How did fossils of giant extinct mammals influence Charles Darwin’s thinking?

During his voyage on the HMS Beagle, Charles Darwin’s discovery of fossil bones from giant extinct mammals—such as Megatherium and Toxodon—played a pivotal role in shaping his scientific thinking.123

These findings influenced him in several key ways:

• Stimulating Evolutionary Inquiry: The extraction of these massive relics, including skulls and armor plates, pushed Darwin to contemplate the primeval world and the causes behind the extinction of such large beasts. This curiosity became a central theme in his research, eventually contributing to his interest in how species originate.12
• Deep Time and Geologic Change: By observing these fossils alongside evidence of rising land and shifting coastlines in South America, Darwin began to think in terms of "deep time," reinforcing the uniformitarian view that Earth’s history was shaped by steady, cumulative forces.1
• The Mystery of Species: The discovery that South American fossil mammals differed significantly from those found in Europe helped highlight the geographic and temporal distribution of life, fueling his later investigations into the "mystery of mysteries"—the simultaneous change and succession of fossil life.13