Evidence of evolution - Rock fossils

Fossils

A fossil is the preserved remains of a dead organism from millions of years ago. Fossils are found in rocks and can be formed from:

  • hard body parts, such as bones and shells, which do not decay easily or are replaced by minerals as they decay
  • parts of organisms that have not decayed because one or more of the conditions needed for decay are absent for example, dead animals and plants can be preserved in amber, peat bogs, tar pits or in ice
  • preserved traces of organisms, such as footprints, burrows and rootlet traces - these become covered by layers of sediment, which eventually become rock
Photo of an ammonite fossil
Ammonite fossils, an example is shown here, are sea creatures that became extinct about 65 million years ago

The fossil record

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.