Why Tightly Coiled Hair May Have Been the Original Human Hair Type
In the previous article, we established that scalp hair likely evolved as a thermoregulatory adaptation — a passive system to protect the brain from solar radiation while minimizing the costly need to sweat. But that still leaves a crucial question unanswered: Why does scalp hair look so different from person to person?
If all hair types provide some protection, does the shape of that hair matter? And if one shape worked best, why did others survive?
A landmark 2023 study published in the Proceedings of the National Academy of Sciences (PNAS) set out to answer exactly these questions. The researchers, led by biological anthropologist Tina Lasisi at Penn State University, used a thermal manikin and human hair wigs to measure how different hair textures affect heat gain on the scalp.
Their findings were striking. Tightly curled hair provided the most effective protection against solar radiation — and may have been the original hair type that allowed early humans to grow larger brains.
The Experiment: Putting Hair to the Test
To understand how hair affects scalp temperature, the researchers used a thermal manikin — a life-sized human model that uses electricity to simulate body heat. The manikin was programmed to maintain a constant surface temperature of 35 degrees Celsius (95 degrees Fahrenheit), matching the average temperature of human skin.
They tested four conditions:
| Condition | Description |
|---|---|
| Nude | No hair (bald scalp) |
| Straight hair | Wigs made from straight human hair |
| Moderately curled hair | Wigs with loose curls |
| Tightly curled hair | Wigs with dense coils |
All wigs were made from real human hair sourced from China, so the only difference was the curl pattern. The manikin was placed in a climate-controlled chamber with simulated solar radiation (about 788 watts per square meter — similar to equatorial sun at midday) and tested at different wind speeds representing standing still (0.3 m/s), walking (1.0 m/s), and running (2.5 m/s).
Then they measured how much heat reached the scalp.
The Results: Curls Win
The results were clear and followed a consistent pattern. The highest solar heat gain was experienced under the nude condition, and straight hair, moderately curled hair, and tightly curled hair showed decreasing heat gain in that order.
In plain English: the tighter the curl, the cooler the scalp.
| Hair Type | Solar Heat Gain | Protection Level |
|---|---|---|
| Nude (no hair) | Highest (baseline) | None |
| Straight hair | Reduced from baseline | Moderate |
| Moderately curled hair | Further reduced | High |
| Tightly curled hair | Lowest of all | Highest |
At a walking speed (1.0 m/s wind), the tightly curled hair condition actually allowed the scalp to lose heat even while the sun was shining — something the nude scalp could not do. At a running speed (2.5 m/s), both tightly curled and moderately curled hair showed net heat loss.
In other words, hair did not just reduce heat gain. Under the right conditions, it actively helped the head cool down.
Why Does Tightly Curled Hair Work Best?
The researchers identified two main reasons tightly curled hair performed best.
First, shape creates space. Tightly curled hair does not lie flat against the scalp. Instead, it lifts away from the skin, creating a gap of trapped air between the hair and the scalp. That air gap acts almost like a protective layer. The hair blocks some sunlight, but because it does not lie flat like a heavy blanket, the scalp can still release heat.
The closest parallel in animal biology is the way fur depth affects heat gain in mammals. In general, deeper fur provides more protection from solar radiation. Tightly curled human hair achieves a similar effect without requiring a thick, heavy coat.
Second, it saves water. Sweating is an expensive way to cool down. It costs water and electrolytes. In a hot, dry environment, losing too much water can be deadly.
The study found that while hair does reduce evaporative cooling (because it traps some sweat against the scalp), it also reduces the amount of sweat needed to offset solar heat gain. The nude scalp could evaporate more sweat — but it needed to. The hair-covered scalp, particularly the tightly curled scalp, required far less sweat to maintain heat balance.
As Lasisi explained to National Geographic: “Sweating isn’t free — you’re losing water and electrolytes. And for our hominin ancestors, that could have been important”.
What This Means for Brain Evolution
Here is where the story becomes truly significant.
About two million years ago, the genus Homo emerged. Around the same time, hominin brains began to grow larger. A larger brain generates more metabolic heat and is more sensitive to overheating than a smaller one.
This created a serious problem. Early humans were walking upright in equatorial Africa, under intense sun, with a brain that could not afford to get too hot. They needed a way to protect their heads without losing water through excessive sweating.
Scalp hair — particularly tightly curled scalp hair — may have been the solution.
As Lasisi explained: “Walking upright is the setup and brain growth is the payoff of scalp hair. Around two million years ago we see Homo erectus, which had the same physical build as us but a smaller brain size. And by one million years ago, we’re basically at modern-day brain sizes, give or take. Something released a physical constraint that allowed our brains to grow. We think scalp hair provided a passive mechanism to reduce the amount of heat gained from solar radiation that our sweat glands couldn’t”.
