Hands Beat Feet. An Old Evolutionary Question is Answered

Scientists long debated whether tool use or upright posture came first. Now they may have the answer

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The hand of a Japanese macaque.

It’s hard to use a socket wrench if you’re walking on all fours. As for chopping wood, throwing a spear or handling a pneumatic drill? Not a chance. That was a problem that would have confronted any pre-human species that aspired to develop the ability to make and use tools—and just maybe to rule the planet a few million years later as a result. But a question always lingered for evolutionary scientists: Did bipedalism appear first, freeing up the hands for tools, or did humans’ early ability to fashion and use objects push us to our feet? Now, a study published in Philosophical Transactions of the Royal Society offers an answer: hands beat feet, the researchers say, with manual and intellectual dexterity occurring earlier in our history than bipedalism.

The new findings are based on work by researchers at the RIKEN Brain Science Institute in Tokyo and the University of Tokyo Museum. The investigators began their work by mapping the brain regions responsible for control of the fingers and toes in both human and monkey subjects, using functional magnetic resonance imaging (fMRI) for the humans and a decidedly more invasive technique involving drilling holes in the skull and implanting electrodes in the brain for the monkeys.

The so-called somatotopic brain maps that resulted from this work showed that when it came to hands, humans and monkeys have more in common than an evolutionarily vain species like us might want  to admit, with both of us having a separate brain site dedicated to each finger on both hands. For feet, things were different: a single region in the brain controls all five of the monkey’s toes, while we have one brain region controlling four of the toes and a separate one for the big toe.

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That, in some respects, ought not be a surprise, since both humans and monkeys exhibit impressive manual dexterity, but only one of us needs dedicated big-toe control to keep us balanced when we’re standing and walking. Nothing in the brain maps indicated which quality arose first, but a simple principle of evolution does—and that’s where the study’s subtle breakthrough lies. Traits that related species have in common almost always came earlier, while ones they don’t share often come later—particularly if they’re traits that would have driven them down opposite evolutionary paths. In this case, that means separate control of the big toe was the more recent innovation.

“In early quadruped hominids, finger control and tool use were feasible, while an independent adaptation involving the use of the big toe for functions like balance and walking occurred with bipedality,” wrote the authors in the study.

The findings were bolstered by additional analysis of the fossilized remains of the Ardipithecus ramidus, a 4 ft. (120 cm) tall, 110-lb. (50 kg) hominin that lived in northern Ethiopia 4.4 million years ago. The architecture of the specimen’s hands suggests a dexterity similar to that of modern humans and modern monkeys, and Ardi lived long before the two species went their separate evolutionary ways.

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“Evolution is not usually thought of as being accessible to study in the laboratory,” said RIKEN neurobiologist Atsushi Iriki in a release that accompanied the paper. “But our new method of using comparative brain physiology to decipher ancestral traces of adaptation may allow us to re-examine Darwin’s theories.”

The new work, of course, has its limitations. Both the scans and the fossil work look only at comparatively gross structures in the skeleton and the motor regions of the brain. Millions of years worth of additional evolution were necessary to give humans the manual, pedal and intellectual ability to thread a needle, perform microsurgery or dance Swan Lake. A better understanding of where we started, however, gives us a better appreciation of how far we’ve come.

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