How Do Fossils Form, and What Do They Tell Us?

Fossils form when remains or traces of organisms are preserved in rock. They tell us what some organisms from millions of years ago were like and provide evidence that life on Earth has changed over time.

How do fossils form?

There are three main ways to remember for AQA GCSE Biology:

• Decay is prevented. Parts of an organism remain because one or more conditions needed for decay are absent.
• Minerals replace parts of the organism. As the original material decays, minerals take its place and preserve its shape.
• Traces are preserved. Footprints, burrows and root marks can remain in rock even when the organism itself does not.

A fossil is therefore not always a preserved body part. A footprint can also be a fossil because it records evidence of an organism’s activity.

What can fossils tell us?

By comparing fossils from different organisms, scientists can see how much, or how little, organisms have changed as life developed on Earth. Fossils are part of the evidence for evolution: change in the inherited characteristics of populations over time. They can also provide information about organisms that are now extinct, meaning no individuals of that species remain alive.

There is an important limit: the fossil record is incomplete. Many early organisms had soft bodies that left few traces, and geological activity has destroyed many traces that did form. Scientists therefore cannot use fossils to give a complete account of every organism that lived or be certain from the fossil record about how life began.

Common exam mistake: Saying that all fossils are bones. Remember the three routes above, especially preserved traces, and explain that fossils provide evidence for change over time rather than a complete record. This content applies to both Foundation and Higher tiers of AQA GCSE Biology.