He is portrayed by Mark Wahlberg.
Leo Davidson was an astronaut officer who was stationed onboard the space station Oberon researching genetic engineering on chimps, orangutans and gorillas. He worked regularly with a chimp named Pericles who he trained to perform basic tasks out in space. When a great electromagnetic storm was detected in space, Pericles was dispatched in a small craft to check it out. Leo didn't like the idea and would have preferred to go himself. However his commander, Vasich, believed in sending a chimp before sending a man to explore something dangerous. When Pericles lost contact while piloting the space pod, Leo snuck onboard another pod and followed him, only to get caught in the same electrical storm. The storm rocketed Leo through and caused his pod to crash on a strange new world ruled by apes but also populated by humans who existed as savages and slaves.
Escaping slavery in ape city, Leo traced a signal from his space station colleagues and tried to find the rescue mission. Instead he found the thousand-year-old remains of the crashed station itself and realized that the apes and humans were the descendants of his crew-mates and cargo. The station was hurled into the planet while searching for him, while he was thrown forward through many centuries of evolution and genetic mutation. Leo led a revolt against the apes, eventually leading the human forces to an important victory. He returned through time to Earth, only to find it also ruled by apes. Leo discovered with horror an honorary shrine in the memory of Thade in the similar style to Abe Lincoln, before being surrounded by ape police, press and public.