How fossils form

1. Some fossils form when the hard parts of an organism, such as bones, shells and teeth, are gradually replaced by minerals as they decay and turn to rock.
2. Some form from parts that have not decayed, because a condition needed for decay (oxygen, moisture or warmth) was missing, for example in amber, ice or peat bogs.
3. Others survive as trace fossils, the marks an organism left behind, such as footprints, burrows and rootlet traces, pressed into soft ground that later turned to rock.

What fossils tell us

1. Fossils dated by the rocks they are found in show what organisms were like at different times in the past.
2. Comparing older and younger fossils shows how much or how little organisms have changed as life developed on Earth.
3. They also show that many species that once lived are now extinct.

Why the fossil record is incomplete

1. A lot of the earliest living things were soft-bodied rather than having hard parts, so very little of them was ever preserved.
2. The few remains that did manage to form have since been broken up by geological activity, such as movements of the Earth's crust.
3. For these reasons, no one can say for certain exactly how life began on our planet.