The researchers say their macabre experiments support the hotly debated
idea that human hands evolved not only for manual dexterity, but also
for fistfights.
However, some scientists vehemently argue that the new research does little to support this notion.
David Carrier, a comparative biomechanist at the University of Utah,
and his colleagues have controversially suggested that fist fighting
might have helped to drive the evolution of not only the human hand, but also the human face and the human propensity to walk upright.
Humans possess shorter palms and fingers, as well as longer, stronger
and more flexible thumbs, than their ape relatives. Scientists have long
thought that these features evolved to help give humans the manual dexterity to make and use tools. [The 7 Biggest Mysteries of the Human Body]
Cadaver arms
Carrier and his colleagues argue that human hands also evolved to act
as effective clubs. To seek more evidence for this idea, they
experimented with nine human male cadaver arms purchased from the
University of Utah's body donor program and from a private supply
company. The arms were not embalmed, and were kept frozen until they
were tested.
The researchers first tied fishing lines to tendons inside the arms.
Then, they secured these lines to guitar-tuner knobs that helped apply
tension to the tendons in order to hold them open for slaps, weakly
clench them into "unbuttressed" fists or strongly curl them into
"buttressed" fists.
Sensors that measured the amount of strain experienced by the bones
were then glued directly onto the metacarpals, or palm bones. "The
metacarpal bones are the bones of the hand that break most frequently
when people fight," Carrier told Live Science.
"It is easy to be distracted by the macabre nature of this experiment,"
Carrier said. However, the experiments required the scientists to
attach sensors directly onto the bones — a procedure too invasive to
attempt on live people, he said.
Then, the researchers mounted the arms on a pendulum and swung them at padded dumbbell weights rigged with sensors.
"Each one of these hands took about a week of work," Carrier said in a
statement. "First, we had to dissect it to expose the muscles; apply one
or more strain gauges; and then attach the lines to all the tendons so
you can control the position of the wrist, thumb and fingers to create a
buttressed fist, unbuttressed fist or open-palm posture. Everything had
to be lined up just right — all the joints, tension in muscles, the
orientation of bones."
After hundreds of punches and slaps using eight arms — one was too
arthritic — the data revealed that humans can safely strike with 55
percent more force with a buttressed fist than with an unbuttressed
fist, and with twice as much force with a buttressed fist than with an
open-hand slap.
These findings suggest that fists can protect hand bones from injuries
and fractures by reducing the level of strain during striking, the
researchers said in a write-up of their research published online Oct.
21 in the Journal of Experimental Biology.
Fighting the finding
The researchers suggested that the shape of human hands evolved to both
improve manual dexterity and make it possible to use fists as clubs
during fights. However, they emphasized that these reasons are not the
only factors that might have shaped the evolution of the human hand. For
instance, evolution favored lengthening the big toe and shortening
other toes so that humans could run more easily, and the same genes
likely influenced hand proportions as well, they said.
Still, not everyone was convinced by these experiments.
"This is a perfect example for how to not use bone-strain data,"
Brigitte Demes, a functional morphologist at Stony Brook University in
New York who did not take part in this research, told Live Science.
"Strain magnitudes in a bone cannot really tell you what kind of loads
and behaviors a bone is adapted for."
Demes noted that the magnitude of strain that bones undergo in live
people during locomotion or chewing "var[ies] greatly between bones and
even within bones that are supposedly well adapted to resist high forces
associated with these activities," Demes said. "Using bone-strain
levels for reconstructing the evolution of complex traits is outright
absurd," she added.
Although not all scientists agree on the implications of the findings,
Carrier defended the experiment, saying it could provide a window into
human evolution and behavior.
"I think a lot of the criticisms we get come from a fear that any evidence of aggressive behavior having
been important in our evolution somehow provides a kind of
justification for bad behavior," Carrier said. "Rather than justifying
aggression, an improved understanding of who we are, of human nature, should help us prevent violence of all kinds in the future."
Source: http://www.livescience.com/52562-hands-evolved-for-fighting.html
