The earliest evidence of bipedalism dates to roughly 7 million years ago, based on a skull found in Chad belonging to a species called Sahelanthropus tchadensis. That puts upright walking remarkably close to the estimated time when the human and chimpanzee lineages split, suggesting our ancestors may have begun walking on two legs almost immediately after branching off from other great apes.
The 7-Million-Year-Old Skull
Sahelanthropus tchadensis was announced in 2002 based on a cranium dated to between 6.7 and 7.2 million years ago. The key piece of evidence is the foramen magnum, the hole at the base of the skull where the spinal cord exits. In Sahelanthropus, this opening is positioned toward the front of the skull and angled downward, which is characteristic of creatures that carry their head balanced on top of an upright spine rather than jutting forward from a horizontal one. Additional analysis of the skull’s internal structure shows basicranial flexion and a strong posterior position of the occipital lobes, both consistent with upright head carriage.
This remains the oldest proposed evidence for bipedalism, though it’s also the most debated. A skull can hint at how the head was held, but it can’t tell you how the creature walked. Without leg or foot bones from the same species, some researchers consider the case incomplete.
Stronger Evidence From Leg Bones
The next major candidate is Orrorin tugenensis, known from fossils found in Kenya’s Lukeino Formation and dated to about 6 million years ago. Orrorin is significant because it preserves something Sahelanthropus does not: three fragments of thighbone. Detailed anatomical comparisons show these femora share a suite of features with later human ancestors and modern humans, but none with chimpanzees or gorillas. In fact, the femur of Orrorin is closer in shape to a modern human thighbone than to those of australopithecines, the group that came millions of years later. This is one of the strongest early indicators of habitual bipedal walking.
A toe bone from Ardipithecus kadabba, dated to 5.2 million years ago, also shows features associated with bipedal walking. But the evidence is limited to that single bone, found 10 miles from the rest of the Ardipithecus kadabba specimens, so it’s treated cautiously.
From Part-Time to Full-Time Walking
Early bipeds almost certainly didn’t walk the way you do. The transition from occasional, or “facultative,” bipedalism to permanent, “obligate” bipedalism played out over millions of years and left clear marks on the skeleton. The distinction comes down to whether a body is simply capable of walking upright or whether it’s built so thoroughly for upright walking that other forms of locomotion become awkward.
By about 4 million years ago, that transition was well underway. A fossil shinbone from Kanapoi, Kenya, belonging to Australopithecus anamensis (dated to 4.12 million years ago), shows an ankle joint oriented perpendicular to the long axis of the leg, just like in modern humans. In chimps and gorillas, this joint is angled significantly, allowing the foot to rotate inward for tree climbing. Every early hominin tibia examined from this period onward shares the human configuration, suggesting these species were committed ground walkers.
Australopithecus afarensis, the species that includes the famous “Lucy” skeleton from about 3.2 million years ago, shows even more dramatic changes. The pelvis had shortened, widened iliac blades that flared out laterally over the hip joint, a shape that anchors the muscles needed to stabilize the body over one leg during each stride. The attachment point for the hamstrings had rotated from a downward orientation (as seen in apes) to a more posterior one, matching the arrangement in modern humans. The overall pelvis was extremely wide relative to body size, wider than in any modern human of similar stature, and broader side to side than front to back.
The Laetoli Footprints
The oldest indisputable proof of bipedalism comes not from bones but from footprints. Discovered in 1977 at Laetoli, Tanzania, a trail of hominin tracks was preserved in volcanic ash dated to about 3.66 million years ago. These prints were made by individuals walking on two feet across an open landscape.
Biomechanical analysis reveals that the Laetoli walkers used a gait that was recognizably bipedal but not quite modern. They likely struck the ground with a more bent-knee posture than a typical modern human stride. Their foot arch was intermediate: shallower than a modern human’s but much more developed than a chimpanzee’s. These prints capture a snapshot of bipedalism still being refined, with important evolutionary changes to stride mechanics occurring in the millions of years that followed.
Why Walk Upright at All?
The question of when bipedalism appeared is inseparable from the question of why. Several theories have been proposed, and the answer is likely a combination of factors rather than a single cause.
One influential idea is that walking upright was originally a feeding posture, not a traveling one. Chimpanzees today stand on two legs when gathering fruit from low branches, and early hominins may have developed bipedalism alongside arm-hanging as a way to efficiently harvest food in open woodlands. The fact that early bipedalism appears to have been somewhat energetically inefficient for long-distance travel supports this view.
Energy savings did become important over time, though. Modeling studies show that under all temperature conditions examined, bipedalism allowed hominins to forage at higher temperatures, cover greater distances, and consume less food and water than a quadruped of similar size. For a creature living in increasingly open, hot environments as African forests shrank, those advantages would have compounded generation after generation.
Thermoregulation offers another piece of the puzzle. When the sun is directly overhead, only about 8% of a biped’s body surface is exposed to direct sunlight, compared to 18% for a quadruped. Earlier models suggested this sun-avoidance effect alone could explain bipedalism’s advantage, but more recent work shows the picture is more nuanced. For a hair-covered individual standing still, posture makes little difference to heat balance because internally generated heat dominates. The thermoregulatory benefit of bipedalism becomes significant only after hair loss and efficient sweating had evolved to near-modern levels. In other words, bipedalism and hair loss likely co-evolved, each making the other more advantageous.
A Timeline of Bipedal Evolution
- 7 million years ago: Sahelanthropus tchadensis skull suggests upright head carriage, the earliest indirect evidence of bipedalism.
- 6 million years ago: Orrorin tugenensis thighbones show habitual bipedal walking.
- 5.2 million years ago: A single Ardipithecus kadabba toe bone shows bipedal features.
- 4.12 million years ago: Australopithecus anamensis shinbone confirms a fully bipedal ankle configuration.
- 3.66 million years ago: Laetoli footprints preserve the oldest undeniable record of bipedal walking, with a gait that was upright but used more leg flexion and a flatter foot than modern humans.
- 3.2 million years ago: Australopithecus afarensis shows a pelvis and lower body extensively remodeled for obligate bipedalism.
The short answer is that bipedalism likely originated around 6 to 7 million years ago in some form, but it took roughly 3 to 4 million years of gradual skeletal remodeling before it began to resemble the efficient, striding walk humans use today.