1. 1795: James Hutton publishes his Theory of the Earth, which proposes that our planet is much older than previously thought.
2. 1815: William Smith completes the first geological map of England and Wales. He observes that each layer in a column of sedimentary rock contains index fossils.
3. 1830: British geologist Charles Lyell publishes the first of three volumes of his Principles of Geology. The book proposes that Earth has gradually changed over time and that processes still acting today have shaped its surface.
4. 1859: Charles Darwin publishes his groundbreaking work On the Origin of Species. The book encourages geologists to begin interpreting their fossil collections in terms of the evolution of life.

In addition to the timeline, there is also image of James Hutton, Charles Darwin, title page of On the Origin of Species and Ammonites (Ammonites come in many forms, each specific to a particular rock layer and time in the past. They are used as index fossils for the Jurassic period.)