Mammals have fur over most of their bodies, but at some point during evolution, we humans lost that fur covering. Researchers at Washington University School of Medicine in St. Louis argue that hair on the head is somehow different from fur because fur stops growing when it reaches a certain length, but our head hair continues to grow. To drive home their argument, they ask in a recent article in the journal Evolutionary Anthropology, "Have you ever seen a chimpanzee getting a haircut?"
When Arthur H. Neufeld, Ph.D., the Bernard Becker Professor of Ophthalmology and Visual Sciences at Washington University School of Medicine in St. Louis, first put forth his idea that human head hair is somehow different from fur, over dinner with his close friend and colleague Glenn C. Conroy, Ph.D., professor of anatomy and anthropology, Conroy told him he wasn't aware of anyone in anthropology studying differences between hair and fur.
"So we talked about it for a while and asked questions like why does human head hair continue to grow? Where did this difference arise in the evolutionary process?" Neufeld recalls. "And the more we looked at it, the more we found that there really isn't anything in the literature."
You're not likely to find a chimpanzee at the barber shop because fur stops growing at a certain length.
One reason might be that under a microscope a hair follicle taken from the leg would look just about the same as one from the head. Our human "fur" if you will — the hair under the arms, on the legs and elsewhere — is anatomically identical to head hair.
Experiments related to hair transplantation, however, have demonstrated that where hair comes from is important in how it grows. Hair transplants always move hair from one part of the head to another because the procedure doesn't work if hair is moved onto the head from another part of the body. Nor does hair behave correctly when transplanted from the head to other parts of the body.
"When researchers transplanted hair from the head to the leg, it kept growing," Neufeld says. "It didn't grow as long as it would on the head, but the hair grew much longer than typical leg hair."
Both head hair and body fur grow in cycles. The hair follicle produces a strand of hair during its active growth phase, called anagen. Then the growth slows, and the follicle "rests" for a while, the telogen phase. Then comes exogen when the hair falls out, and the follicle begins growing a new strand of hair as the anagen phase begins again. Hair on the leg usually grows for 19 to 26 weeks and then falls out. Hair on the head keeps growing for two to six years.
Neufeld and Conroy hypothesize that although the hair follicle itself might be the same, the hair follicle growth cycle must be regulated differently on the head than elsewhere on the body.
Arthur Neufeld (left) and Glenn Conroy
"Interestingly, anthropologists have thought for years about why humans are basically hairless," Conroy says. "Why and how did we lose our body hair? That's a big question, and a lot of different theories have been put forward to explain it. But it took someone like Art — who is from outside of the field of anthropology — to ask why different types of human hair apparently have evolved differently."
There is a slight difference in the levels of keratin in human hair. Keratins are fibrous proteins that form the chemical basis of hair, fingernails, rhinoceros horns and other external features of many animals. The keratin content of human head hair is different than what's found in fur on chimpanzees and gorillas, our closest biological relatives.
A potential reason for that difference is a segment of human DNA called a pseudogene. The actual gene, known as ΦhHaA, makes a keratin protein in chimps and gorillas. Although the same DNA sequence is preserved in humans, human cells don't use it to make the protein. That's why scientists call it a pseudogene.
"I don't have any sense that this necessarily is a clue," Neufeld says. "It's just the only article I was able to find in the literature that distinguished human hair from chimpanzee or gorilla hair, in terms of molecular biology and biochemical makeup."
Neufeld and Conroy have ideas about how to identify differences between head hair and fur. They say studies will be difficult because there can be no animal model for experiments. But they believe it might be possible, for example, to take a human hair follicle from the head and another from the leg and conduct gene-chip experiments to learn about differences in the genes that regulate the activity of those follicles. A similar study involving follicles from the human head and from chimpanzee skin also might provide clues.
But even if it's possible to identify the factors that make head hair grow differently than body hair, scientists may never know why humans evolved head hair that's so different from our closest animal cousins.
"If I had to guess, I would think a lot of this somehow has to do with sexual selection," Conroy says. "But how continuously growing hair plays into sexual selection is anybody's guess."
Neufeld adds, "I was almost bald by the time I was in my early twenties, so if it is sexual selection, it would be interesting to figure out what differences are being signaled between my baldness and Glenn's full head of hair,"
We may never know the answer, but at least Neufeld and Conroy have asked the question.
Neufeld AH, Conroy GD. Human head hair is not fur. Evolutionary Anthropology, vol. 13:3, p. 89, June 2004.