Charles Darwin's voyage on the HMS Beagle was a journey that would revolutionise our understanding of the natural world and our place in it. The magisterial work it spawned, The Origin of Species, is widely associated with the flora and fauna of the Galapagos Islands. Less well known is Darwin's passion for geology and how his fossil discoveries in South America – by demonstrating the relationship between extinct lifeforms and living species – shaped his theory of evolution. This is the story of those fossil-hunting adventures in the 1830s, the pioneering science behind the fossils he found, and how these remarkable discoveries played a crucial role in forging Darwin's revolutionary ideas.