Stone Age people, unlike their Neandertal contemporaries, had heel bones spring-loaded for long runs, a new study suggests. Neandertals weren’t left in the dust, though. The backs of their feet gave them a leg up on power walking, say anthropologist David Raichlen of the University of Arizona in Tucson and his colleagues. In ancient Homo sapiens, as in people today, a short lower heel stretched the Achilles tendon taut, Raichlen’s team concludes. That arrangement increased the tendon’s spring-like action during running and reduced energy consumption, enabling extended excursions.