ever wonder why you never see a chimpanzee getting a haircut?
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Ever wonder why you never see a chimpanzee getting a haircut?

Researchers at the Washington University School of Medicine in St. Louis addressed that question in a recent article in the journal Evolutionary Anthropology. According to their research, hair found on the human head behaves differently from the body hair (or fur) that covers the bodies of most mammals. While body hair or fur stops growing once it reaches a predetermined length, human head hair enjoys a much lengthier growth cycle.

It was Arthur Neufeld, a professor of ophthalmology, who first raised the idea of a human hair/animal fur difference over dinner with his friend Glenn Conroy, a professor of anatomy and anthropology.

"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.”

Viewed under a microscope, a hair follicle taken from the leg would look identical to one from taken the head. In fact, the hair found all over the human body is anatomically identical to the hair found on the human head. Despite this similarity, however, scientists already know that the two types of hair behave differently. Hair transplants always involve moving hair from one part of the head to another because the procedure is unsuccessful if hair is moved from the body to the head and vice versa. Hair transplanted from the head to the leg will continue to grow, not as long as it would on the head, but much longer than other hair on the leg. Correspondingly, hair follicles transplanted from the leg to the head only produce short hair fibers.

All hair grows in cycles. The hair follicle produces hair during its active growth phase (anagen). Next, the growth slows and the follicle rests (telegen). Finally, the hair strand falls out (exegen) and the follicle enters anagen again. While hairs found on the leg grow for 19 to 26 weeks before falling out, hair found on the head can grow for two to six years before falling out.

Neufield and Conroy believe that while hair follicles are the same all over the body, the growth cycle of hair follicles on the head must be regulated differently. They turned to research on animal fur to look for clues to solve this head-hair puzzle, and discovered that human hair and animal fur have different levels of keratin (the fibrous proteins that form the chemical basis of hair, fingernails, animal horn, etc.)

One possible reason for the difference in keratin levels between human hair and gorilla or chimpanzee fur may be traced to differences in our DNA. Chimps and gorillas have a gene (known as FhHaA) that makes keratin protein; humans have the same gene, but do not use it to make keratin protein. Neufield and Conroy hope to do further research, conducting gene chip experiments and comparing hair follicles from chimps and humans, as well as hair follicles from different parts of the human body.

Though scientists may soon pinpoint the factors that make head hair grow differently than body hair in humans, they may not be able to determine the reason for the evolutionary difference between human head hair and animal head hair. Conroy posits it may tie into sexual selection, but is uncertain any research will lead to a definitive answer to this intriguing question.

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