University of Utah (USA) school of medicine researchers measured the forces when martial artists hit a punching bag. They found that the structure of the fist provides support that increases the ability of the knuckles to transmit force, while protecting the delicate bones of the hand. Making a fist increased the stiffness of the knuckles by a factor of four.
     In their paper, biology Professor and senior author David Carrier and researcher Michael H Morgan point out that the human hand has also been shaped by the need for manual dexterity, but say a number of different hand proportions are compatible with enhanced ability to manipulate objects.
     "There may, however, be only one set of skeletal proportions that allows the hand to function both as a mechanism for precise manipulation and as a club for striking," the researchers write. "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.
     Compared with apes, humans have shorter palms and fingers and longer, stronger, flexible thumbs - features long thought to have evolved so our ancestors had the manual dexterity to make and use tools.
     Humans strengthen and stabilise 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, which are locked in place by the palm at the base of the thumb. Professor Carrier commented: "The question for me is 'why wasn't this discussed 30, 40 years ago.' As far as I know it isn't in the literature."
     Morgan and Carrier cite other arguments that fighting helped shape human hands:
     Among apes, only humans hit with a clenched fist. Gorilla hands are closer in proportion to human hands than those of other apes, yet chimps are better known for tool-making and dexterity. Morgan and Carrier believe aggression was also 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."
     In primates, the difference in body size between males and females is greater if there is more male competition. "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 notes a third theory to explain the proportions of human hands may also be true: Natural selection for walking and running among led to shorter toes and a longer big toe, and the responsible genes also led to shorter fingers and longer thumbs.

Edited from EurekAlert!, BBC News, LiveScience (19 December 2012)