It's tempting to picture human evolution as a single package deal — walking upright and thinking with a bigger brain arriving together, each making the other useful. The fossil record says otherwise. Bipedal walking shows up in the hominin lineage roughly six to seven million years ago, while brain size stayed close to that of a modern chimpanzee for millions of years afterward, only beginning a sustained increase around two million years ago. Whatever pushed our ancestors upright, it wasn't a bigger brain.
What the fossils actually show, in order
Early hominins like Sahelanthropus and Ardipithecus, and clearer bipedal walkers like Australopithecus afarensis — the species that includes the famous "Lucy" skeleton, dated to about 3.2 million years ago — show skeletal adaptations for upright walking: a forward-positioned spinal opening at the base of the skull, a shortened pelvis shaped for balance over two legs, and leg and foot bones built for striding rather than the knuckle-walking or tree-climbing gait of other apes. Their brains, meanwhile, stayed roughly 400 to 500 cubic centimetres — in the same range as a modern chimpanzee, not meaningfully larger. Sustained brain growth toward modern human size doesn't clearly begin until later species, particularly Homo erectus, around 1.9 million years ago onward.
Two separate evolutionary pressures, not one
The gap matters because it undercuts a once-popular assumption that walking upright freed the hands, which then enabled tool use, which then drove brain growth, all as one connected story. The timeline doesn't support that clean sequence — bipedalism was already established for millions of years before brains started growing, and the earliest confirmed stone tools appear around 3.3 million years ago, itself well before the major brain size increase. Researchers now generally treat upright walking and brain enlargement as responses to different pressures at different times — bipedalism plausibly linked to changing habitats and the energetic efficiency of walking versus climbing, brain growth plausibly linked to later dietary shifts, social complexity, and tool sophistication — rather than as two steps of a single, tidy evolutionary plan.
What we're still unsure about
Exactly why bipedalism emerged at all remains genuinely debated among palaeoanthropologists — competing hypotheses point to energy-efficient long-distance walking, freeing hands to carry food or infants, thermoregulation in open habitats, or some combination, and the fossil and environmental evidence doesn't yet single out one cause decisively. The hominin fossil record is also still being actively revised: new finds regularly shift the dates and relationships between species, and which fossils belong on the direct line to modern humans versus on side branches that left no living descendants is an active, evolving area of research rather than a fixed family tree.
This sits inside Human Evolution & the Hominin Lineage, one of seven topics in Evolution, one of six domains in Biology, one of seventeen subjects the app can quiz you on.