is a lifting of selection pressures and a move into sexual selection, but without investigation, we won’t know for sure. It seems like this defining trait which holds so much power to individuals deserves a little more explanation.
Cabanac, M., & Brinnel, H. (1988). Beards, baldness, and sweat secretion. European Journal of Applied Physiology and Occupational Physiology, 58(1–2), 39–46. https://doi.org/10.1007/bf00636601
Frost, P. (2006). European hair and eye color: a case of frequency-dependent sexual selection?. Evolution and Human Behavior, 27(2), 85-103.
Lasisi, T., Smallcombe, J. W., Kenney, W. L., Shriver, M. D., Zydney, B., Jablonski, N. G., & Havenith, G. (2023). Human scalp hair as a thermoregulatory adaptation. Proceedings of the National Academy of Sciences, 120(24). https://doi.org/10.1073/pnas.2301760120
Loury, E. (2012, May 3). The Origin of Blond Afros in Melanesia. Science. https://www.science.org/content/article/origin-blond-afros-melanesia
Norkin, L. (2018, August 3). We Asked Women Across the Country All About Their Hair. In Style. https://www.instyle.com/beauty/splitting-hairs-survey-american-women-and-hair
Yildirim, A. M., Yousaf, A., Fang, W., & Kolodney, M. S. (2022, November). A cross-sectional study of male balding patterns in people of color. Journal of the American Academy of Dermatology. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9511210/
World of Paleoanthropology 