How do we know evolution happened? Scientists do not guess — they examine multiple independent lines of evidence. From ancient bones buried in rock to the striking similarities in limbs across wildly different animals, the evidence is overwhelming.
Imagine you are a palaeontologist and you uncover a fossilised bone in the Australian outback.
Fossils are the preserved remains or traces of organisms from the past. They provide direct, physical evidence that life on Earth has changed over time — and that many organisms once existed that are now extinct.
How fossils form: Most fossils form when organisms are buried quickly by sediment (mud, sand, volcanic ash). Over millions of years, minerals replace organic material, turning bone or shell into rock. Impressions, footprints, burrows and even chemical traces can also become fossils.
What fossils can tell us:
Limitations of fossil evidence:
Australia has one of the world's most remarkable fossil records. The thylacine (Thylacinus cynocephalus), or Tasmanian tiger, left fossils showing it once ranged across mainland Australia and New Guinea — not just Tasmania. Megafauna such as Diprotodon (a rhino-sized wombat relative), Procoptodon (a giant short-faced kangaroo) and Megalania (a seven-metre goanna) roamed Australia until roughly 40,000–50,000 years ago. Their fossils show that Australian mammals once reached enormous sizes, likely due to the absence of large placental predators. Naracoorte Caves in South Australia and Riversleigh in Queensland are UNESCO World Heritage fossil sites preserving this incredible history.
The bones of your arm, a whale's flipper, a bat's wing and a horse's leg look dramatically different on the outside — but beneath the surface, they share a remarkably similar pattern. This is powerful evidence of common ancestry.
Homologous structures are body parts that have the same underlying bone structure but different functions. They indicate common ancestry with divergence:
All four contain a humerus, radius and ulna, wrist bones and finger bones. The differences are in proportion and shape, not in basic architecture. This pattern makes no sense unless these animals inherited the same limb blueprint from a common ancestor and modified it for different purposes.
Analogous structures have similar functions but different underlying structures. They indicate convergent evolution — independent solutions to the same environmental challenge:
Vestigial structures are reduced or non-functional body parts that were functional in ancestors. They are evolutionary "leftovers":
If you examine the early embryos of fish, reptiles, birds and mammals, you will see striking similarities. These similarities are difficult to explain unless all vertebrates share a common ancestor.
At early stages, vertebrate embryos show:
These similarities do not mean that a human embryo "recapitulates" its entire evolutionary history (an outdated idea called Haeckel's biogenetic law, which was an oversimplification). Rather, they show that developmental genes are deeply conserved across species. The same genetic toolkit builds bodies in fish, frogs, birds and humans — because we all inherited it from a common ancestor.
In 2004, palaeontologists discovered Tiktaalik roseae in Arctic Canada — a 375-million-year-old fossil with features intermediate between fish and tetrapods (four-limbed land animals). It had fish-like scales and fins, but also a flat skull with eyes on top, a neck (fish do not have necks), and wrist bones inside its fins. Tiktaalik is a classic transitional fossil: it shows how limbs evolved from fins, exactly as evolutionary theory predicted.
1 The wing of a bat and the wing of a bird
2 The flipper of a dolphin and the flipper of a penguin
3 The human tailbone (coccyx)
4 The forelimbs of a mole and the forelimbs of an anteater
5 The reduced eyes of cave-dwelling fish
1 Fossils of Diprotodon (a giant wombat relative) have been found across Australia. What can these fossils tell us about where and when Diprotodon lived?
2 No complete Diprotodon soft tissues have been found. Explain why this is expected, and state what we therefore cannot know from fossils alone.
3 Explain why the discovery of a transitional fossil like Tiktaalik is powerful evidence for evolution.
1. What do homologous structures indicate?
2. Which is an example of a vestigial structure?
3. Why are there gaps in the fossil record?
4. The wings of a bat and the wings of an insect are...
5. Embryological evidence supports evolution because...
6. Describe two things fossils can tell us about past life, and one limitation of fossil evidence. 3 MARKS
7. Distinguish between homologous and analogous structures, using an example of each. 4 MARKS
8. Explain how comparative anatomy and fossil evidence together provide stronger support for evolution than either line of evidence alone. 5 MARKS
Go back to your Think First responses at the top of the lesson.