In other words: by reducing the need to sweat, tightly curled hair may have helped early humans stay cool enough — and hydrated enough — to support the evolution of larger brains.
Nina Jablonski, Evan Pugh University Professor of Anthropology at Penn State and a co-author of the study, put it this way: “Humans evolved in equatorial Africa, where the sun is overhead for much of the day, year in and year out. Here the scalp and top of the head receive far more constant levels of intense solar radiation as heat. We wanted to understand how that affected the evolution of our hair. We found that tightly curled hair allowed humans to stay cool and actually conserve water”.
When Did Curly Hair Evolve?
If tightly curled hair provided such a clear advantage, when did it appear in human evolution?
The study’s authors suggest that the selection pressure for tightly curled hair likely occurred sometime around two million years ago, when Homo erectus was the dominant hominin and brain sizes were beginning to increase.
The ubiquity of tightly curled hair across African populations — despite Africa having the greatest genetic diversity of any continent — also suggests that this trait is ancient. If tightly curled hair had arisen recently, it would not be so widespread across genetically diverse populations.
Lasisi is cautious about making definitive claims. “We don’t expect that it would have been homogenous,” she told National Geographic. At a later point in human evolution, curly hair may have lost its evolutionary advantage in some populations, and straight hair may have been favored by different types of genetic selection.
“Maybe once we had those larger brains, we also had all these cultural adaptations to avoid overheating, like better sources of water,” she said. “And at that point, maybe there wasn’t such a selective pressure for curly hair”.
A Note of Caution: Don’t Over-Simplify
It is important to be careful here. Hair type is not a simple climate map. You cannot look at one person’s hair and explain their entire ancestry, environment, or history. Evolution is never that clean.
Human groups move, mix, split apart, and meet again. Some traits spread because they were useful. Some changed because of chance. Some became common because a small group carried them into a new place. And some may have been shaped by what people found attractive.
The PNAS study provides strong experimental evidence for a thermoregulatory advantage of tightly curled hair. But the researchers are careful not to overstate their findings. They note that many factors — including genetic drift, population history, and sexual selection — also shape human hair variation.
For example, while tightly curled hair may have been the original state for humans living in equatorial Africa, straight hair eventually became common in many East Asian and Native American populations. That does not mean straight hair has one simple cause. It may involve cold environments, population history, sexual selection, genetic drift, and other factors that are still being studied.
What This Means for Understanding Your Hair
The hair on your head is not just decoration. It is not just style. And if you have tightly curled or coily hair, it is not just a “texture” — it may be carrying the signature of an ancient adaptation that helped your ancestors survive under the equatorial sun.
Joseph Graves, an evolutionary biologist at North Carolina Agricultural & Technical State University who was not involved in the study, told New Scientist: “Any mechanism that could help cool the body, and at the same time save precious water, would definitely have been acted on strongly by natural selection”.
The study is, as evolutionary biologist Elizabeth Tapanes at the University of California San Diego put it, “a great leap forward about thinking why we have so much hair on our heads”.
But here is the most important takeaway: tightly curled hair is not primitive, not unmanageable, not “difficult.” It is a highly specialized adaptation that may have been essential to the evolution of the human brain itself.
Every curl pattern tells a story. And the story of tightly curled hair is one of survival, water conservation, and brain growth.
Looking Ahead
In the next article, we will go beneath the skin to explore how hair actually gets its shape — the follicles, the growth patterns, and the microscopic structures that determine whether a strand comes out straight, wavy, curly, or coiled.
But for now, sit with this thought. If you have curly or coily hair, your ancestors may have had an evolutionary advantage that helped them survive under the hottest sun on Earth — and that same advantage may have helped make us human.
References
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), e2301760120. https://doi.org/10.1073/pnas.2301760120
Metcalfe, T. (2023, June 20). Why curly hair was an evolutionary advantage. National Geographic. https://www.nationalgeographic.com/premium/article/curly-hair-evolutionary-advantage-brain-protect
BBC Newsround. (2023, June 11). Curly hair kept early humans cool and made our brains bigger, study finds. BBC. https://news-app-global.api.bbc.co.uk/newsround/65879881
Turner, V. S. (2023, June 22). Curly hair keeps the head coolest. Smithsonian Magazine. https://www.smithsonianmag.com/smart-news/curly-hair-keeps-the-head-coolest-but-any-hair-is-better-than-none-180982424/
Nottingham Post. (2023, June 7). Scientists discover there’s a good reason why some people have curly hair. Nottingham Post. https://www.nottinghampost.com/news/health/scientists-discover-theres-good-reason-8506155
Disclaimer: This article was researched and drafted with the assistance of AI. All sources are real and verifiable. Readers are encouraged to check the references themselves and draw their own conclusions.
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