…biologists have uncovered differences in muscle architecture between chimpanzees and humans. But evolutionary biologist Alan Walker, a professor at Penn State University, thinks muscles may only be part of the story.
In an article published in the April issue of Current Anthropology, Walker argues that humans may lack the strength of chimps because our nervous systems exert more control over our muscles. Our fine motor control prevents great feats of strength, but allows us to perform delicate and uniquely human tasks.
Walker’s hypothesis stems partly from a finding by primatologist Ann MacLarnon. MacLarnon showed that, relative to body mass, chimps have much less grey matter in their spinal cords than humans have. Spinal grey matter contains large numbers of motor neurons—nerves cells that connect to muscle fibers and regulate muscle movement.
More grey 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. We can engage just a few muscle fibers for delicate tasks like threading a needle, and progressively more for tasks that require more force. Conversely, since chimps have fewer motor neurons, each neuron triggers a higher number of muscle fibers. So using a muscle becomes more of an all-or-nothing proposition for chimps. As a result, chimps often end up using more muscle than they need.
“[A]nd that is the reason apes seem so strong relative to humans,” Walker writes.
