A video describing an amphioxus, a worm-like animal, that is our ancestor with numerous common features including the precursor to the vertebrate backbone. Learn how some chordates like tunicates, salps, and larvaceans, have remained simple creatures without backbones. But others like the vertebrates have four times as many genes as their simple chordate ancestors, which allowed an explosion of new forms. Also, understand how fish evolved skulls and jaws and dominated the seas. Some fish evolved limb-like fins and crawled out onto land. Reptiles (lizards, snakes) and dinosaurs flourished on land. When the dinosaurs went extinct 65 million years ago, mammals inherited the land. Humans are most closely related to the great apes. [15:43]