1. Bat wing and bird wing: Analogous structures. Both are used for flight but have completely different underlying anatomy — bat wings are modified mammal forelimbs with bones, while bird wings are modified forelimbs covered in feathers with different bone proportions [1 mark]. This indicates convergent evolution, not common ancestry [0.5 mark].
2. Dolphin flipper and penguin flipper: Analogous structures. Both are used for swimming but dolphin flippers contain bones (mammalian heritage) while penguin flippers are modified wings with different bone structure (bird heritage) [1 mark].
3. Human tailbone: Vestigial structure. It is a reduced remnant of a tail that was functional in our primate ancestors [1 mark].
4. Mole and anteater forelimbs: Homologous structures. Both are modified mammal forelimbs with the same bone pattern (humerus, radius/ulna, carpals, phalanges), inherited from a common mammalian ancestor [1 mark].
5. Reduced eyes of cave fish: Vestigial structure. Eyes were functional in ancestors but reduced in dark caves where vision provides no advantage [0.5 mark].
3. Tiktaalik as transitional fossil: Transitional fossils are powerful because they show intermediate characteristics predicted by evolutionary theory [1 mark]. Tiktaalik had fish-like scales and fins but also a neck, flat skull and wrist bones — features of tetrapods [1 mark]. Its discovery confirmed a specific prediction: that limbs evolved from fins in a particular time period and environment [1 mark]. This is strong evidence because it matches multiple independent lines of evidence (anatomy, geology, molecular clocks) [1 mark].
1. A — Homologous structures indicate common ancestry. Option B describes analogous structures. Option C is false. Option D confuses homology with same function.
2. C — Vestigial structures are reduced remnants. Options A and B describe functional structures. Option D is also functional.
3. B — Gaps exist because fossilisation is rare and many fossils are undiscovered. Option A is a common misconception. Option C is false. Option D is backwards.
4. B — Bat and insect wings are analogous: same function, different structure. Option A confuses analogous with homologous. Options C and D are irrelevant.
5. C — Vertebrate embryos share similar structures from common ancestry. Option A is false. Option B describes Haeckel's outdated recapitulation theory. Option D is false.
Q6 (3 marks): Fossils can tell us: (1) the existence of organisms that are now extinct [1 mark]; (2) the body structure and size of ancient organisms, or their behaviour from footprints and bite marks [1 mark]. One limitation is that soft tissue rarely fossilises, so we rarely see skin, muscles or organs; alternatively, the fossil record is incomplete because not all environments favour fossilisation [1 mark].
Q7 (4 marks): Homologous structures have the same underlying bone structure but different functions, indicating common ancestry [1 mark]. For example, the forelimbs of humans, whales, bats and horses all contain a humerus, radius/ulna, carpals and phalanges, modified for different purposes [1 mark]. Analogous structures have similar functions but different underlying structures, indicating convergent evolution [1 mark]. For example, insect wings (chitin, no bones) and bat wings (modified mammal forelimbs) both enable flight but evolved independently [1 mark].
Q8 (5 marks): Comparative anatomy shows that living species share structural patterns — such as homologous forelimbs — that are best explained by inheritance from a common ancestor [1 mark]. Fossil evidence provides a historical timeline, showing how structures changed over millions of years and revealing extinct intermediate forms [1 mark]. Together, these lines of evidence are stronger because they are independent — one examines living organisms, the other examines extinct ones [1 mark]. When both predict the same evolutionary relationships, the conclusion is much more robust than if only one line existed [1 mark]. For example, the anatomical similarity between whale flippers and human arms is supported by fossil whales with hind limb remnants, showing whales descended from land-dwelling ancestors [1 mark].
Test your knowledge of fossils, comparative anatomy and embryology in this fast-paced quiz battle. Correct answers power your attacks!
Climb platforms using your knowledge of fossils, homology and vestigial structures. Pool: Lesson 13.
Tick when you have finished all activities and checked your answers.