For this latest study, he and co-author Michael Morgan, a medical student, conducted three experiments. First, they analyzed what happened when men, aged from 22 to 50, hit a punching bag as hard as they could. The peak stress delivered to the bag - the force per area - was 1.7 to 3 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 said.
The second and third experiments determined that buttressing provided by the human fist increases the stiffness of the knuckle joint fourfold. It also doubles 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.
In terms of the size and shape of hand anatomy, the scientists point out that humans could have evolved manual dexterity with longer thumbs, but without the fingers and palms getting shorter.
Gorilla hands are closer in proportion to human hands than are other apes' hands, but they and no other ape - aside from us - hits with a clenched fist.
The researchers additionally point out that humans use fists during threat displays. There is also a difference in body size between males and females, particularly evident with hands and arms. This, Carrier said, is "consistent with the hand being a weapon."
Human males tend to be more physically violent than women, with men being ten times more likely to commit homicide than females in the U.S., Carrier said. But the research, nonetheless, applies to women as well.
"The bottom line is that women need to fight and defend themselves too," Morgan told Discovery News. "Women need to fight off attackers and defend themselves from rape."
Defending children may even help to explain human hand anatomy, since both men and women are often driven to protect their offspring, in addition to fighting with others over territory, resources and for other reasons.
"It can be argued that modern man exists in a world devoid of the evolutionary and selective pressures to which aggression was a beneficial trait," Morgan said. "Our aggressive behavior remains, but no longer serves an evolutionary purpose."