Have you seen a news clip or a video showing a volcano erupting, or an earthquake shaking a city? One of the interesting things about those events is that they occur today in the same way that they have in the past. Scientists look at modern-day geologic events—whether as sudden as an earthquake or as slow as the erosion of a river valley—to get a window into past events. This is known as uniformitarianism: the idea that Earth has always changed in uniform ways and that the present is the key to the past.

The principle of uniformitarianism is essential to understanding Earth’s history. However, prior to 1830, uniformitarianism was not the prevailing theory. Until that time, scientists subscribed to the idea of catastrophism. Catastrophism suggested the features seen on the surface of Earth, such as mountains, were formed by large, abrupt changes—or catastrophes. When discussing past climates, opponents to uniformitarianism may speak of no-analog changes. This idea suggests that certain communities or conditions that existed in the past may not be found on Earth today.

The idea of catastrophism was eventually challenged based on the observations and studies of two men—James Hutton and Charles Lyell. Hutton (1726–1797) was a Scottish farmer and naturalist. In his observations of the world around him, he became convinced natural processes, such as mountain building and erosion, occurred slowly over time through geologic forces that have been at work since Earth first formed. He eventually turned his observations and ideas into what became known as the Principle of Uniformitarianism.

Among the scientists who agreed with Hutton was Charles Lyell. Lyell (1797–1875) was a Scottish geologist. In 1830, he published a book, Principles of Geology, that challenged the idea of catastrophism, which was still the dominant theory despite Hutton’s work. Lyell believed Hutton was correct about the gradually changing processes shaping Earth’s surface. He found his own examples of these processes in his examination of rocks and sediments. For example, he discovered evidence that sea levels had risen and fallen in the past, that volcanoes may exist atop older rocks, and that valleys form slowly by the erosional power of water. The combined efforts of Lyell and Hutton became the foundation of modern geology.

Charles Darwin, the founder of evolutionary biology, looked at uniformitarianism as support for his theory of how new species emerge. The evolution of life, he realized, required vast amounts of time, and the science of geology now showed Earth was extremely old. If there had been plenty of time for mountains to rise up and erode away, then there had also been enough time for millions of species to emerge, and either evolve into new species or go extinct. Science’s conceptions of both geology and biology had entered a new day.