Bipedalism is a form of locomotion defined by movement on two rear limbs or legs, a trait that sets the human lineage apart from most other mammals. While many animals can stand upright or take a few steps on two legs, humans have developed an anatomical structure that makes this mode of travel our primary means of movement. The transition to habitual upright walking is considered a defining moment in human evolution, profoundly influencing the anatomy and behavior that followed. This shift required a complete reorganization of the skeleton to support the body’s weight and maintain balance over a narrow base.
Defining Bipedalism and Its Forms
Bipedalism describes any terrestrial locomotion using two limbs, but the term encompasses a spectrum of behaviors in the animal kingdom.
Humans are considered obligate bipeds, meaning two-legged walking is the standard and most efficient form of locomotion for the species. Other obligate bipeds include birds, whose forelimbs are specialized as wings, leaving their hindlimbs as the sole means of ground travel.
In contrast, facultative bipedalism describes the ability to walk on two legs only temporarily, usually in response to a specific need or circumstance. Animals like chimpanzees, gibbons, and various species of lizards are considered facultative bipeds. For instance, a chimpanzee may walk bipedally for a short distance to carry food or use a threat display, but its primary mode of travel remains quadrupedal knuckle-walking.
The Specialized Anatomy for Upright Walking
The entire body structure was reshaped to stack the torso and head directly above the feet, minimizing the energy needed to maintain balance.
The pelvis underwent one of the most profound transformations, evolving from the long, narrow structure seen in apes to a shorter, broader, and bowl-shaped form in humans. This wide, flared structure provides a stable platform for the internal organs and allows the gluteal muscles to reorient. These muscles function to stabilize the pelvis and prevent it from tipping sideways during the single-leg stance phase of walking.
The vertebral column also adapted by developing a distinctive S-shaped curve, consisting of the backward-curving thoracic spine and the forward-curving lumbar spine. This configuration positions the body’s center of gravity directly over the hip joints, absorbing the shock of walking. The femur, or thigh bone, exhibits the valgus angle, slanting inward from the hip to the knee joint, which brings the knees and feet closer to the midline of the body.
Finally, the foot transformed from a grasping appendage to a rigid, weight-bearing platform with an arched structure. This longitudinal arch acts as a shock absorber and a spring, storing and releasing energy during the push-off phase of the stride. The big toe, or hallux, became non-opposable and aligned with the other toes, providing a firm lever for propulsion rather than a grasping tool.
Key Theories on the Origin of Bipedalism
The evolutionary pressure that first prompted early hominins to adopt bipedalism led to several competing explanations.
The Savanna Hypothesis suggests bipedalism arose as hominins moved from dense forests to more open, arid grasslands. Upright posture would have allowed early hominins to see over tall grasses, helping them spot predators or locate distant food sources.
The Carrying Hypothesis focuses on the freeing of the forelimbs. Standing upright allowed hominins to carry objects such as food gathered from foraging sites, tools, or infants safely. The ability to transport resources would have been a significant advantage for provisioning a family or a group.
The Thermoregulation Hypothesis proposes that bipedalism evolved as a mechanism for regulating body temperature in the hot, open environment. By standing upright, a hominin reduces the surface area of the body directly exposed to the intense midday sun. Furthermore, the upright stance raises the body higher off the ground, where wind speed is greater and the air is cooler, facilitating heat dissipation.
Energetic Costs and Advantages
The primary advantage of obligate bipedalism is its remarkable energetic efficiency for walking long distances. Studies comparing human walking to quadrupedal or bipedal walking in chimpanzees show that humans expend about 75% less energy than chimpanzees when traveling at a walking pace.
This low metabolic cost stems from the specialized anatomy, which allows the legs to function like inverted pendulums, minimizing muscle effort. This efficiency provided an advantage for early hominins who needed to forage across large territories or engage in endurance running.
However, the upright stance introduced several biomechanical vulnerabilities not seen in quadrupeds. The stacked vertebral column places considerable strain on the lower back, increasing the risk of intervertebral disc issues and chronic pain. The inward-slanted femur and the need to absorb the full body weight on two limbs make the knee and hip joints susceptible to wear and tear over a lifetime. Furthermore, the narrow human pelvis, adapted for efficient walking, created the obstetric dilemma, which imposes a constraint on the size of the birth canal, making childbirth more difficult compared to other primates.