HUMAN EVOLUTION

Walking Upright

~3.7 to 3.2 million years ago

Phanerozoic · Cenozoic · Neogene · Pliocene

Footprints in volcanic ash at Laetoli record the weight passing from heel to toe, the way it does in our own stride. A long argument about the foot itself turned on a single bone, a complete fourth metatarsal of Australopithecus afarensis: its head twisted against its base as a transverse arch imposes, both ends angled as a lengthwise arch requires, and no trace of the flexible midfoot an ape uses to grip. The shoulders and arms of the same species were still built for climbing.

Why it matters

Walking upright came first, long before large brains and earlier than either stone tools or meat. It was not a change of habitat either, since the same animal still went up trees. What the arch does is turn the foot into a stiff lever and cushion the shock of each step, which is what makes a long walk cheap, and it is the long walk that our lineage kept.

Dating & uncertainty

The Laetoli footprints at 3.66 Ma and the Lucy skeleton at 3.2 Ma bracket the evidence. How much time Australopithecus still spent in trees has been debated since the 1980s and is not settled.

Sources

1. Raichlen et al. (2010), PLoS ONE · Laetoli footprints preserve earliest direct evidence of human-like bipedal biomechanics
2. Ward et al. (2011), Science · Complete fourth metatarsal and arches in the foot of Australopithecus afarensis
3. Stern & Susman (1983), American Journal of Physical Anthropology · The locomotor anatomy of Australopithecus afarensis

Earth history as one day

23:58:501 min 10 s before midnight

if Earth’s 4.54 billion years were compressed into a single day