Why Humans Have Hair on Their Heads

“There are naked mammals, there are furry mammals, but we’re the only weirdos that have a naked body and fur on our heads,” says Tina Lasisi, a University of Michigan evolutionary biologist.

The origins of this peculiar human characteristic have been poorly understood, but new research by Nina Jablonski, a professor of anthropology at the Pennsylvania State University - building on her previous work with Lasisi - suggests a genetic mechanism developed around the time the first humans evolved, allowing hair follicles to remain in their growth phase for an exceptionally long time.

The ability to grow long hair wasn’t some evolutionary accident, according to Jablonski and Lasisi. It was more of an evolutionary imperative: it kept our ancestors alive in equatorial Africa as they moved out of the forests and shifted to walking upright on two feet. This directly exposed their heads, and the growing brains within, to the intense heat and solar radiation of the savannah sun.

Starting around 2 million years ago, around the time the species Homo erectus began thriving, one of our early upright-walking ancestors likely shed most of its body hair - a thermoregulatory nightmare in the blazing sun of the open savannah - and developed more sweat glands to keep its body and growing brain cool.

In addition to those sweat glands, Lasisi thinks our ancient ancestors had already begun developing an additional evolutionary trump card: tightly curled locks that created air gaps to minimize heat absorption and maximize heat loss.

The ability to stand on two feet and manage the heat allowed Homo erectus to roam further and hunt more efficiently. Thanks to this combination, some researchers think the species’ descendants began to develop the larger brains that are the evolutionary hallmark of modern humans.

Experts think that once our ancestors spread out of Africa into Europe and Asia, moving into less sun-intensive environments, the evolutionary pressure for that dark, tightly curled scalp hair lessened, and the myriad hair textures and hues we see today developed over time.

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