Knuckle walking is a distinct method of movement among certain land animals. This unique form of quadrupedal locomotion involves supporting body weight on the backs of the fingers, specifically the middle knuckles, rather than on flat palms or fingertips. This specialized gait is relatively uncommon, primarily observed in a few select groups. It allows these animals to navigate their environments while keeping their hands free for other purposes.
Animals That Knuckle Walk
The most widely recognized animals that employ knuckle walking are the great apes of Africa: gorillas and chimpanzees. Both species use this movement when traveling on the ground. While gorillas primarily rely on knuckle walking, chimpanzees also engage in it, often in conjunction with arboreal movements.
Knuckle walking is also observed in other species such as giant anteaters and platypuses. Giant anteaters curl their long, sharp digging claws under their paws to protect them while moving across the ground. Similarly, platypuses knuckle walk to keep their webbed front feet from interfering with terrestrial movement, as their webbing extends past their fingers.
The Biomechanics of Knuckle Walking
Knuckle walking in African apes involves specific anatomical adaptations in the wrist and hand, allowing weight to be borne on the dorsal surfaces of the middle phalanges (the bones in the middle of the fingers). Gorillas and chimpanzees exhibit differences in their knuckle-walking mechanics. Gorillas maintain a straighter, more extended elbow posture, which provides stability for their large body size. Their wrist joints remain stiff, with limited extension, as they swing their limbs.
Chimpanzees, in contrast, show more variability in their wrist and elbow postures, often using more wrist ulnar deviation (bending the wrist towards the pinky finger) and a slightly more flexed elbow during locomotion. Their metacarpophalangeal joints, where the fingers meet the hand, can extend significantly during this gait. While finger flexor muscles were once hypothesized to buffer ground impact forces, current biomechanical analysis suggests they do not play a significant role in impact absorption during knuckle walking.
Evolutionary Significance
Knuckle walking likely evolved in African apes as an adaptation to their mixed arboreal and terrestrial lifestyles. This gait offers a protective mechanism for the delicate fingers and palms, preventing wear and tear that would occur if they walked on flat hands. By keeping the palms off the ground, the hands remain adapted for fine manipulation, such as gripping branches for climbing, foraging for food, or using tools.
For large-bodied apes like gorillas, knuckle walking provides a stable quadrupedal platform for moving on the ground, where their size makes extensive arboreal locomotion challenging. The wrist morphology, with features that create a stable, locked wrist, supports the considerable body weight transferred through the forelimbs. This locomotion allowed these apes to transition to more terrestrial environments while retaining the versatile hand structure beneficial for their arboreal activities.
Knuckle Walking in Human Evolution
The role of knuckle walking in the direct lineage to modern humans has been a subject of scientific discussion. While some early research suggested that human ancestors might have passed through a knuckle-walking phase, more recent evidence points away from this hypothesis. The current consensus suggests that bipedalism, or upright walking on two legs, evolved from a more generalized arboreal ape ancestor, rather than directly from a knuckle-walking one.
Fossil evidence, such as that from Ardipithecus ramidus, indicates adaptations for upright walking in early hominins without clear signs of sustained knuckle-walking. Differences in wrist bone morphology between African apes and hominins further support the idea that knuckle walking evolved independently in the chimpanzee and gorilla lineages after their divergence from the human lineage. The transition to bipedalism in human evolution represents a distinct locomotor shift, setting our lineage apart from the knuckle-walking adaptations seen in our ape relatives.