A fossil is the preserved remains of a dead organism from millions of years ago.
There are several ways in which fossils can be formed:
Fossil remains have been found in rocks of all ages. Fossils of the simplest organisms are found in the oldest rocks, and fossils of more complex organisms in the newest rocks. This supports Darwin's theory of evolution, which states that simple life forms gradually evolved into more complex ones.
Evidence for early forms of life comes from fossils. By studying fossils, scientists can learn how much (or how little) organisms have changed as life developed on Earth.
There are gaps in the fossil record because many early forms of life were soft-bodied, which means that they have left few traces behind. What traces there were may have been destroyed by geological activity. This is why scientists cannot be certain about how life began. They do not have enough evidence.
Fossils provide a snapshot of the past and allow us to study how much or how little organisms have changed as life developed on Earth. For example, fossils of dinosaurs show how they had hollow bones and so do modern birds. This is one piece of evidence that scientists have used to determine that birds and dinosaurs are related.