Evidence for Evolution, Fossils and Anatomy
A 560-million-year-old Dickinsonia fossil found in South Australia's Flinders Ranges in 1946 is the oldest confirmed animal on Earth, and it has no living relatives.
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Q1 · Whales have tiny hip bones that are not used for walking. What might this suggest about whale ancestors? Write your prediction.
Vestigial structures are remnants of features that served a function in an organism's ancestors.
Q2 · The fossil record is incomplete, not every species that ever lived has left fossils. Why do you think some organisms are more likely to become fossils than others?
Think about what conditions might preserve remains over millions of years.
● Know
- Definitions of fossil, fossil record, homologous, analogous and vestigial structures
- That fossils provide direct evidence of past life but have limitations
- That embryological similarities suggest shared ancestry
● Understand
- How different lines of evidence independently support evolution
- Why homologous structures indicate common ancestry while analogous structures indicate convergent evolution
- Why gaps in the fossil record do not weaken the theory of evolution
● Can do
- Identify and classify structures as homologous, analogous or vestigial
- Evaluate the strengths and limitations of fossil evidence
- Explain how multiple evidence lines together provide stronger support
A whale swimming in the Southern Ocean has two small internal bones buried inside its body, the remnants of hips and hind limbs it no longer uses. Look at the arm bones of a human, a bat wing and a whale flipper side by side and you see the same five-finger structure rearranged. The fossil record and comparative anatomy provide two of the most visually compelling lines of evidence for evolution. Fossils preserve the remains of ancient organisms, showing us what lived in the past and how forms changed over time. A fossil is not just a bone, it is a data point that can be dated, placed in a sequence, and compared to both older and younger fossils to reveal trends.
Homologous structures are features shared by related species because they inherited them from a common ancestor. The forelimbs of humans, bats, whales and horses all contain the same bones, humerus, radius, ulna, carpals, metacarpals and phalanges, arranged in the same order. In each species, these bones have been modified for different functions: grasping, flying, swimming and running. This pattern makes no sense unless these species share a common ancestor that also had these bones.
Whales and dolphins are fully aquatic mammals, yet they retain small pelvic bones and sometimes even tiny hind limb bones embedded in their body wall. These bones are not used for walking and play no role in swimming. They are vestigialevolutionary leftovers from four-legged ancestors that lived on land roughly 50 million years ago. Their presence is powerful evidence that whales evolved from terrestrial mammals.
Australian fossils: The Riversleigh fossil deposits in Queensland are one of the world's richest fossil sites, preserving mammals, birds and reptiles from the last 25 million years. These fossils document the evolution of Australia's unique marsupial fauna, including the ancestors of kangaroos, koalas and the extinct marsupial lion. The site is a UNESCO World Heritage area and a treasure trove for evolutionary biologists.
Match each feature to the type of evolutionary evidence it represents.
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.
Transitional fossils are fossils that show intermediate characteristics between older and younger species. They are not 'missing links' in the sense of half-one-animal-half-another; rather, they possess a mosaic of traits, some ancestral and some derived. Archaeopteryx, for example, had teeth and a long bony tail like a dinosaur, but also feathers and wings like a bird. It is not 'the ancestor' of all birds, but it shows the kind of form that existed during the dinosaur-to-bird transition.
Embryology provides another line of evidence. Early embryos of fish, amphibians, reptiles, birds and mammals all show pharyngeal arches, a notochord and a tail. In fish, the pharyngeal arches become gill supports. In humans, they become parts of the jaw, ear and throat. These similarities suggest that all vertebrates develop using the same basic genetic toolkit, inherited from a common ancestor.
Tiktaalik roseae, discovered in Arctic Canada in 2004, is a transitional fossil between fish and four-legged land animals. It had scales and fins like a fish, but its fin bones correspond to the arm bones of terrestrial animals, humerus, radius and ulna. It could prop itself up in shallow water. Tiktaalik was not aiming to become a land animal; it was simply well-adapted to its shallow-water niche, and its descendants eventually colonised land.
Australian palaeontology: The discovery of Obdurodon, an extinct platypus with teeth, in Riversleigh fossils shows how monotremes have evolved over millions of years. Modern platypuses lack teeth as adults, but their fossil ancestors had well-developed molars. This fossil evidence helps trace the evolutionary history of one of Australia's most iconic animals.
