Final answer:
Darwin discovered fossils of extinct giant mammals in the Andes, supporting the notion that Earth and its organisms change over time, aiding his theory of evolution.
Step-by-step explanation:
On his voyage on the HMS Beagle, Charles Darwin discovered fossils of gigantic extinct mammals like the ground sloth in the Andes Mountains of Argentina. This was crucial evidence that living organisms had looked significantly different in the past and suggested that both the Earth's surface and living things on it change over time. These discoveries in the Andes contributed to Darwin's developing theory of evolution, especially his understanding of how species could undergo gradual changes over extended periods.