Knuckle-walking is a form of quadrupedal locomotion involving a specific posture of the forelimbs. It is an adaptation that balances the needs of ground travel with other functions of the hands and fingers. This method of movement is a subject of scientific interest due to its limited use among animal species and its potential connections to primate history.
What is Knuckle-Walking?
Knuckle-walking is a method of movement where an animal supports its body weight on the knuckles of its forelimbs. Specifically, the animal curls its fingers and presses the dorsal, or back, side of the middle phalanges—the bones in the middle of the fingers—against the ground. This posture keeps the palms and sensitive undersides of the fingers off the ground, and the outer fingers are often held clear entirely.
This form of locomotion is associated with African apes, namely gorillas, chimpanzees, and bonobos. For these primates, it serves as a compromise, allowing them to use their hands for terrestrial movement while preserving the long fingers needed for gripping branches and manipulating food. While other animals, such as giant anteaters and platypuses, also employ a form of knuckle-walking, it is for reasons related to their own anatomies, like protecting large digging claws or swimming webbing.
The way gorillas and chimpanzees knuckle-walk shows subtle differences. Gorillas keep their wrist and hand joints in a straighter, more “columnar” alignment. In contrast, chimpanzees use a more extended wrist posture, a variance attributed to the greater amount of time they spend climbing in trees compared to the more terrestrial gorillas. This highlights how the gait varies based on an animal’s lifestyle and anatomy.
Anatomy of a Knuckle Walker
A primary adaptation for knuckle-walking is the locking mechanism of the wrist joints. In gorillas, the forearm and wrist bones can lock together, creating a solid, weight-bearing column that prevents the joint from collapsing under the animal’s mass. This structure is different from the more flexible wrist of a human.
The bones of the hand are also adapted for this purpose. The metacarpals, the bones that form the palm area, are thick and robust in knuckle-walkers. The ends of the radius bone in the forearm are shaped to form a more secure and stable base. This reinforcement helps dissipate ground-reaction forces during movement, protecting the smaller bones and joints of the hand from damage.
These anatomical adaptations are a functional compromise. The long arms, short legs, and stiff backs of apes are suited for climbing but make sustained upright walking difficult. Knuckle-walking allows these animals to be mobile on the ground without forfeiting the traits needed for life in the trees.
The Link to Human Ancestors
The question of whether human ancestors were knuckle-walkers has been a long-standing debate in paleoanthropology. One hypothesis suggested that bipedalism evolved from an ancestor that was already adapted to moving on the ground via knuckle-walking. This idea was supported by anatomical similarities in the wrist, elbow, and shoulder shared by humans and African apes.
Some researchers pointed to features in the fossilized wrist bones of early hominins like Australopithecus anamensis and Australopithecus afarensis as evidence of a knuckle-walking past. They argued that specific morphologies in the distal radius (the end of the forearm bone at the wrist) were retained from a knuckle-walking ancestor, even as these species began walking upright.
However, the current scientific view has shifted. The discovery of older hominid fossils, such as Ardipithecus ramidus, has provided new insights. Ardipithecus specimens show adaptations for upright walking but lack the specialized skeletal features of a knuckle-walker. This suggests that knuckle-walking may have evolved independently in gorillas and chimpanzees after their lineage split from the one that led to humans.