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BY BRIDGET ALEX
On a hot
day, an
average Joe
can sweat
12 liters;
maximum
rates for
humans
are over
four times
that of
chimps.
Today most scholars lean the other way.
They contend fur loss was not for sex, but
survival — specifically, survival of the
sweatiest. Reduced body hair enhanced
the cooling capacity of sweat, a crucial
adaptation in our ancestors’ hot, savanna-
like environments.
“Humans can dump heat ... whereas
other mammals, when you chase them,
overheat,” says Harvard University evolu-
tionary biologist Daniel Lieberman.
Homo sapiens also stand out as the
most perspiring primate. On a hot day, an
average Joe can sweat 12 liters; maximum
rates for humans are over four times that
of chimpanzees. The disparity is partially
explained by relative abundances of two
types of sweat glands: eccrine, the source
of watery sweat exuded through pores,
and apocrine, which secrete viscous liq-
uid from within hair roots. Chimp skin
comprises roughly two eccrine per one
apocrine, but ours is almost all eccrine.
“Humans have just gone wild with those
glands,” says Lieberman. Sweat cools the
body through evaporation, drawing heat
away from the skin. Thick fur impedes this
process; bare skin promotes it.
Researchers think fur loss coincided
with eccrine gland gain in our evolution-
ary story, as paired adaptations for better
sweating. But the details on when and how
are still patchy.
SWEATING: THE DETAILS
Lieberman has proposed two scenarios for
when and why fur was sacrificed for sweat.
Both assume the changes occurred after
hominins — the evolutionary branch of
apes leading to humans — began walk-
ing upright and traded tropical forests for
open, sunnier habitats, some 7 million
years ago. Fur usually shields mammals
from damaging solar radiation. But bipeds
could forgo this protection because only
their scalps are exposed to direct rays. And
this likely explains why hominins kept hair
on their heads.
The first scenario places fur loss within
a few million years of the origins of biped-
alism, when our ancestors were merely
3 to 4 feet tall, with chimpanzee-sized
brains. Since two legs are slower than four,
Getting Naked
Our human ancestors lost the primate pelt —
researchers investigate what they gained.
In a lineup of primates, humans are easy to spot. We’re the
naked ones.
While all our living evolutionary cousins sport fur coats, Homo
sapiens alone are naturally nude, aside from diminutive body hairs
and dense tufts over our heads, underarms and genitals.
This has vexed scientists since Charles Darwin. In his 1871 The
Descent of Man, Darwin asserted that fur loss is “an inconvenience
and probably an injury to man.” Dismissing the possibility that
nakedness evolved via natural selection — as a trait that improved
survival — he attributed it to sexual selection — a trait fancied
by mates. By this view, ancestors with less body hair were more
attractive and reproductively fruitful.
O
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ORIGIN STORY