The human hand is a beautiful product of evolution. Each one a finely crafted arrangement of 27 bones, our hands are among the most dexterous in the animal kingdom, and are every bit as capable of threading a needle as they are grasping the oar of a canoe. But newly published findings suggest our palms and fingers may have evolved into their present shapes for more brutish purposes — namely beating the living crap out of one another.
Few anatomical structures can compete with the range of precision afforded by a human hand. Stout, square palms. Short fingers (relative to the longer digits of our hominoid cousins). Long, strong, flexible thumbs. When combined, these features give rise to a shape that is uniquely suited for two different hand grips: the precision grip, in which objects are held and manipulated with the fingertips, and the power grip, where an object is held firmly by fully wrapped fingers and thumb.
Our capacity for manual manipulation is a large part of what makes us human, and is thought to have played an important role in the evolution of the hand itself. But in the latest issue of The Journal of Experimental Biology, researchers Michael Morgan and David Carrier suggest another driving force in the evolutionary history of the hand-shape we know today: the ability for our ancestors — and males, specifically — to hold their own during hand-to-hand combat.
The same hand-proportions that allow us to dominate at Jenga and grip a bat also allow us to make a closed fist. Unlike a chimpanzee, whose long fingers and stout thumb form a loose, open doughnut-shape when curled, a human is capable of instantly transforming his arm and hand into what amounts to a knobby-ended cudgel. And when you get right down to it, which would you rather have at your disposal during a violent encounter: a knobby-ended cudgel, or a stick with a donut on the end of it? (The image featured here compares the anatomy of a chimp hand with that of a human.)
Reason would suggest that the cudgel is the way to go. To verify the pugilistic merits of the human fist, Morgan and Carrier asked a range of male test subjects — all of them with boxing or martial arts experience — to participate in a series of physical tests (more later on the choice to use all male subjects). In the first test, subjects were asked to strike a punching bag as hard as they could, both with an open palm, as well as with a clenched fist. Surprisingly, the researches found that a fist did not deliver more total force per blow. The striking surface area of a fist, however, was found to be one-third less than the area of the whole hand. "This means that if the total force applied in a strike is the same, then the stress in the targeted tissue will be 1.7 to 3.0 times greater in a fist strike than in a palm strike," write the researchers. In other words: a clenched fist dramatically increases the potential for injury.
Additional tests looked at whether finger and thumb placement provided significant support and protection to a hand under pressure. Test subjects were first asked to make a fist and push the first joint of the index finger firmly against a device that measured the rigidity of the knuckle joint. Test subjects repeated this process for each of the three fist postures shown here (note the placement of the fingers relative to the palm and the positioning of the thumb over the fingers):
Morgan and Carrier found that positioning the fingertips against the central palm and wrapping the thumb across the backs of the pointer and middle fingers served as a supportive "buttress" for the hand, and locked the digits into a solid shape that facilitated the transfer of energy from the fingers to the wrist. This finger positioning not only quadrupled the rigidity of the first knuckle joint, it also doubled the ability to deliver "punching" force, relative to the more loosely-arranged conformations.
No other hominoid employs this clenched-fist configuration, yet to us humans it feels very natural. A clenched fist is used in fighting styles practiced all over the world, and is universally recognized as a sign of aggression. Even infants are known to use a ‘closed hand' to communicate anxiety and distress.
That most male hominoids still compete with one another over mates suggests that bigger forelimbs would have been evolutionarily advantageous to our forebears, giving rise to the dramatic physiological differences that we see in males and females today. Such differences are especially common in the upper bodies of men and women, including the hands. The ratio between the lengths of the pointer and ring fingers, for example, is lower in males than in females. Among mammals, note the researchers, physiological differences between the sexes are often greatest in those characteristics that improve a male's ability to dominate over other males. Repeating the present study with all female test subjects could help shed light on whether these physiological differences between male and female hands actually arose out of a need for our male ancestors to resolve contention with optimally buttressed fists.
"There appears to be a paradox in the evolution of the human hand," the researchers ultimately conclude. "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." They continue:
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.
The researchers findings are published, free of charge, in the latest issue of The Journal of Experimental Biology.
Top image via Shutterstock; all other images via Morgan and Carrier