The finding came from new research conducted by researchers at the University of Utah and was published in the Journal of Experimental Biology.
The palms of humans are shorter than the palms of apes, and humans also have stronger, longer, and more flexible thumbs. It has been believed that these features have evolved so that human ancestors had the manual dexterity to survive by making and utilizing tools.
"The role aggression has played in our evolution has not been adequately appreciated," explained University of Utah biology Professor David Carrier, senior author of the study.
"There are people who do not like this idea, but it is clear that compared with other mammals, great apes are a relatively aggressive group, with lots of fighting and violence, and that includes us," Carrier said. "We're the poster children for violence."
It has been long since questioned "about whether we are, by nature, aggressive animals," he added. "Our anatomy holds clues to that question. If we can understand what our anatomy has evolved to do, we'll have a clearer picture of who we were in the beginning, and whether aggression is part of who we are."
Although Carrier accepts that our hands evolved for better manual dexterity, "the proportions of our hands also allow us to make a fist," which keep fragile hand bones, ligaments, and muscles safe while in a hand-to-hand battle.
As humans evolved, "an individual who could strike with a clenched fist could hit harder without injuring themselves, so they were better able to fight for mates and thus more likely to reproduce." People also fought for water, land, shelter and food to help their family, and "over pride, reputation and for revenge," he revealed.
"If a fist posture does provide a performance advantage for punching, the proportions of our hands also may have evolved in response to selection for fighting ability, in addition to selection for dexterity," explained Carrier.
In order to determine the benefits that a human fist offers during combat, Carrier and his colleague, Micheal H. Morgan, a University of Utah medical student, conducted three trials.
Three Trials & The ResultsThe first trial looked into whether a fist helps humans hit harder. There were 10 males between the ages of 22 and 50 involved in the study who were all familiar with boxing or martial arts. They were asked to hit a punching bag as hard as possible.
Each participant hit the bag 18 times (3 of each for 6 types of hit: side punches and slaps, overhead hammer fists and slaps, and forward punches and palm shoves). In order to measure the force of each hit, the punching bag was instrumented - it could tell how effective a blow was.
The results showed that the highest force was the same for both a punch thrown with a fist or a slap with an open hand. The team also found that with only a third of the surface area of the hand, a fist produced the same force.
The highest stress thrown at the bag (the force per area) was 1.7 to 3 times larger when a man hit with a fist as opposed to a slap.
"Because you have higher pressure when hitting with a fist, you are more likely to cause injury" to body parts, such as: bones, tissue, eyes, jaw and the teeth, Carrier explained.
The other 2 trials, which also consisted of 10 male participants, set out to determine whether a fist offers buttress to keep the hand safe while punching.
The stiffness of the knuckle joint of the first finger was calculated, and how force is passed from the fingers to the thumb was analyzed. The calculations were made with regular, buttressed fists and when incomplete fists were not buttressed.
There are two ways that humans buttress (fortify and stabilize) fists in which apes are not able to:
- The 4 fingertips meet the part of the palm nearest the fingers.
- The thumb lays across the index and middle fingers, ending about the middle of the ring finger. Those fingers, therefore, are stuck in that position by the palm at the foundation of the thumb.
The team calculated the force and identified how much the index finger had to flex. In order to measure the force that was passed from the fingers to the thumb, the participants were asked to go in the position of a one-handed push-up while pressing their knuckles on a block that was on another force transducer.
Results from the last two trials showed that the stiffness of the knuckle joint increased four times (or flexing decreased four times) by the buttressed fists.
As a result of the clenched fist allowing the force to pass from the fingers to the thumb, the capability of the fingers to convey the force of the punch increased two times.
"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 revealed.
There is another speculation that may explain the size of human hands, other than dexterity and aggression. The natural selection among human ancestors for running and walking caused people to have a longer big toe and shorter toes, and the reason people have shorter fingers and longer thumbs also resulted from the accountable genes.
Fists and AggressionIn order for apes to be able to climb trees, their lengthened fingers and hands evolved.
"The standard argument is that once our ancestors came out of the trees, the selection for climbing was gone, so selection for manipulation became dominant, and that's what changed the shape of our ancestors' hands. Human-like hand proportions appear in the fossil record at the same time our ancestors started walking upright 4 million to 5 million years ago. An alternative possible explanation is that we stood up on two legs and evolved these hand proportions to beat each other."
If the only factor was manual dexterity, Carried said, people could have evolved manual dexterity with thumbs that were longer and without palms and fingers getting smaller.
However, he pointed out, "there is only one way you can have a buttressed, clenched fist: the palms and fingers got shorter at the same time the thumb got longer."
The scientists listed other examples to show that fighting assisted in the forming of human hands:
- Humans are the only apes that hit with their fists clenched. Gorilla hands and human hands are the most comparable in size, which is contradictory because dexterity and tool-making are more associated with chimps. Therefore, Morgan and Carrier assume that aggression also played a part in the development of the hands of gorillas.
- Fists are shown as a sign of threat by humans. "If you are angry, the reflexive response is to form a fist," Carrier said. "If you want to intimidate somebody, you wave your fist."
- Sexual dimorphism (the body size of males is different than that of females) is larger when there is more competition among males in a primate species. "Look at humans and gorillas. The difference between the sexes is mainly in the upper body and the arms, and especially the hands. It's consistent with the hand being a weapon," Carrier added.
The authors concluded:
"It is arguably our most important anatomical weapon, used to threaten, beat and sometimes kill to resolve conflict. Yet it is also the part of our musculoskeletal system that crafts and uses delicate tools, plays musical instruments, produces art, conveys complex intentions and emotions, and nurtures.
More than any other part of our anatomy, the hand represents the identity of Homo sapiens. Ultimately, the evolutionary significance of the human hand may lie in its remarkable ability to serve two seemingly incompatible but intrinsically human functions."
Written by Sarah Glynn