What is bipedalism?
Bipedalism is the ability to walk upright on two legs. It evolved in the hominin lineage roughly 6-7 million years ago and is the earliest defining trait of the human family. Walking upright freed the hands for tool use and carrying, and set the stage for the evolution of larger brains.
Bipedalism — habitual upright walking on two legs — is the foundational adaptation of the human lineage. It evolved approximately 6 to 7 million years ago, millions of years before stone tools, brain expansion, or language. In many ways, standing up was the first step toward becoming human.
The reasons bipedalism evolved remain actively debated among paleoanthropologists. Leading hypotheses include energy efficiency (walking on two legs uses less energy than quadrupedal locomotion over long distances), improved ability to spot predators and food sources over tall grass, thermoregulation (an upright body exposes less surface area to the tropical sun), and the advantage of freeing hands for carrying food, tools, or infants. The truth likely involves a combination of factors.
The consequences were profound. Free hands could carry objects, manipulate tools, and eventually create complex technologies. The restructuring of the pelvis required for upright walking narrowed the birth canal, which may have contributed to the evolution of smaller-brained, more helpless infants. This in turn demanded more intensive parenting and social cooperation — potentially driving the evolution of the complex social behaviors that characterize our species.
Key fossil evidence for early bipedalism includes the famous "Lucy" (Australopithecus afarensis, c. 3.2 million years ago) and the Laetoli footprints in Tanzania (c. 3.6 million years ago) — a trail of footprints preserved in volcanic ash that shows unmistakably human-like upright walking millions of years before the first stone tools.