Darwin's Dangerous Idea documents Charles Darwin's voyage aboard the HMS Beagle where he discovered evidence that would lead to his theory of evolution by natural selection. The film shows how Darwin was struck by similarities between animals on different islands in the Galapagos, and how this, along with his studies of geology and observations of animal breeding, led him to understand that all life on Earth descended from common ancestors and that natural selection drives evolution. It also demonstrates how modern science like DNA analysis has supported Darwin's theory and shown that humans share a common ancestor with chimpanzees.