Evidence for Evolution: Fossils and Extinction

The theory of evolution by natural selection is now widely accepted by scientists. It became accepted because of the large amount of evidence that supports it, gathered from several independent sources.

When Charles Darwin first published his theory, it was not fully accepted. One reason was that the mechanism of inheritance was unknown — Darwin could not explain how characteristics were passed from parents to offspring. That gap has since been filled.

Three main lines of evidence now support evolution:

Evidence	What it shows
Inheritance through genes	Characteristics are passed to offspring in genes, providing the mechanism Darwin lacked
The fossil record	A record of how organisms have changed over millions of years
Antibiotic resistance in bacteria	Evolution by natural selection happening fast enough to observe directly

Key idea: A theory becomes accepted not from one proof, but from a large body of evidence pointing the same way. Evolution is supported by genetics, fossils and observed changes in living organisms.

This lesson covers the fossil and bacterial evidence, then extinction — the permanent loss of species over geological time.

Bacteria reproduce very quickly, so evolution by natural selection can be seen happening over a short time. This makes antibiotic resistance a powerful, observable piece of evidence for evolution.

Worked walk-through — how a resistant population evolves:

1. Within a population of bacteria there is variation — random mutations mean a few bacteria happen to carry a gene that makes them resistant to an antibiotic.
2. An antibiotic is used. It kills the non-resistant bacteria, but the resistant ones survive.
3. The surviving resistant bacteria reproduce, passing the resistance gene to their offspring.
4. Over generations, the proportion of resistant bacteria rises until the whole population is resistant.

This is natural selection: variation, a selection pressure (the antibiotic), differential survival, and inheritance of the advantageous gene.

Exam tip: The antibiotic does not create resistance. The mutation already exists by chance; the antibiotic only selects for the bacteria that already carry it. Saying "the antibiotic made them resistant" is a common error and is unlikely to score.

MRSA (methicillin-resistant Staphylococcus aureus) is a real example of bacteria that evolved resistance in this way.

Fossils are the remains of organisms from millions of years ago, found in rocks. They give us direct evidence of what lived in the past and how organisms have changed over time.

There are three main ways fossils form:

Formation route	What happens	Typical example
Prevented decay	Parts of the organism do not decay because one or more conditions needed for decay are absent (e.g. no oxygen, no moisture, low temperature, no suitable microorganisms)	Mammoths preserved in ice; insects in amber
Mineral replacement	As the organism's hard parts slowly decay, they are replaced by minerals, forming a rock-like copy of the original	Petrified bones, shells and wood
Preserved traces	The organism itself is not preserved, but traces it left behind are	Footprints, burrows, rootlet traces

Definition: Decay happens when microorganisms break down dead material. It needs oxygen, moisture, warmth and microorganisms. Remove one of these and decay slows or stops — which is exactly why some remains survive long enough to fossilise.

Note that traces such as footprints are still classed as fossils even though no part of the actual organism remains.

Use the three routes to reason about a given specimen. The key is to ask what was preserved and how.

Scenario A — A dinosaur leg bone, now solid stone, found in sandstone. The original bone has been turned to a rock-like material. The hard part has been replaced by minerals as it decayed. This is the mineral replacement route.

Scenario B — A mosquito with intact soft body parts, sealed inside amber (hardened tree resin). The whole insect, including soft tissue, has survived. The amber sealed it away from oxygen and microorganisms, so decay could not take place. The organism's parts did not decay because a condition for decay was absent. This is the prevented decay route.

Scenario C — A line of three-toed prints pressed into mudstone, with no bones nearby. No part of the animal is present — only the marks it made while walking. These are preserved traces (footprints). This is the preserved trace route.

Exam tip: Read the clue in the question. "Replaced by minerals / turned to stone" → mineral replacement. "No oxygen / amber / ice / peat bog" → prevented decay. "Footprints / burrows / rootlets" → preserved traces.

The fossil record tells us much about how life developed, but it has a large gap at the very beginning. Scientists cannot be certain how life on Earth began, because there are very few valid fossils from the earliest periods.

There are two main reasons for this gap:

• Many early organisms were soft-bodied. Soft tissue decays quickly and rarely fossilises, because there are no hard parts to be replaced by minerals or to leave clear traces. These organisms left few traces behind.
• The traces that did form were largely destroyed. Over the enormous timescales involved, geological activity — such as movement of rock, heat, pressure and erosion — destroyed most of the early traces that existed.

Key idea: An incomplete fossil record is not evidence against evolution. It reflects how fossils form: soft-bodied organisms rarely fossilise, and ancient traces are often destroyed. Absence of early fossils is expected, not surprising.

This is also why the fossil record is described as incomplete: we have a sample of past life, not a full catalogue. We can still learn from fossils how much, or how little, different groups of organisms have changed as life developed on Earth.

Extinction is the permanent loss of all members of a species — it occurs when there are no remaining individuals of that species still alive anywhere. Extinction is permanent: once the last individual dies, the species cannot return.

Several factors may contribute to a species becoming extinct. Often more than one acts at the same time.

Factor	Why it can cause extinction	Example
New predators	A new predator hunts a species that has no defences against it	Ground-nesting birds wiped out by introduced rats or cats
New diseases	A new pathogen spreads through a species with no resistance	Amphibian populations lost to the chytrid fungus
New, more successful competitors	Another species competes better for the same food, space or other resources	A native species out-competed by an introduced one
Environmental change over geological time	Slow changes in climate, sea level or habitat over very long timescales	Cooling climates reducing suitable habitat
A single catastrophic event	One sudden event destroys habitats or food supplies on a huge scale	A massive volcanic eruption or an asteroid collision

Worked walk-through — match the factor to the scenario:

A small island has one species of flightless bird. Sailors arrive and release rats, which eat the birds' eggs. Within decades the bird is gone. The trigger is an organism that was not there before and now eats the species. This is a new predator. (You could also argue the rats are competitors for ground nests, but predation on eggs is the clearer driver here — justify whichever you choose using the detail in the question.)

1. Saying the antibiotic "creates" resistance

The resistance mutation arises by chance before the antibiotic is used. The antibiotic only selects for bacteria that are already resistant. Describe it as selection, not creation.

2. Forgetting that traces count as fossils

Footprints, burrows and rootlet traces are fossils even though no part of the organism's body is preserved. Do not limit fossils to bones and shells.

3. Mixing up the three formation routes

Match the clue to the route: replaced by minerals (mineral replacement), conditions for decay absent such as no oxygen, ice or amber (prevented decay), and footprints/burrows/rootlets (preserved traces).

4. Treating the patchy fossil record as evidence against evolution

The gap exists because early organisms were soft-bodied and left few traces, and geological activity destroyed most of those traces. This explains the gap; it does not undermine evolution.

5. Defining extinction loosely

Extinction is the permanent loss of all members of a species — not "when a species becomes rare" or "endangered". State that no individuals remain alive.

6. Giving only one cause of extinction

The specification expects a range of contributing factors: new predators, new diseases, new competitors, environmental change over geological time, and a single catastrophic event such as a volcanic eruption or asteroid collision. List several and link each to the scenario.