New Delhi: Tightly-wound curls do much more than just look pretty. A new study shows that curly hair was an evolutionary mechanism for “thermoregulation” and helped early humans “stay cool and actually conserve water”.
The study, conducted by a team of researchers from the Pennsylvania State University in the US, the Loughborough University in the UK, and the University of Australia in Sydney, looked closely at the role that human scalp hair plays in biology and why it evolved in the otherwise “hairless” human body.
It was published in the Proceedings of the National Academy of Sciences (PNAS) on 6 June.
In the study, researchers said while tightly-curled hair has always been seen as thick or been compared to “wool”, contrary to popular belief, it doesn’t “trap” heat but protects it from heat exposure.
In fact, that’s the function of all forms of scalp hair, according to the study’s authors — it is an evolutionary mechanism meant to protect the human brain from overheating.
“We find evidence for a significant reduction in solar radiation influx to the scalp in the presence of hair. Particularly, we find that hair that is more tightly curled offers increased protection against heat gain from solar radiation,” the study states.
As early humans evolved to walk upright in equatorial Africa, the tops of their heads increasingly took the brunt of solar radiation, Tina Lasisi, one of the authors, said in a news release.
“The brain is sensitive to heat, and it generates heat, especially the larger it grows. Too much heat can lead to dangerous conditions like heat stroke. As humans lost much of their body hair, they developed efficient sweat glands to keep cool, but sweating comes at a cost in lost water and electrolytes,” said Lasisi.
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Thermoregulation — why hair evolved
For the study, researchers used thermal manikins that were manipulated to imitate human body conditions, such as having a constant surface temperature of 95 degrees Fahrenheit (or 35 degrees Celsius) and even sweating.
Manikins are different from mannequins and are human body forms usually used in medical or art studies. Thermal manikins are specially designed forms that can be heated to simulate heat transfer between humans and the environment.
The manikins were given four different kinds of hair conditions — no hair, straight hair, moderately curly hair and tightly curled hair — and placed in a temperature and humidity-controlled wind tunnel.
The wigs used in the experiment were all black human hair of Chinese origin, according to the study.
The experiments were conducted by creating an ambient temperature of 86 degrees Fahrenheit (or 30 degrees Celsius) heat and 60 per cent relative humidity — all conditions that replicate environments in equatorial Africa.
Scientists then simulated solar radiation by directing two lamps to the manikins and calculated heat loss at various speeds and after wetting the scalp to mimic sweating.
“Sweating works in tandem with a seemingly hairless body to create a highly-effective cooling system. But sweating also increases the need for fluid replacement and thus can lead to dehydration. Therefore, scalp hair may have offered significant additional benefits to a person,” the study said.
Researchers found that all hair, regardless of its texture, provides protection against solar radiation. But tightly-curled hair offered the best protection.
“Tightly curled human hair form does not lay flat on the scalp and therefore increases the distance between the surface of the hair and the surface of the scalp,” the authors said, suggesting that evolution of hair “might be explained by thermoregulation pressures experienced in hot and arid environments”.
“Around 2 million years ago, we see Homo Erectus, which had the same physical build as us but a smaller brain size,” Lassi, quoted earlier, said in the news release. “And by 1 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 provides a passive mechanism to reduce the amount of heat gained from solar radiation that our sweat glands couldn’t.”
But authors concede there are limitations to the study. For one thing, the wigs used in the experiments only represent a narrow spectrum of human hair. In addition, the study also incorporated a simulation of sweat, which may differ from an actual sweating response.
For another, the study does not take into account actual physiological responses that a person has to heat stress such as changing skin temperature, although the research says these results provide a good starting point for discussion and further research on the evolutionary function of human scalp hair.
(Edited by Uttara Ramaswamy)
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