When we got down out of the trees and started living in the Savannah, we exposed ourselves to a lot more sunlight. To help us cope with this added heat, we reduced the amount of hair we had, just like the Vervet and Patas monkeys* we shared the plains with.
Well, almost like them...
Unlike these monkeys, we started walking upright. This meant our bodies acted like giant cooling towers, and over time we lost most of our hair to take advantage of this position. The other two primates still move on all fours, so only the tops of their heads are bare.
This provided another benefit: Without hair, ticks and fleas had a hard time living on our bodies, protecting us from diseases. Even in cold climates we've kept our bare skin.
Scientists estimate this happened around 1.7 million years ago. During this time there's a detectable shift in human skin color to help us deal with our new bare skin. Pheomelanin, a reddish color that provides no UV protection, was phased out while eumelanin, which does protect us from UV rays, was phased in.
So, why do we have hair on our heads and the Savannah monkeys don't? Since the rest of our body worked so well removing heat, we kept the hair at the top of our heads to protect our skulls from impacts. Hair on other areas of the body was kept as a scent marker.
*That's not the only similarity we have with them.
Posted 1991 day ago