Appliance of Science: Why did humans lose their hair?

In a world where hair removal is a big industry we sometimes forget how little hair we actually have, writes Dr Naomi Lavelle.

Unlike other primates most of our skin is exposed. There are many advantages to a fur covered body, it protects the skin and provides warmth, so why have we lost ours?

Our earliest ancestors, hominins were ape-like in appearance, with fur all over their bodies. So what happened during human evolution to make them lose their hair? Humans are not in fact as naked as we think, we have as much hair (and hair follicles) as our chimpanzee relatives. Chimps however have thick hairs all over their bodies, that we call fur. On most parts of our bodies we have very fine hairs called vellus hair. So why the shift from fur to hair?

There is no definite answer, but there are many theories:

Aquatic apes

The aquatic ape theory proposes that our ancestors had to adapt to a semi-aquatic life when they moved into open savannah regions. In the dry season lakes became their source of food, leading them to wade in water, a process that might have been impeded by the fur on our bodies, resulting in an evolutionary bias to a more hair-free body.

This theory meets with much opposition as there are many examples of fur bodied animals that life an aquatic, or semi-aquatic lifestyle.

Less to bug you

Another proposal is that our loss of fur reduced the infection of lice and other parasites. Perhaps a smoother body became the sign of a healthier one, a more attractive option in a mate; sexual selection for hair loss.

All the more to see

Could our hair loss be the result of a far more subtle and intuitive selection process? A need to see our skin? We can tell a lot from slight changes in skin colour; from these nonverbal communications we can gain insights into another person’s mood, health and emotions. A surprising reinforcement of this theory can be found in our eyes. Unlike many mammals, we have three, not two, types of cone cells in our eyes. Cone cells are involved in colour vision and having three allows us detect more subtle colour changes. Interestingly, primates with bare faces and bare rumps also tend to have three cones.

Too hot to handle

The most popular theory as to why we lost our fur has to do with thermoregulation. Environmental pressures lead our ancestors out of shady forests and into exposed savannah grasslands meaning that they had to hunt over longer distances, exposed to the heat of the day. There is evidence to suggest that these new conditions lead to a number of physiological changes allowing us to run more, an increase in the number of sweat glands on our bodies to allow us sweat more and a loss of fur to allow that sweat evaporate off our bodies as a form of heat loss and temperature control.

So how did our ancestors stay warm during the cold nights?

The answer proposed is that they learned to light fires and moved to wearing animal skins although the timelines in support of this theory are often debated.

Putting it all together

As with so many questions in science, there are many theories but no definite answers. It is likely that the evolution of human hair loss was influenced by more than one factor, a combination of selective advantage and environmental pressures.

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