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.