Lieberman describes chimps hunting, killing, and eating a monkey—a sight which may remind humans of their own violence and physical inferiority. Humans’ physical inferiority compared to apes started when humans became bipedal. Researchers have yet to find the “missing link,” or last common ancestor (LCA), between humans and apes, but they infer that the LCA lived in African rain forests, which are not conducive to fossilization. A simple graphic shows that gorillas and humans diverged around 9 million years ago and that humans and chimps diverged 7 to 8 million years ago. Hypothetically, the LCA would have been similar in appearance to chimps and gorillas.

Since the 1990s, four early hominin species have been discovered: Sahelanthropus tchadensis from 7.2 million years ago, Orrorin tugenesis from 6 million years ago, Ardipithecus kadabba from 5.8 million years ago, and Ardipithecus ramidus from 4.4 million years ago. The best representative of the latter species is Ardi, a partial female skeleton. Few fossilized remnants have been found, but they provide insight into hominin evolution after the LCA. The fossils suggest early hominins were bipedal and ape-like. Lieberman suggests Ardi is an intermediate biped. Signs of bipedalism in Ardi include changes to the shape and orientation of the pelvis, enlarged hip joints, S-shaped spines, and arched feet.