Upright walking was a trade-off, not an upgrade

Bipedalism is often framed as a triumphant step toward humanity, but Lieberman shows it was a costly compromise. Walking on two legs freed the hands and reduced the energy cost of covering long distances compared to a quadrupedal ape, which mattered enormously for early hominins searching for scattered food across African savannas.

The cost was significant: a narrower, less stable base of support, a spine and pelvis under new structural strain, and vulnerability to back and joint problems that quadrupeds mostly avoid. Childbirth also became more difficult and dangerous, since an upright pelvis had to balance the competing demands of walking efficiently and passing an increasingly large-brained infant's head through the birth canal.

Every major evolutionary shift in this story, Lieberman keeps returning to, bought a survival advantage at a real biological price — bipedalism didn't make us perfect, it made us better suited to one set of pressures while creating new vulnerabilities that persist today, including much of modern lower-back pain.