Geologic time marks the period from the birth of the planet Earth to the time of written history. Fossils are the remains of plants and animals that have been preserved in rock. Fossils are usually a skeleton, shell, or plant fiber that has been buried and turned into rock over time. Fossils can also be tracks and traces of these animals, or in some rare cases, the actual plant or animal can be preserved, such as an insect preserved in amber. The photo above shows a trilobite fossil preserved in rock.
Geologists use fossils to determine the age of the rock layer that it was found in.
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.