Fossils are the remains of organisms from millions of years ago, found in rocks. They can form in three main ways.
First, fossils can form from parts of an organism that have not decayed because one or more of the conditions needed for decay are absent. For example, there may be no oxygen, or the temperature may be too low. Second, parts of the organism can be replaced by minerals as they decay, which forms a rock copy of the original. Third, fossils can be preserved traces of organisms, such as footprints, burrows and rootlet traces.
The fossil record is incomplete. Many early forms of life were soft-bodied, so they left few traces behind, and what traces there were have mainly been destroyed by geological activity. This is why scientists cannot be certain about how life began on Earth.
We can learn from fossils how much or how little different organisms have changed as life developed on Earth. Fossils can be arranged in order of age, and information from charts, tables and evolutionary trees can be used to see how organisms are related and how they have changed.