The size of the prey's third toe is important because, among birds, long third toes are helpful for grasping branches and perching in trees.

Still, heel and toe depths at Laetoli provide a glimpse of efficient walking “long before the emergence of our own genus, Homo, ” Jungers says.

To get a sense of just how long, he says, imagine a giant with her head in Baltimore and her toe off the coast of South Africa.