HUMAN EVOLUTION

Walking Upright

~3.7 to 3.2 million years ago

Phanerozoic · Cenozoic · Neogene · Pliocene

Footprints in volcanic ash at Laetoli record a heel striking first and the weight rolling forward, the way ours does. A single bone settled the anatomy behind it: a complete fourth metatarsal of Australopithecus afarensis, twisted at one end in the way that only a transverse arch produces, angled at both in the way a lengthwise arch requires, and with 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, 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 store and return energy at every step, which is what makes a long walk cheap, and the long walk is the thing 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 argued 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