Why Chimps Are Stronger Than Humans
This composite photograph - an homage to iconic, if outdated, "Evolution of Man" imagery - depicts the measurement of oxygen consumption during walking in quadrupedal and bipedal chimpanzees and in humans.
Credit: Cary Wolinsky
The brutal attack last month by a pet chimpanzee on a Connecticut woman was a stark reminder that chimps are up to four times stronger than humans.

One reason, a new study suggests, is that we pay a price for our fine motor skills.

Humans may lack the strength of chimps — our closest relatives on the tree of evolution — because our nervous systems exert more control over our muscles, says evolutionary biologist Alan Walker, a professor at Penn State University. Our fine motor control prevents great feats of strength, but allows us to perform delicate and uniquely human tasks, Walker writes in the April issue of the journal Current Anthropology.

Walker's hypothesis stems partly from a finding by primatologist Ann MacLarnon, who showed that relative to body mass, chimps have much less gray matter in their spinal cords than humans have. Spinal gray matter contains large numbers of motor neurons — nerves cells that connect to muscle fibers and regulate muscle movement.

Chimps are not without fine motor skills, of course. In fact separate study in January found that a relatively new brain area, developed in humans and other primates, gives us all an advantage in this realm.

More gray matter in humans means more motor neurons, Walker proposes. And having more motor neurons means more muscle control.

Our surplus motor neurons allow us to engage smaller portions of our muscles at any given time, she explained. We can engage just a few muscle fibers for delicate tasks like threading a needle, and more for tasks that require more force. Since chimps have fewer motor neurons, each neuron triggers a higher number of muscle fibers and using a muscle becomes more of an all-or-nothing proposition. As a result, chimps often end up using more muscle than they need.

"That is the reason apes seem so strong relative to humans," Walker writes.

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