Who proposed that our planet is millions of years old and has experienced significant development and change?

The concept that our planet is millions of years old and has undergone significant development and change is best associated with the ideas presented in uniformitarianism. This principle, largely articulated by geologist Charles Lyell in his work "Principles of Geology," suggests that the geological processes observed in the present have been consistent over time, implying that Earth has a long history characterized by gradual changes.

This approach contrasts with catastrophism, which posits that Earth's features are shaped primarily by catastrophic events. Uniformitarianism emphasizes the slow and continuous processes that can be observed today, providing a framework for understanding the Earth's geological past.

While "Theory of the Earth" is a generalized term that may encompass various ideas about Earth's age and development, it does not specifically highlight the importance of gradual change over vast time periods, which is a hallmark of uniformitarianism. Therefore, this principle stands out as the explanation for the significant geologic changes and the ancient age of the planet.