Human Evolution and Hair Loss
Humans May Be a Race of Sexually Mature Juveniles
Why did humans lose their hair over time? Human ancestors may have developed sexually before they developed physically and so ended up hairless.
One of the striking features about the human race is its lack of hair. If you compare humans to our closest relative, the chimpanzee, the difference is pretty obvious. Why did we become so much less hairy?
There are a number of theories out there. One is that when we left the trees and headed out to the savannah, having no hair helped keep us cool. Or that we lived a semi-aquatic life for awhile and so lost our hair. Or that we lost our hair to gain protection from parasites.
These are all interesting ideas. But none of them explains how we lost our hair. An intriguing theory that offers an explanation is called neoteny.
Neoteny is just a way to say that an organism has reached sexual maturity before it has reached physical maturity. This sounds strange but can and does happen in nature. And if it happened with our ancestors, it would certainly explain why humans lack fur.
Neoteny in Nature
The classic example of neoteny is the mole salamander. Some species of mole salamanders will shift when they are ready to have babies based on what the environment is like.
For example, if the weather is dry and the pond is drying up, a salamander may become sexually mature as a tadpole. When these eggs hatch, the weather will again determine when the next generation is ready to have babies.
Sometimes, though, a species will stay more or less permanently stuck in the immature form. A species of salamander called Axolotl is one such case. At some point in the past, this species of salamander became arrested in the tadpole stage physically. But it still can become sexually mature in this stage even though it is essentially a tadpole.
Neoteny in People
Some scientists think people may be like the Axolotl. The idea is that at some point in our past, a group of our ancestors became sexually mature before they became physically mature and it stuck. This group of sexually mature juveniles went on to become us.
There aren’t any hard facts to support this idea but there is some intriguing circumstantial evidence. For example, a baby chimpanzee looks remarkably like a human baby. They are both hairless (except on the top of the head) and their skull structures are similar. Also, baby chimpanzees and baby humans have an incredible capacity to learn.
As each species grows, they start to lose these similarities. Chimps get much hairier and their skull structure changes significantly. Chimpanzees also stop learning well as adults whereas humans can keep right on learning into adulthood.
We do seem like chimpanzees that never grew up. Unfortunately there isn’t any easy way to prove this theory—skin and hair just don’t fossilize well. We’ll have to wait for scientists to find the change(s) that keep us from developing hair. From that scientists may be able to tell if the change should have a big effect like neoteny. Or a little effect like hair loss.
Related Book: Sudden Origins: Fossils, Genes, and the Emergence of Species, Jeffrey H. Schwartz (Wiley, 1999)
