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Curly Hair May Have Been Key to Human Brains Growing So Big

A study published by Penn State University researchers analyzed how human hair texture can help regulate body temperatures. Humans evolved in Equatorial Africa—where the sun is often overhead throughout the year, meaning the scalp receives more levels of solar radiation. And hair texture may have been important in the early evolution of the human brain.

By using wigs and a human-shaped model that used electricity to mimic body heat, the scientists analyzed how different hair types affected body temperature. During the study, they measured how much body heat was lost and used lamps on the model’s head to take the place of solar radiation. These lamps mimicked conditions in Equatorial Africa, with about 86 degrees Fahrenheit and 60 per cent humidity. They then did tests using different hair types, such as straight, moderately curled and tightly curled.

The researchers found that tight curls kept the body cool the best out of all hair types. “We hypothesized that tightly curled scalp hair would provide some benefits, but the extent of these benefits was uncertain,” Tina Lasisi, the study’s lead author, told sources.

“Previous studies on mammalian coats have shown that hair can limit the amount of sunlight reaching the skin, but we were particularly surprised by the significant reduction in solar heat radiation impact provided by tightly coiled hair,” she said.

Larger brains may have developed under these conditions because of this. That’s because large brains are sensitive to heat and also generate a lot of heat. Too much heat can lead to dangerous consequences like heat strokes. This means curly hair could have served as an adaptation for passive thermal regulation, Lasisi said.

“Once humans developed large brains, they could employ other behavioural and social strategies to cope with the heat, potentially diminishing the relative advantage of curly hair,” she said. “This could have led to a diverse distribution of hair textures worldwide. Furthermore, since straight hair better retains heat, populations in colder environments may have experienced selective pressure for straight hair.”

This research gives scientists some insight into how the role of human scalp hair developed, without putting humans in danger. “Hair texture can change slightly depending on environmental conditions such as humidity, but not drastically,” Lasisi said. “Very straight hair won’t suddenly become tightly coiled or vice versa. However, hair may become somewhat straighter or curlier depending on how individual hair fibres interact and the weight of the hair strands, as they hold varying amounts of water.”

She said the research primarily focused on “the biophysical aspects of heat transfer in relation to hair textures. The next step is to understand the implications of our findings in the context of human physiology.

“Future research should aim to answer these questions by incorporating our data into mathematical models of human physiology or conducting experiments with human subjects who have different hair textures to examine the impact on their thermal regulation,” Lasisi said.

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