Human hands evolved
for punching, not just
dexterity
Men whacked punching bags for a University of Utah study
that suggests human hands evolved not only for the manual dexterity needed to
use tools, play a violin or paint a work of art, but so men could make fists
and fight.
Compared
with a chimpanzee hand, at left, the human hand, at right, has shorter
fingers and palms and a longer, stronger more flexible thumb. That not only
allows fine manipulation of tools and other objects, but allows humans to
make a clenched fist, which apes cannot. A new University of Utah study argues
that human hands evolved not only for manual dexterity, but for fighting
[Credit: Denise Morgan for the University of Utah]
|
Compared with apes, humans have shorter palms and
fingers and longer, stronger, flexible thumbs -- features that have been long
thought to have evolved so our ancestors had the manual dexterity to make and
use tools.
"The role aggression has played in our evolution
has not been adequately appreciated," says University of Utah biology
Professor David Carrier, senior author of the study, scheduled for publication Dec. 19 by the Journal of
Experimental Biology.
"There are people who do not like this idea, but it
is clear that compared with other mammals, great apes are a relatively aggressive
group, with lots of fighting and violence, and that includes us," Carrier
says. "We're the poster children for violence."
Humans have debated for centuries "about whether we
are, by nature, aggressive animals," he adds. "Our anatomy holds
clues to that question. If we can understand what our anatomy has evolved to
do, we'll have a clearer picture of who we were in the beginning, and whether
aggression is part of who we are."
Carrier agrees that human hands evolved for improved
manual dexterity, but adds that "the proportions of our hands also allow
us to make a fist," protecting delicate hand bones, muscles and ligaments
during hand-to-hand combat.
As our ancestors evolved, "an individual who could
strike with a clenched fist could hit harder without injuring themselves, so
they were better able to fight for mates and thus more likely to
reproduce," he says. Fights also were for food, water, land and shelter to
support a family, and "over pride, reputation and for revenge," he
adds.
"If a fist posture does provide a performance
advantage for punching, the proportions of our hands also may have evolved in
response to selection for fighting ability, in addition to selection for
dexterity," Carrier says.
So Carrier and study co-author Michael H. Morgan -- a
University of Utah medical student -- conducted their study to identify any
performance advantages a human fist may provide during fighting.
The research was funded by the National Science
Foundation.
Three Experiments and the findings
The first experiment tested the hypothesis that humans
can hit harder with a fist. So, Carrier and Morgan had 10 male students and
nonstudents -- ages 22 to 50 and all of them with boxing or martial arts
experience -- hit a punching bag as hard as they could.
Each subject delivered 18 hits, or three of each for six
kinds of hits: overhead hammer fists and slaps, side punches and slaps, and
forward punches and palm shoves. The bag was instrumented to allow calculation
of the force of the punches and slaps.
To the researchers' surprise, the peak force was the
same, whether the bag was punched with a fist or slapped with an open hand.
However, a fist delivers the same force with one-third of the surface area as
the palm and fingers, and 60 percent of the surface area of the palm alone. So
the peak stress delivered to the punching bag -- the force per area -- was 1.7
to three times greater with a fist strike compared with a slap.
"Because you have higher pressure when hitting with
a fist, you are more likely to cause injury" to tissue, bones, teeth, eyes
and the jaw, Carrier says.
The second and third experiments -- which each also
involved 10 male subjects -- tested the hypothesis that a fist provides buttressing
to protect the hand during punching.
To do that, the researchers measured the stiffness of
the knuckle joint of the first finger, and how force is transferred from the
fingers to the thumb. Both measurements were made with normal, buttressed fists
or when partial fists were not buttressed.
 |
These
three views of a clenched human fist show how we buttress the fist to reduce
the chance of hand injury when punching. The four fingertip pads touch the
pads at the top of the palm, and the thumb wraps in front of the second,
third and part of the fourth finger, which are locked in place by the palm at
the base of the thumb. A new University of Utah study showed how a fist punch
provides a performance advantage compared with an open hand slap, suggesting
human hands evolved for fighting as well as for manual dexterity [Credit:
Denise Morgan for the University of Utah]
|
Humans buttress -- strengthen and stabilize -- fists in
two ways that apes cannot: The pads of the four fingertips touch the pads at
the top of the palm closest to the fingers. And the thumb wraps in front of the
index and middle fingers, and to some extent the ring finger, and those fingers
are locked in place by the palm at the base of the thumb.
To measure stiffness of the second knuckle joint, the
study's 10 male subjects slowly pushed a pressure transducer, with clenched
fists or with fingers bent but the fist unclenched. Researchers measured the
force and also how much the index finger flexed.
Force transfer from fingers to the thumb also was
measured, but in this case the subjects got in a one-handed pushup position,
with their knuckles pushing down on a block placed on a different force
transducer.
The second and third experiments found that buttressing
provided by the human fist increased the stiffness of the knuckle joint
fourfold (or reduced flexing fourfold), and also doubled the ability of the
fingers to transmit punching force, mainly due to the force transferred from
the fingers to the thumb when the fist is clenched.
"Because the experiments show the proportions of
the human hand provide a performance advantage when striking with a fist, we
suggest that the proportions of our hands resulted, in part, from selection to
improve fighting performance," Carrier says.
Carrier notes that besides dexterity and aggression, a
third theory to explain the proportions of human hands also may be true:
Natural selection for walking and running among human ancestors led to shorter
toes and a longer big toe -- and the responsible genes also led to shorter
fingers and longer thumbs.
How selection favored fists and aggression
Apes' elongated fingers and hands evolved so they could
climb trees.
"The standard argument is that once our ancestors
came out of the trees, the selection for climbing was gone, so selection for
manipulation became dominant, and that's what changed the shape of our
ancestors' hands," Carrier says. "Human-like hand proportions appear
in the fossil record at the same time our ancestors started walking upright 4
million to 5 million years ago. An alternative possible explanation is that we
stood up on two legs and evolved these hand proportions to beat each
other."
Carrier says that if manual dexterity was the only
driving force, humans could have evolved manual dexterity with longer thumbs
without the fingers and palms getting shorter. But, he adds, "there is
only one way you can have a buttressed, clenched fist: the palms and fingers
got shorter at the same time the thumb got longer."
Morgan and Carrier cite other arguments that fighting
helped shape human hands:
-- No ape hits with a clenched fist other than humans.
Gorilla hands are closer in proportion to human hands than are other apes'
hands -- a paradox since chimps are better known for tool-making and dexterity.
So Morgan and Carrier also believe aggression was a factor in the evolution of
gorillas' hands.
-- Humans use fists as threat displays. "If you are
angry, the reflexive response is to form a fist," Carrier says. "If
you want to intimidate somebody, you wave your fist."
-- Sexual dimorphism -- a difference in body size
between males and females -- is greater if there is more male-male competition
in a primate species. "Look at humans and gorillas. The difference between
the sexes is mainly in the upper body and the arms, and especially the
hands," Carrier says. "It's consistent with the hand being a
weapon."
Carrier and Morgan write that the human hand is paradoxical.
"It is arguably our most important anatomical
weapon, used to threaten, beat and sometimes kill to resolve conflict,"
they say. "Yet it is also the part of our musculoskeletal system that
crafts and uses delicate tools, plays musical instruments, produces art,
conveys complex intentions and emotions, and nurtures."
"More than any other part of our anatomy, the hand
represents the identity of Homo sapiens. Ultimately, the evolutionary
significance of the human hand may lie in its remarkable ability to serve two
seemingly incompatible but intrinsically human functions."
Source: University of
Utah [December 19, 2012]
