The first steps toward walking upright

What happened

The Smithsonian's Human Origins Program traces the oldest evidence for walking on two legs back to two of the earliest known species in the human lineage. Sahelanthropus tchadensis, dated to about 7 to 6 million years ago, may have walked on two legs, based on features of its skull. Around 6 million years ago, Orrorin tugenensis left behind a thigh bone whose upper portion resembles that of other large apes, but whose angled neck closely resembles a modern human's, forming what the Smithsonian describes as a strong bridge with the hip capable of supporting the body's weight while walking upright. For a long stretch afterward, from at least 6 to 3 million years ago, early human ancestors combined apelike and humanlike ways of moving, still capable of climbing trees while increasingly walking upright on the ground.

How we know

The Smithsonian Human Origins Program dates Sahelanthropus to about 7 to 6 million years ago and describes the skull evidence for possible bipedalism; it separately describes the Orrorin tugenensis femur and the anatomical reasoning connecting its shape to upright walking. Because the fossil evidence for these earliest species is limited, the interpretation that either walked upright is treated by researchers as likely rather than certain, so this event is marked as debated.

Sources

• Smithsonian Institution. Walking upright (Smithsonian Human Origins Program) (2024) · Reputable sourcehumanorigins.si.edu · The domain "humanorigins.si.edu" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
• Earliest evidence of hominin bipedalism in Sahelanthropus tchadensis (peer-reviewed, via PubMed Central) · Peer-reviewed (author-declared)pmc.ncbi.nlm.nih.gov · Cited as a "journal" source (no stronger domain match). · Link is live and its text matches the event's key terms (Jul 2026)