Three predecessor species anchor this look at what actually separates humans from the rest of the animal kingdom: Australopithecus afarensis, Homo erectus, and early Homo sapiens. Archaeologists and makeup artists rebuild the face of Lucy, the famous afarensis skeleton who stood upright roughly four million years ago, and set the recreation walking through modern London to make the timescale visible. Homo erectus gets the same treatment: a model of the species, which appeared around 1.8 million years ago with a brain about two-thirds the size of ours but the first mastery of stone tools and fire, is dropped into a present-day butcher's shop to see whether modern hands can still work ancient tools. The film then follows early Homo sapiens through an ice age that shrank populations and forced changes in toolmaking, communication, and problem-solving, changes it argues set the template for everything that came after. Interviews with paleoanthropologists carry the argument between reenactments, tracking bipedalism and brain size as the two traits that mark the line between ape and human.