Fossils

Inheritance, variation and evolution · The development of understanding of genetics and evolution · note 2 of 4

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FossilsSpec 4.6.3.5

In short

Fossils are the remains of organisms from millions of years ago, found in rocks. They form from parts of organisms that have not decayed, from parts replaced by minerals as they decay, or as preserved traces such as footprints and burrows. The fossil record is incomplete because many early life forms were soft-bodied and geological activity destroyed most traces.

Fossils are the 'remains' of organisms from millions of years ago, which are found in rocks.

How fossils may form

From parts of organisms that have not decayed because one or more of the conditions needed for decay are absent.
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.

Why the fossil record is incomplete

Many early forms of life were soft-bodied, which means that they have left few traces behind. What traces there were have been mainly destroyed by geological activity. This is why scientists cannot be certain about how life began on Earth.

Evolutionary trees

You may need to extract and interpret information from charts, graphs and tables, such as evolutionary trees. Evolutionary trees use fossil data for extinct organisms and current classification data for living organisms.

Exam tip:

Fossils are evidence for how organisms have changed over time, but gaps mean the record is incomplete. Quote both ideas in an evaluation.

Evolutionary tree with time running upwards from a common ancestor at the bottom to four living species A to D at the present day, with two extinct branches ending in a cross. — Branches ending in a cross are extinct species, known only from fossils.

Written and checked against the AQA GCSE Combined Science (8464) specification · Updated October 2026

Frequently asked questions

What is the evidence for evolution?

The evidence for evolution includes the fossil record, which shows how much or how little organisms have changed as life developed on Earth, the discovery that characteristics are passed on to offspring in genes, and antibiotic resistance in bacteria, which shows natural selection happening in a short time. This is why the theory is now widely accepted.

How are fossils formed?

Fossils form in three ways: from parts of organisms that have not decayed because one or more of the conditions needed for decay are absent, from parts that are replaced by minerals as they decay, or as preserved traces such as footprints, burrows and rootlet traces. Fossils are found in rocks and show how organisms have changed.

How does antibiotic resistance develop in bacteria?

Antibiotic resistance develops by natural selection. A mutation produces a strain of bacteria that is resistant to antibiotics. When antibiotics are used, non-resistant bacteria are killed but the resistant bacteria survive and reproduce, so the resistant population rises. Because bacteria reproduce at a fast rate, resistant strains such as MRSA can spread quickly, with no effective treatment.

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