At one time people assumed that the earth and its inhabitants were fixed and unchanging. However, as they began to understand the nature of events such as volcanic eruptions, earthquakes, and erosion it became obvious that the earth was changeable. Furthermore, the presence of the fossilized remains of extinct organisms showed that the nature of the kinds of things that inhabited the earth had changed as well. These ideas challenged core beliefs and began a long process of re-forming the way we look at the history of the earth and life on it. In particular, people needed to develop an understanding that the history of the earth and its life was very long and should be measured in billions of years rather than hundreds or thousands of years.