Why do we have hair in such random places? - Nina G. Jablonski
We have lots in common with our closest
primate relatives.
But comparatively, humans seem a bit...
underdressed.
Instead of thick fur covering our bodies,
many of us mainly have hair
on top of our heads—
and a few other places.
So, how did we get so naked?
And why do we have hair where we do?
Human hair and animal fur
are made of the same stuff:
filaments of the protein keratin that grow
out of organs known as follicles,
which go through cycles
of growth and shedding.
Across mammalian species, hairs have been
modified for numerous purposes,
ranging from the soft fluff
covering rabbits
to the rigid quills protecting porcupines.
But for many mammals,
hair grows in two layers
consisting of a shorter undercoat
of ground hairs
covered by longer guard hairs.
Together, they help insulate
the animal’s body and protect its skin.
Human hairs, on the other hand, are
kind of a combination of these hair types.
Unfortunately,
hair is rarely found in fossils,
making it hard for researchers
to pinpoint when and how
our ancient ancestors lost their coats.
But scientists have developed
some working hypotheses.
It seems that,
millions of years ago in Africa,
early hominins first transitioned out of
trees and adopted a more active lifestyle.
Keeping cool became
increasingly important.
Eventually, they developed
more sweat glands,
which helped them lose heat by evaporating
moisture through the skin.
In fact, humans have 10 times more sweat
glands than chimpanzees, for instance.
But efficiently losing heat by sweating is
harder to do when you’re covered in fur.
Scientists believe that early humans
lost much of their coat around this time
to help their sweat evaporate faster.
However, if losing our hair
was so advantageous,
why do we have any left at all?
It seems that there are unique uses
for hair in different parts of our bodies.
When it comes to the tops of our heads,
temperature regulation
likely played a part again.
Since early humans
began venturing into the open,
their heads would’ve been exposed
to the scorching sun.
Thicker, longer-growing hair protects
our sensitive scalps
and keeps our brains from overheating.
Dark tightly curled hair is most effective
at keeping solar radiation off of skin.
Other kinds of head hair evolved
as humans moved to different places.
Meanwhile, researchers think eyebrows
are especially useful for communication
because they sit atop active facial
muscles that convey our feelings.
Eyelashes have been shown to minimize
airflow over our eyeballs,
preventing them from drying
out and catching debris.
And maybe facial hair proved helpful in
distinguishing identity from a distance,
but we really don’t know.
Evidence is stubbly at best.
Why we have hair in other regions is...
more pungent.
Our armpits, nipples, and pubic areas
are dotted with apocrine glands.
They produce oily, smelly secretions
which the thick, curly hair
that often grows in these spots
helps disperse.
The secretions that waft off these hairy
patches may be useful for identification.
For example, several studies have shown
that people are able to identify
their own armpit odors as well as those
of people they’re close with.
The final type of notable human hair is
the vellus hair that covers our bodies.
We don't know if these hairs serve
any purpose themselves,
but the follicles vellus hair grows from
are essential banks of stem cells
that repair damaged skin after injury.
They’re also important sites
of nerve endings that convey signals
of gentle touch to the brain.
In fact, although it’s much finer,
humans have roughly the same density
of body hair as apes of comparable sizes.
So despite all this talk
of human nakedness,
we're not actually as hairless as we look.
