Eccrine
glands
Apocrine
glands
Sebaceous
glands
CHIMP
HUMAN
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some genes may play a role in both hair and sweat
gland development. The team found that less hairy
mice also had more eccrine glands, and that the gene
En1 appeared to be responsible for increasing sweat
gland development while reducing hair production.
The discovery pointed to a potential genetic link
between fur loss and sweat gland gain in mammals.
The team also compared human and primate skin by
counting follicles and glands in biopsies from cadavers.
The resulting 2018 Journal of Human Evolution paper
showed H. sapiens have 10 times more eccrine glands
than chimpanzees, but similar hair follicle density. The
reason we seem to have less hair: Most human follicles
develop what’s known as vellus hair — the fine, barely
visible type — rather than terminal hair, the thick,
pigmented strands found on our scalps and on the
bodies of most other primates.
Building on the earlier work with mice, Kamberov
is narrowing in on specific genetic variants that could
explain the hair and sweat gland differences between
chimps and humans. Her team is now genetically engi-
neering mice with those variants to see if they develop
chimplike or humanlike skin traits. “We’re really at the
cusp of knowing the answer,” says Kamberov.
HIRSUTE PURSUIT
As scientists inch closer to explaining how we lost
our fur, another mystery looms: What about hair
that sprouts at puberty? Why do ample patches cover
underarms, genitals and faces to varying degrees
depending on age, sex and heredity?
Some researchers see this hair as an evolutionary
accident, perhaps a side effect of hormone activity:
These
small,
two-legged
hominins
could have
stayed safe
by foraging
during
midday
heat, when
most
carnivores
rest.
“we became easy pickings for any carnivore,” says
Lieberman. But, if they were efficient at reducing heat
through sweating, these small, two-legged hominins
could have stayed safe by foraging during midday heat,
when most carnivores rest.
Alternatively, perspiration-prone skin may have
been advantageous later in our origin story, when
hominins became hunters. Lieberman hypothesizes
that roughly 2 million years ago, Homo erectus pursued
prey by trekking and endurance running through the
hottest hours of the day. The animals would eventually
overheat, allowing sweaty hominins to nab their meal.
The scenarios are plausible, but impossible to test
with fossils, the traditional source of evidence for
understanding human evolution: No early hominin
skin or hair has survived the ages. So Lieberman and
other Harvard scientists launched a project to find the
cause of human fur loss in our genes.
GETTING DOWN TO THE DNA
Geneticists have the technology to see how evolution-
ary change happened at a basic biological level — in
this case, how minor alterations to the DNA code
shared by all primates could lead to uniquely human
sweat and skin. However, finding these genetic vari-
ants wasn’t easy, says Yana Kamberov, who joined the
research team while at Harvard and now leads it at the
University of Pennsylvania. “We had no idea where
to look in the genome. We didn’t know what to look
for,” she says.
The researchers began by comparing DNA from
mouse breeds that grow differing amounts of hair on
their paws — previous research had suggested that
Hair-Raising Development
Humans have about as much hair as chimps, but ours isn’t as thick. Chimps
have more oil-secreting apocrine glands; ours occur only in a few areas of the
body. We also have 10 times more eccrine glands, the source of watery sweat.
Shine a Light on It
Fur protects skin from solar radiation,
but so does walking upright, which
exposes less of the body to the sun; it
may be why we lost the primate
pelt but not our head hair.
ORIGIN STORY