How did the evolution of hairlessness help us to stay alive and create bigger brains?
Theories abound as to why we became relatively hairless as compared to other species. One theory about our abandonment of fur includes the avoidance of parasites. As we became less nomadic and inhabited caves and other dwelling sites for longer periods of time, we allowed more contact with parasites or other animals. One example is bat bugs,who feed on sleeping bats and crossed species onto early hairy humans, giving rise to bed bugs. Our sedentary and group lifestyle also provided a host for fleas and their transmissible diseases such as the plague; hosting fleas is a singular fact to us and no other primates. Losing hair was a major buzz-kill.
The issue is survival and living long enough to mate. Even without vectors, hair favors the spread of disease. As with most dermatologists I have treated children and others with severe bacterial and fungal hair infections that may have resulted in mortality in our early ancestors.
Each of us possesses between two million and five million eccrine glands to discharge sweat. Hairless skin and watery sweat help humans eliminate excess heat very efficiently rather than collecting warmth in the fur found in all other primates.
The invention of clothing allowed us to change our clothing to control our temperatures and protect the skin from other ectoparasites such as ticks and lice. Due to changes in weather patterns and dietary changes that incorporated more meat, human hunters needed to travel greater distances for diminished food supplies. Losing hair along with increased ability to sweat to avoid overheating were life-saving adaptations. Combined with the fact that hairlessness helped with the prevention of infections, the trait of being hairy eventually had more costs than benefits.
What about bigger brains? According to Nina Jablonski, “the loss of most of our body hair and the gain of the ability to dissipate excess body heat through eccrine sweating helped to make possible the dramatic enlargement of our most temperature-sensitive organ, the brain.” In Jablonski’s research she noted that Australopithecines had a brain that was 400 cubic centimeters on average and the modern brain is three times that size. The higher caloric intake of meat and other foods also contributed towards the high energy demands of the growing brain. Losing our body hair, increasing the number of our eccrine sweat glands, and boosting our calories all contributed to our gray matter expansion. To anticipate your question, shaving off all your hair will not make you smarter.
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For more information, look at my YouTube series on the Brain-Skin Connections:
