Just in time for Halloween, gore-resistant scientists are swinging frozen human cadaver arms like battering rams — in the name of science, of course.
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]
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."