# Evidence for Evolution

> Fossils and dating, comparative anatomy, embryology, biogeography and biochemical evidence such as DNA and protein comparisons.

From HSC Biology on WhizzNotes, free study notes for NSW students: https://whizznotes.com/biology/evidence-for-evolution/. Updated 3 November 2026.
Year 11 and Year 12 boxes go deeper than the Year 10 text around them.

### Story time: The fish with legs
In 2004, a team in the Canadian Arctic was looking for a fossil they predicted should exist. Fish lived in water about 380 million years ago. Four-legged animals were on land by about 365 million years ago. So somewhere in rocks about 375 million years old, there should be a fish starting to look like a land animal.

They searched rocks of exactly that age for six summers. Then they found it: *Tiktaalik*, a fish with gills and scales, but also a neck, lungs and fins with wrist bones. Evolution had made a prediction, and the rocks confirmed it.

**In short:** Several kinds of evidence show that species have evolved from common ancestors. Fossils show life changing over time, homologous structures and similar embryos show shared ancestry, and the more similar two species' DNA and proteins are, the more closely related they are.

## Starting point: what you already know

- Natural selection changes populations over many generations (see the previous chapter).
- Rocks form in layers, with younger layers on top of older ones (Year 9 and 10 Earth science).
- DNA codes for proteins.

## Fossils

A **fossil** is the preserved remains or trace of a living thing from long ago. Fossils usually form when a dead organism is buried quickly in sediment, and minerals slowly replace its hard parts.

The **fossil record** shows

- simple life appearing first and more complex life later,
- groups appearing in an order (fish, then amphibians, then reptiles, then mammals and birds),
- **transitional fossils** that share features of two groups, like *Tiktaalik* (fish to land animal) and *Archaeopteryx* (dinosaur to bird).

## Dating fossils

**Relative dating** puts fossils in order. Fossils in lower rock layers are usually older. **Index fossils** (species that lived for a short time but were widespread) help match layers in different places.

**Absolute dating** gives an age in years using **radioactive isotopes** that decay at a fixed rate. The **half-life** is the time for half the isotope to decay.

### Worked example: Using half-life
Carbon-14 has a half-life of about 5730 years. A bone has $\frac{1}{4}$ of the carbon-14 a living animal would have. How old is it?

$1 \rightarrow \frac{1}{2}$ is one half-life, and $\frac{1}{2} \rightarrow \frac{1}{4}$ is a second. So the age is about $2 \times 5730 = 11\,460$ years.

Carbon-14 works for things up to about 50 000 years old. Older rocks are dated with isotopes such as potassium-40.

## Comparative anatomy

**Homologous structures** have the same basic bone pattern but different uses. A human arm, a bat wing, a whale flipper and a cat leg all have one upper bone, two forearm bones, wrist bones and fingers. This points to a **common ancestor**. It is evidence of **divergent** evolution.

**Analogous structures** do the same job but are built differently, like a bird wing and an insect wing. They show **convergent** evolution, not close relationship.

**Vestigial structures** are reduced parts with little or no use now, but which were useful in ancestors: the human tailbone, whale hip bones, and the tiny wings of an emu.

## Comparative embryology

Early embryos of fish, birds and mammals look very alike. All have tail-like and gill-slit-like structures at some stage. Shared development suggests shared ancestry.

## Biogeography

**Biogeography** is the study of where species live. Australia has most of the world's **marsupials** and all of its living **monotremes** (except a few in New Guinea). This makes sense because Australia split from the supercontinent **Gondwana** and drifted alone for about 45 million years. Its animals evolved in isolation. Related plants such as southern beeches are found in Australia, South America and New Zealand, which were all joined in Gondwana.

## Biochemical evidence

All living things use DNA, the same genetic code and the same 20 amino acids. That itself points to a single origin of life.

The more closely two species are related, the more similar their **DNA sequences** and **proteins** (like haemoglobin or cytochrome c). Humans and chimpanzees share about 98.8% of their DNA.

| Species compared to humans | Differences in the haemoglobin beta chain (out of 146 amino acids) |
|---|---|
| Chimpanzee | 0 |
| Gorilla | 1 |
| Rhesus monkey | 8 |
| Dog | 15 |
| Frog | 67 |

Fewer differences mean a more recent common ancestor.

**Mind map**

```text
Evidence for evolution
  Fossils
    Order in rock layers
    Transitional: Tiktaalik
    Relative and absolute dating
  Anatomy
    Homologous: same plan
    Analogous: same job
    Vestigial: leftover
  Embryology
    Embryos look alike
  Biogeography
    Gondwana
    Australian marsupials
  Biochemistry
    Same genetic code
    DNA and protein similarity
```

### Year 11: Building a phylogenetic tree
A **phylogenetic tree** (cladogram) shows how species are related. Species that share more features or DNA are placed on nearby branches. The point where two branches split is their **most recent common ancestor**.

Using the haemoglobin table: chimpanzees branch off closest to humans, then gorillas, then monkeys, then dogs, and frogs furthest away.

Different lines of evidence (fossils, anatomy, DNA) usually give the same tree. When independent evidence agrees, scientists are very confident in the conclusion.

### Year 12: Molecular clocks
Mutations in some DNA sequences build up at a roughly steady rate. Counting the differences between two species and dividing by the rate gives an estimate of **when** they split, called a **molecular clock**. Mitochondrial DNA, which is passed only from mothers, has been used to trace the movement of early humans, including the arrival of Aboriginal peoples' ancestors in Australia more than 60 000 years ago.

## Quick summary

- Fossils show life changing over time. Transitional fossils link groups.
- Relative dating uses rock layers. Absolute dating uses radioactive half-life.
- Homologous structures show common ancestry. Analogous structures show convergent evolution. Vestigial structures are leftovers.
- Similar embryos, the spread of species (Gondwana) and DNA and protein similarity all support evolution.
- Fewer differences in DNA or protein means more closely related.

## Quiz

Try these questions from the chapter quiz.

1. A fossil with features of both fish and land animals is called
   - A. an index fossil
   - B. a transitional fossil
   - C. a vestigial fossil
   - D. an analogous fossil
   - Answer: B. a transitional fossil. It shows a transition between two groups.

2. In undisturbed rock layers, the oldest fossils are usually
   - A. at the top
   - B. at the bottom
   - C. in the middle
   - D. on the surface
   - Answer: B. at the bottom. Older layers are laid down first.

3. A human arm and a bat wing have the same bone pattern. These are
   - A. homologous structures
   - B. analogous structures
   - C. vestigial structures
   - D. index fossils
   - Answer: A. homologous structures. Same basic plan, different use.

The whole chapter quiz is on the page, for signed-in students: https://whizznotes.com/biology/evidence-for-evolution/#quiz