Men whacked punching bags for a University of Utah study that suggests human hands evolved not only for the manual dexterity needed to use tools, play a violin or paint a work of art, but so men could make fists and fight.
Compared with apes, humans have shorter palms and fingers and longer, stronger, flexible thumbs – features that have been long thought to have evolved so our ancestors had the manual dexterity to make and use tools.
"The role aggression has played in our evolution has not been adequately appreciated," says University of Utah biology Professor David Carrier, senior author of the study, scheduled for publication Dec. 19 by the Journal of Experimental Biology.
"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 says. "We're the poster children for violence."
Humans have debated for centuries "about whether we are, by nature, aggressive animals," he adds. "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."
Carrier agrees that human hands evolved for improved manual dexterity, but adds that "the proportions of our hands also allow us to make a fist," protecting delicate hand bones, muscles and ligaments during hand-to-hand combat.
As our ancestors 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," he says. Fights also were for food, water, land and shelter to support a family, and "over pride, reputation and for revenge," he adds.
"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," Carrier says.
So Carrier and study co-author Michael H. Morgan – a University of Utah medical student – conducted their study to identify any performance advantages a human fist may provide during fighting.
The research was funded by the National Science Foundation.
Three Experiments and the Findings
The first experiment tested the hypothesis that humans can hit harder with a fist. So, Carrier and Morgan had 10 male students and nonstudents – ages 22 to 50 and all of them with boxing or martial arts experience – hit a punching bag as hard as they could.
Each subject delivered 18 hits, or three of each for six kinds of hits: overhead hammer fists and slaps, side punches and slaps, and forward punches and palm shoves. The bag was instrumented to allow calculation of the force of the punches and slaps.
To the researchers' surprise, the peak force was the same, whether the bag was punched with a fist or slapped with an open hand. However, a fist delivers the same force with one-third of the surface area as the palm and fingers, and 60 percent of the surface area of the palm alone. So the peak stress delivered to the punching bag – the force per area – was 1.7 to three 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 says.
The second and third experiments – which each also involved 10 male subjects – tested the hypothesis that a fist provides buttressing to protect the hand during punching.
To do that, the researchers measured the stiffness of the knuckle joint of the first finger, and how force is transferred from the fingers to the thumb. Both measurements were made with normal, buttressed fists or when partial fists were not buttressed.
Humans buttress – strengthen and stabilize – 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, and to some extent the ring finger, and those fingers are locked in place by the palm at the base of the thumb.
To measure stiffness of the second knuckle joint, the study's 10 male subjects slowly pushed a pressure transducer, with clenched fists or with fingers bent but the fist unclenched. Researchers measured the force and also how much the index finger flexed.
Force transfer from fingers to the thumb also was measured, but in this case the subjects got in a one-handed pushup position, with their knuckles pushing down on a block placed on a different force transducer.
The second and third experiments found that buttressing provided by the human fist increased the stiffness of the knuckle joint fourfold (or reduced flexing fourfold), and also doubled 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.
"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.
Carrier notes that besides dexterity and aggression, a third theory to explain the proportions of human hands also may be true: Natural selection for walking and running among human ancestors led to shorter toes and a longer big toe – and the responsible genes also led to shorter fingers and longer thumbs.
How Selection Favored Fists and Aggression
Apes' elongated fingers and hands evolved so they could climb trees.
"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," Carrier says. "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."
Carrier says that if manual dexterity was the only driving force, humans could have evolved manual dexterity with longer thumbs without the fingers and palms getting shorter. But, he adds, "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."
Morgan and Carrier cite other arguments that fighting helped shape human hands:
Carrier and Morgan write that the human hand is paradoxical.
"It is arguably our most important anatomical weapon, used to threaten, beat and sometimes kill to resolve conflict," they say. "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."
University of Utah: http://www.unews.utah.edu/
This press release was posted to serve as a topic for discussion. Please comment below. We try our best to only post press releases that are associated with peer reviewed scientific literature. Critical discussions of the research are appreciated. If you need help finding a link to the original article, please contact us on twitter or via e-mail.
Pigs ‘edited’ with a warthog gene to resist African swine fever could help spawn GM animal farms in the UK
Mouse House to make naturalist biopic, six years after box-office failure of Creation, starring Paul Bettany
International team spends 10 years making inroads into treatment of bacterium which kills up to half of those it infects
You may not know it, but you probably have some Neanderthal in you. For people around the world, except sub-Saharan Africans, about 1 to 3 percent of their DNA comes from Neanderthals, our close cousins who disappeared roughly 39,000 years ago.
Research at Yale plotted what happened in the brains of two scientists as they held a conversation
From medicines to jet fuel, we have so many reasons to celebrate the microbes we live with every day
Genome sequencing indicates Kennewick Man is Native American, reopening the bitter battle over whether he should be reburied or studied
In the article on the discovery of dinosaurs (They’re back, Review, 6 June) you state: “In Sussex, a local doctor uncovered fragmentary remains of what appeared to be two more species of colossal extinct land reptiles.” You grossly underplay the contribution of Lewes-born Gideon Mantell, geologist and palaeontologist, author and diarist, friend to princes and international scholars as well as local doctor. Mantell not only discovered (aided by his wife) the first remains of the iguanodon in 1824 but named it – as it resembled the tooth of an iguana. This was the first known land dinosaur, Mary Anning having identified the first sea-living dinosaur.Mantell went on to put together more pieces of the jigsaw with extra fossil discoveries. In contrast to Richard Owen, whose models form the basis for the Crystal Palace dinosaurs, Mantell stated correctly that iguanodon would have walked on their back legs, using their forearms to fight or gather food. He did, however, attribute the thumb spike to a nose horn though later corrected this assumption. The Natural History Museum has a display on Gideon and his wife Mary’s contribution as well as the large “Mantell-piece” of Iguanodon fossils that he had on show in his museum in Brighton. He sold it, along with many more priceless items, to the British Museum in 1838. Gideon Mantell’s reputation deserves better than your throwaway remark. Debby MatthewsLewes, East Sussex Continue reading...
Unique triangular hairs help keep Saharan silver ants cool at 70°C by manipulating the physics of light
Most animals wouldn't confront a fearsome predator like a lion. But through sophisticated group work, hyenas launch successful raids