Bipedalism, the ability to walk upright on two legs, is the trait that launched the human lineage. It appeared roughly 7 million years ago in our earliest ancestors and set off a cascade of changes to the skeleton, brain, and behavior that ultimately produced modern humans. No other primate walks the way we do, and understanding when and why our ancestors stood up is one of the central questions in the story of human evolution.
The Earliest Evidence of Walking Upright
The oldest known species with adaptations for bipedalism is Sahelanthropus tchadensis, which lived between 6.7 and 7.2 million years ago in what is now Chad. When its skull was first described in 2002, researchers noted that the opening where the spinal cord meets the skull (called the foramen magnum) sat toward the base of the skull rather than the back, a hallmark of creatures that carry their heads balanced on top of an upright spine. More recent analysis of limb fossils confirmed that Sahelanthropus was indeed an early biped that evolved from an ancestor resembling modern chimpanzees.
The most famous direct proof of bipedalism in the fossil record comes from the Laetoli footprints in Tanzania, discovered in 1977. These tracks, preserved in volcanic ash roughly 3.66 million years old, remain the earliest indisputable evidence of bipedal walking. They were left by Australopithecus afarensis, the same species as the well-known fossil “Lucy.” Analysis of the prints shows a gait that was recognizably human but not identical to ours: the walkers struck the ground with slightly more bent knees and hips than modern people do, and their foot arches were flatter, sitting somewhere between a chimpanzee’s flexible foot and the stiff, arched foot we have today.
Why Our Ancestors Stood Up
Scientists have proposed several explanations for why bipedalism evolved, and the answer is likely a combination of pressures rather than a single cause.
The most enduring idea is the savanna hypothesis. As African forests shrank and gave way to open grasslands between 6 and 8 million years ago, early hominins that could walk efficiently on two legs had an advantage over those that stayed in the trees. Standing upright freed the hands for carrying food and tools, allowed better visibility over tall grass, and made long-distance travel between scattered food sources more practical.
A related theory focuses on heat. Walking upright in an open equatorial landscape drastically reduces the amount of body surface exposed to direct overhead sunlight. A quadruped’s entire back absorbs solar radiation at midday, while an upright biped exposes only the top of its head and shoulders. This would have allowed early hominins to remain active during the hottest parts of the day, foraging and traveling when quadrupedal competitors needed to rest in shade.
Energy savings likely reinforced the shift. When humans walk on two legs versus crawling on all fours at the same speed, quadrupedal movement costs about 254% more energy. Even compared to chimpanzees walking quadrupedally, human bipedal walking is significantly cheaper per unit of distance, making it a far more efficient way to cover the long stretches between food and water sources on an open landscape.
How the Skeleton Transformed
Walking on two legs demanded a near-total redesign of the skeleton, from skull to toes. In quadrupedal apes, the foramen magnum sits toward the back of the skull because the head projects forward from the spine. In bipeds, it shifts to the underside of the skull so the head balances directly on top of the vertebral column. Among all primates studied, humans have the most anteriorly positioned foramen magnum, and this pattern holds across other bipedal mammals too: kangaroos and bipedal rodents show the same forward shift compared to their four-legged relatives.
The pelvis underwent the most dramatic transformation. Apes have tall, narrow hip blades oriented toward the back. The human pelvis is short, wide, and bowl-shaped, with hip blades that curve around the sides of the body. This shape does two critical things: it anchors the muscles that stabilize the trunk over a single supporting leg during each step, and it cradles the internal organs that would otherwise press downward under gravity. Research published in Nature in 2025 traced this change to two specific shifts in how the hip bone develops before birth. One involves a rotation in the direction that cartilage cells grow, and the other changes where and when bone begins to harden. These developmental innovations are unique to humans among all primates, and possibly all mammals.
The foot changed just as profoundly. Apes have a grasping foot with a mobile big toe, useful for climbing but poor for walking. The human foot traded that flexibility for a stiff longitudinal arch that acts like a spring and a lever. During walking, the arch deforms under your body weight, storing energy in the connective tissues along the sole. Then, during push-off, small muscles in the foot actively stiffen the forefoot, turning it into a rigid platform that transmits propulsive force to the ground. Without that active stiffening, your ability to push off powerfully with each step would be significantly impaired.
The Obstetric Dilemma
Bipedalism did not come without costs, and one of the most consequential affects every human birth. When the hip blades shortened and the sacrum dropped to create an efficient walking pelvis, the birth canal became narrower and more convoluted. At the same time, human brain size was increasing dramatically over the course of evolution, meaning bigger-headed babies needed to pass through a smaller opening.
The result is what anthropologist Sherwood Washburn called the “obstetrical dilemma” in 1960. Unlike other primates, whose infants pass through the birth canal in a relatively simple path, human babies must rotate and flex through a series of tight turns to navigate the twisted pelvic canal. One proposed solution to this conflict was to give birth earlier in development, before the head grows too large. This may explain why human newborns are so helpless compared to other primate infants: they arrive neurologically and physically immature, essentially completing outside the womb a stage of development that other species finish before birth.
Back Pain as an Evolutionary Legacy
Humans suffer from spinal problems far more often than other primates, and bipedalism is a major reason why. Walking upright loads the lower spine with compressive forces it was never originally “designed” for in our quadrupedal ancestors. The lumbar vertebrae bear the brunt of this, and not everyone’s spine is equally well adapted to the task.
A study in BMC Evolutionary Biology found that people who develop herniated discs tend to have vertebrae shaped more like those of chimpanzees: rounder vertebral bodies with shorter, wider attachment points. People whose vertebrae are more distinctly human in shape, with features better suited to handling vertical loads, are less prone to these injuries. In other words, natural variation in how “bipedal-adapted” your spine is can influence your risk of back problems. The combination of round vertebral bodies and short supporting structures appears to provide less support during upright posture, making certain individuals more vulnerable to the forces that bipedalism generates.
Bipedalism as the Starting Point
What makes bipedalism so central to human history is that it preceded nearly every other trait we think of as distinctly human. Our ancestors walked upright millions of years before they developed large brains, made stone tools, controlled fire, or developed language. Freeing the hands from locomotion created the conditions for tool use. Efficient long-distance walking enabled new foraging strategies. The metabolic savings of bipedal travel may have freed up energy that could be invested in growing a larger, more demanding brain.
The timeline makes this clear: bipedal adaptations appear by 7 million years ago, stone tools by about 3.3 million years ago, significant brain expansion by around 2 million years ago, and controlled fire use by roughly 1 million years ago. Each of these milestones built on the foundation that upright walking established. Bipedalism was not just one change among many. It was the first domino, the adaptation that made the rest of human evolution possible.