GabyBiologists at the University of Pittsburgh offer a clear answer: humans aren’t the only mammals with sparse hair. Elephants, rhinoceroses, and naked mole rats also have very little hair, and marine mammals like whales and dolphins are nearly hairless. Prehistoric mammals that lived during the time of the dinosaurs were quite hairy, but over hundreds of millions of years a small group of mammals — including humans — evolved to have less hair. Biologists who study the genes that control hairiness say the difference comes down to whether certain genes are turned on or off.
The Benefits of Abundant Hair
Fur serves many important functions. It keeps animals warm, protects their skin from the sun and injuries, and helps them blend into their environment. Dense hair also helps animals sense their surroundings, for example by detecting nearby objects. Humans have hair all over their bodies, but it is generally fine and sparse compared with furrier mammals. One exception is the hair on our heads, which protects the scalp from the sun. The thicker hair that grows under the arms and in the genital area probably reduces skin friction and helps cooling by allowing sweat to evaporate. So hair can be quite beneficial. There must be a significant evolutionary reason why humans lost so much of it.
Why Did Humans Lose Hair?
The split between the human and chimpanzee lineages began around 7 million years ago. Scientists have several theories about why humans lost most of their body hair. Humans have far more sweat glands than chimpanzees and many other mammals, and sweating helps us cool down: when sweat evaporates from the skin, it removes thermal energy from the body. That cooling system was probably critical for early human ancestors living on the hot African savanna.
Many furry mammals still thrive in hot climates today. Ancient humans may have hunted these animals by tiring them out through long-distance running. Humans didn’t need to run faster than their prey; they only had to track it and keep going until the quarry overheated and collapsed. The ability to sweat freely without a thick coat of fur made humans more resilient on those long chases.
Genes That Control Hairiness
To understand hairiness across mammals, the research team compared genetic data from 62 species, from humans to armadillos, dogs, and squirrels. By mapping DNA, the researchers focused on genes linked to keeping or losing body hair. They found that humans still carry all the genes needed for a full coat of hair — those genes are simply muted or turned off. Genes can therefore produce dramatic changes in hair growth. In the real world, members of Homo sapiens can occasionally grow a lot of hair all over their bodies. This condition is called hypertrichosis, or “werewolf syndrome.” The hair growth in those cases is not under conscious control and is extremely rare. In the 1500s, a Spaniard named Petrus Gonsalvus was born with hypertrichosis. As a child, he was sent in a cage and presented as a gift to Henry II of France. Soon the king realized that Petrus was an ordinary person who could be educated. Eventually, Petrus married. His story helped inspire the tale of “Beauty and the Beast.”
