Human hands evolved, at least in part, for fighting, with a surprising new study that involved using cadaver arms and fists.
Researchers used actual cadaver arms to punch and slap padded dumbbells in experiments that provide evidence in favor of the hotly debated theory that humans, and especially men, are hardwired both mentally and physically to fight at times. The findings are reported in the Journal of Experimental Biology.
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"The idea that aggressive behavior played a role in the evolution of the human hand is controversial," explained senior author David Carrier in a press release.
"Many skeptics suggest that the human fist is simply a coincidence of natural selection for improved manual dexterity," added Carrier, who is a biology professor at the University of Utah. "That may be true, but if it is a coincidence, it is unfortunate."
"As an alternative," he continued, "we suggest that the hand proportions that allow the formation of a fist may tell us something important about our evolutionary history and who we are as a species. If our anatomy is adapted for fighting, we need to be aware we always may be haunted by basic emotions and reflexive behaviors that often don't make sense - and are very dangerous - in the modern world."
Carrier and his team used nine male cadaver arms for the study. The arms from the deceased were purchased from the university's body donor program and from a private supply company.
Human Hands Evolved for Punching
The scientists tested the hypothesis that a clenched fist protects the metacarpal (palm or hand) bones from injury and fracture by reducing the level of strain during striking. To do this, they placed each of the cadaver arms in a pendulum-like apparatus that enabled the arms to swing toward, and punch, a padded, force-detecting dumbbell.
"Metacarpals are bones in the hand that break most often - not finger bones, but bones of the palm," Carrier said. "When guys get mad and punch a wall, or when bars close and they punch each other, the bones that break most often are the metacarpals."
The scientists caused the arms and hands to punch with either a clenched/buttressed fist, where the thumb is locked around the index and middle fingers, which are curled tightly to the palm; or with an unbuttressed fist, where the thumb is outward and not touching the loosely folded fingers. The researchers also caused the arms to side-slap the target with an open hand.
After hundreds of punches and slaps using eight of the arms (one was too arthritic), the researchers determined that humans can safely strike with 55 percent more force with a fully buttressed fist than with an unbuttressed fist, and with twofold more force with a buttressed fist than with an open hand slap.
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In their paper, the team wrote that the evolutionary significance of the hands of humans and their ancestors "may be that these are the proportions that improved manual dexterity while at the same time making it possible for the hand to be used as a club during fighting."
Carrier believes that aggression played a key role in human evolution. He said history shows the use of fists to strike was common in many human cultures, and the prehistoric record provides consistent, ancient evidence for the behavior that still happens today.