The brutal chimpanzee attack on a Connecticut woman in 2009 served as a stark reminder of the significant strength disparity between humans and chimpanzees. Chimpanzees possess up to four times the strength of humans. But why are humans so weak compared to our closest primate relatives? A leading hypothesis suggests a trade-off: we sacrificed brute strength for fine motor control.
The Trade-Off: Strength for Dexterity
Evolutionary biologist Alan Walker, a professor at Penn State University, proposes that the key to understanding human weakness lies within our nervous systems. Our nervous systems exert a greater degree of control over our muscles than those of chimpanzees. This enhanced control allows for the intricate and precise movements necessary for tasks like writing, playing musical instruments, or threading a needle—activities unique to humans. However, this fine motor control comes at a cost: reduced maximum strength.
Walker’s hypothesis is partly based on research by primatologist Ann MacLarnon, who discovered that chimpanzees have significantly less gray matter in their spinal cords relative to their body mass compared to humans. Spinal gray matter houses motor neurons, nerve cells that connect to muscle fibers and control muscle movement.
More Motor Neurons, More Control, Less Strength
The abundance of gray matter in the human spinal cord translates to a higher number of motor neurons. Walker argues that this surplus of motor neurons allows humans to activate smaller portions of their muscles with greater precision. For delicate tasks, we can engage only a few muscle fibers. For tasks requiring more force, we engage more fibers, but still with a finer level of control compared to chimps.
Chimpanzees, with fewer motor neurons, have a less nuanced system. Each neuron activates a larger number of muscle fibers, making muscle activation more of an “all-or-nothing” process. This results in chimps often using more muscle force than necessary for a given task. This difference in neural control explains, at least in part, why chimpanzees appear so much stronger than humans. Their muscular system is geared towards power, while ours is optimized for precision.
A New Brain Area for Fine Motor Skills
Interestingly, separate research has identified a relatively recent brain area developed in both humans and other primates that enhances fine motor skills. While chimpanzees possess this brain area, the differences in spinal cord gray matter and motor neuron density contribute significantly to the strength discrepancy between the two species.
Conclusion: Evolutionary Trade-offs
The relative weakness of humans compared to chimpanzees highlights a fundamental principle of evolution: trade-offs. While we may not possess the raw strength of our primate relatives, we have evolved a unique level of fine motor control that has enabled the development of complex tool use, intricate communication, and sophisticated cultural practices that define our species. Our strength lies not in brute force, but in the dexterity and precision of our movements.
