We know that if a fossil once lived on Earth 260 million years ago, the layer that it was fossilized in must be at least 260 million years old. Geologists can then determine that layers around the world that contain that same fossil must be 260 million years old. The first life appearred about 3.2 billion years ago in the form of algae, a simple single celled plant. Scientists know that life on Earth has changed over time and that the life has become increasingly more complex. Scientists also know that these changes didn't occur over night, sometimes it took millions of years for these plants and animals to evolve.
Scientists use two major types of events to break up the Earth's history.
These events are biological changes and physical changes.