Who proposed the concept of the Earth being millions of years old based on geological findings?

The concept of the Earth being millions of years old based on geological findings is attributed to James Hutton. Hutton is often referred to as the "father of modern geology" for his ideas about the processes that shape the Earth over vast periods of time. He introduced the notion of deep time, suggesting that the Earth is far older than previously thought, and this idea was founded on his observations of sedimentary rock formations and the gradual processes of erosion and sedimentation.

Hutton's work laid the groundwork for later geologists and was fundamental in changing the perception of Earth's history, influencing figures like Charles Lyell, who further popularized these concepts. In contrast, Charles Darwin focused on biological evolution, while Herbert Spencer applied evolutionary ideas to sociology but did not specifically address geological time in the same foundational manner as Hutton did. This makes Hutton the key figure behind the early understanding of Earth’s extensive geological timescale.