A vestigial structure is a feature that has lost most or all of its original function through evolution. It does not mean the structure is completely useless, many vestigial features have been co-opted for secondary functions. The human appendix, for example, is a remnant of a larger caecum used for digesting cellulose in our herbivorous ancestors. Today it is vestigial in its digestive role, but it may serve minor immune functions.
Other human vestigial traits include the tailbone (coccyx), a remnant of the tail our primate ancestors had; goosebumps, caused by muscles that once raised fur for insulation or threat display; and wisdom teeth, extra molars that our ancestors needed for tough plant diets but that our smaller jaws often cannot accommodate. These features are not 'mistakes', they are signatures of our evolutionary past, recorded in our anatomy.
Some snakes, such as boas and pythons, have tiny pelvic bones and even small claw-like spurs near their tails. These are vestigial hind limbs, remnants of their four-legged lizard ancestors. The spurs are sometimes used in mating, but they are clearly reduced structures that no longer function as walking legs. Their existence is inexplicable without evolution.
Australian wildlife: The emu and cassowary are flightless birds that retain small, useless wing bones. Their ancestors could fly, but millions of years on predator-free Australian islands removed the selection pressure for flight. The wings shrank through disuse, becoming vestigial, yet another example of how evolutionary history is written in the body.
Tiktaalik, A Fish with Wrists
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.
Classify the Structures
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
Fossil Analysis, Australian Megafauna
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.
Copy Into Your Book
▼Fossil Evidence
- Fossils = preserved remains/traces of past life
- Show extinct species, body structure, behaviour, environment
- Limitations: soft tissue rare, incomplete record, gaps expected
- Transitional fossils show intermediate forms
Comparative Anatomy
- Homologous = same structure, different function = common ancestry
- Analogous = same function, different structure = convergent evolution
- Vestigial = reduced/no function = inherited from ancestor
Embryological Evidence
- Vertebrate embryos share pharyngeal arches, tails, segmentation
- Reflects conserved developmental genes from common ancestor
- Not exact recapitulation, but deep genetic homology
Australian Fossils
- Thylacine: once widespread across mainland
- Megafauna: Diprotodon, Procoptodon, Megalania
- Naracoorte Caves and Riversleigh: World Heritage sites
At the start of this lesson you were introduced to Dickinsoniaa 560-million-year-old organism discovered in South Australia's Flinders Ranges, among the oldest known animal fossils on Earth. That example was chosen to show you that the fossil record is real, physical and found right here in Australia.
Now that you have worked through fossil evidence, comparative anatomy and homologous structures, explain how these three types of evidence together build a stronger case for evolution than any single line of evidence could alone. What did you find most convincing, and why?
Q1. Describe two things fossils can tell us about past life, and one limitation of fossil evidence. 3 MARKS
Q2. Distinguish between homologous and analogous structures, using an example of each. 4 MARKS
Q3. Explain how comparative anatomy and fossil evidence together provide stronger support for evolution than either line of evidence alone. 5 MARKS
Revisit Your Initial Thinking
Go back to your Think First responses at the top of the lesson.
- Did you correctly identify that fossils can reveal body structure, behaviour, environment and timing?
- Did you recognise that gaps in the fossil record are expected because fossilisation is rare, not evidence against evolution?
- Write one sentence summarising the most important new concept you learned about how anatomy provides evidence for evolution.
Model answers (click to reveal)
Comprehensive Answers
▼Activity 1, Classify the Structures
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].
Activity 2, Fossil Analysis
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].
Multiple Choice
1. AHomologous structures indicate common ancestry. Option B describes analogous structures. Option C is false. Option D confuses homology with same function.
2. CVestigial structures are reduced remnants. Options A and B describe functional structures. Option D is also functional.
3. BGaps 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. BBat and insect wings are analogous: same function, different structure. Option A confuses analogous with homologous. Options C and D are irrelevant.
5. CVertebrate embryos share similar structures from common ancestry. Option A is false. Option B describes Haeckel's outdated recapitulation theory. Option D is false.
Short Answer Model Answers
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 independentone 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].
Jump Through Evidence!
Climb platforms using your knowledge of fossils, homology and vestigial structures. Pool: Lesson 13.