The ability to walk on two legs, known as bipedalism, developed as a gradual evolutionary transformation over millions of years. This fundamental shift in locomotion is one of the defining traits separating the human lineage from other primates. The transition involved complex anatomical changes, likely driven by changing environmental conditions. Understanding when this occurred requires looking at the fossil record for evidence of this change in posture and movement.
The Evolutionary Timeline of Bipedalism
The earliest potential evidence for hominin bipedalism stretches back 6 to 7 million years ago (mya) in the late Miocene epoch. Sahelanthropus tchadensis (7 mya) is considered the oldest known bipedal ancestor, based on the position of its foramen magnum. Following this is Orrorin tugenensis (6 mya), whose femur structure suggests upright walking, although the extent of its bipedalism remains debated.
This early bipedalism was likely facultative, meaning hominins could walk upright when needed but still spent significant time in trees. The genus Australopithecus, appearing around 4 mya, represents a major step toward more consistent upright walking. Species such as Australopithecus afarensis (4 to 2.8 mya) showed clear signs of habitual bipedalism alongside retained tree-climbing adaptations.
The shift toward obligate bipedalism, where upright walking became the primary and most efficient mode of travel, continued with the evolution of the genus Homo around 2.8 mya. By 2 million years ago, anatomical features supporting a human-like stride were much more pronounced, though not fully modern. Bipedalism was one of the earliest characteristics to evolve in the human lineage, preceding the increase in brain size and the development of sophisticated tool use.
Fossil and Trace Evidence of Early Walking
Scientists identify bipedalism by examining specific skeletal modifications that support an upright posture. One feature is the forward and downward position of the foramen magnum at the base of the skull, indicating the head was balanced atop a vertical spine. The pelvis also shifts from the tall, narrow structure seen in apes to a shorter, wider, bowl-shaped structure in hominins like Australopithecus, which helps stabilize the torso over one leg during walking.
In the lower limbs, the femur angles inward from the hip to the knee—the bicondylar angle—which positions the feet directly beneath the center of gravity. This arrangement allows the hominin to balance efficiently on one leg while the other swings forward. The foot structure also changed, losing the grasping big toe and developing longitudinal arches that act as shock absorbers and rigid levers for push-off.
The most direct physical record of early walking comes from trace evidence, specifically the Laetoli footprints discovered in Tanzania, dated to about 3.66 million years ago. These tracks, preserved in hardened volcanic ash, provide an indisputable snapshot of hominin gait. The footprints show a distinct heel-strike and toe-off pattern, suggesting the individuals—likely Australopithecus afarensis—were walking with a stride kinematically similar to modern human walking, though perhaps with a slightly more flexed limb posture.
Selective Pressures Driving Bipedalism
The question of why our ancestors transitioned to walking on two legs is addressed by several competing theories. One prominent idea centers on energy efficiency, suggesting that bipedal locomotion is significantly more economical for covering long distances than the quadrupedal knuckle-walking of apes. This advantage became pronounced as environments shifted and hominins needed to travel further across open grasslands to find scattered food resources.
Another major theory focuses on thermoregulation and survival in the hot, open savanna. Standing upright exposes less body surface area to direct overhead sunlight, reducing heat absorption. Being higher off the ground also allowed for greater exposure to cooling breezes, a significant advantage in the increasingly arid environments of East Africa.
The ability to carry items was likely another powerful selective pressure. Freeing the hands allowed early hominins to transport gathered food, water, or tools back to a central location or carry infants, increasing the survival rate of offspring and the efficiency of resource acquisition. Finally, the “vigilance” hypothesis proposes that standing upright allowed hominins to see over tall grass, helping them spot predators or distant food sources more easily.
Defining Modern Human Gait
Modern human gait is characterized by highly specialized anatomical features that make it remarkably energy-efficient, a form known as obligate bipedalism. Walking is defined by a “pendulum-like” motion where the body’s center of gravity is maintained with minimal vertical or side-to-side displacement. This efficiency is achieved through a fully developed, arched foot that stores and releases elastic energy with each step.
The knee joint can fully extend and “lock” momentarily, allowing the leg muscles to relax during the stance phase, further reducing energy expenditure. The narrow human pelvis ensures the legs remain close to the midline of the body, minimizing the lateral shift of the body’s weight. This specialized, straight-legged stride contrasts sharply with the hypothesized “bent-knee, bent-hip” gait used by earlier hominins like Australopithecus, whose less efficient movement was a compromise with their continued ability to climb trees.