Fossils are the remains of organisms from millions of years ago, found in rocks. They can form in three main ways.
- 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.
- When parts of the organism are replaced by minerals as they decay.
- As preserved traces of organisms, such as footprints, burrows and rootlet traces.
We can learn from fossils how much or how little different organisms have changed as life developed on Earth. Scientists can arrange fossils in order of age and build evolutionary trees.
The fossil record is incomplete. Many early forms of life were soft-bodied, so they left few traces behind. What traces there were have mainly been destroyed by geological activity. This is why scientists cannot be certain about how life began on Earth.