AQA GCSE Biology: Fossils and Evidence for Evolution

Evolution is supported by several lines of evidence, and fossils are one of the most important. This guide covers the main evidence for evolution, how fossils form, what the fossil record shows, and why the record from the earliest life is patchy. The theme is what the evidence can, and cannot, tell us.

Evidence for evolution

There are three main lines of evidence for evolution:

• characteristics being passed on through genes;
• the fossil record;
• the observable evolution of antibiotic resistance in bacteria.

Each one does a different job, and it is worth knowing which.

Inheritance is what makes evolution possible at all. A characteristic that helps an organism survive is only useful to the species if it can be handed on, and genes are the mechanism for handing it on. Without inheritance an advantage would die with the individual and nothing would ever accumulate.

The fossil record is the long-term evidence. It shows that the organisms alive in the distant past were not the same as the ones alive now, and that the change was gradual rather than sudden.

Antibiotic resistance is the short-term evidence, and the most convincing kind, because it can be watched. Bacteria reproduce so quickly that natural selection produces a measurable change in a population within months, which is covered in the extinction and antibiotic resistance guide.

A note on the word theory. In everyday speech a theory is a guess, but in science it is an explanation that has been tested against evidence and has survived. Evolution is a theory in the second sense.

How fossils form

Fossils are the preserved remains of organisms from millions of years ago. They form in three main ways.

• From the absence of decay conditions, so the organism does not rot away.
• By mineralisation, where minerals gradually replace the hard parts of an organism.
• As preserved traces, such as footprints and burrows.

Decay is caused by microorganisms, and they need warmth, moisture and oxygen to work. Take any of those away and a body does not rot. That is why whole organisms are found in ice, in peat bogs, which are too acidic and too low in oxygen, in amber, which seals an insect away from the air, and in very dry places.

Mineralisation replaces the hard parts, so most fossils are of bones, shells and wood. Soft parts almost never survive, which matters for the next section.

Traces are not the organism at all. A footprint, a burrow or a set of droppings tells you something no bone can: what the animal was doing, how it moved, and what it ate.

What the fossil record shows

Fossils are found in layers of rock, and the deeper layers are older. That gives a sequence, and the sequence is the point.

• Species have changed over time, gradually rather than all at once.
• Many species that once lived are no longer alive, which is evidence for extinction.
• Organisms appear in a particular order, with simpler forms in the oldest rocks.

Why the fossil record is incomplete

Scientists cannot be certain about how life began, because the fossil record from the earliest times is very incomplete.

• Early organisms were soft-bodied, so they decayed completely and left few fossils.
• Geological activity over billions of years has destroyed many traces.
• Many fossils simply remain undiscovered.

The first reason is the important one. The earliest living things had no bones and no shells, and mineralisation needs hard parts, so the very organisms we would most like to study are the ones least likely to have been preserved.

This is a standard three mark question, worded as "suggest why scientists cannot be certain about how life on Earth began". It wants those three reasons, not an opinion.

How this fits together

The fossil record shows how species have changed and how many have died out completely. Extinction, and the living example of antibiotic resistance, are the subject of the extinction and antibiotic resistance guide.