Diversity of Life and Evolutionary Thinking
Australia holds 10% of Earth's species in 0.5% of its land area, yet the CSIRO reported over 1,900 of those species threatened in 2022, the highest rate on any continent.
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Q1 · Australia has over 1 million species, many found nowhere else. What does "biodiversity" mean, and why might a country with many unique species need special conservation laws?
Think about what "variety of life" includes, different species, genetic differences within species, and the ecosystems they form.
Q2 · Scientists group all living things into a "tree of life". Before reading on, predict: do you think bacteria and humans share any ancestors? Explain your reasoning.
Consider what it means for two very different organisms to be related.
● Know
- Definitions of biodiversity, classification, taxonomy, evolution and species
- That Australia is a megadiverse nation with high endemism
- That Aboriginal and Torres Strait Islander Peoples have sophisticated classification systems
● Understand
- Why evolution is called the unifying theory of biology
- How classification helps us make sense of biodiversity
- How Indigenous ecological knowledge reflects deep understanding of species relationships
● Can do
- Use the Linnaean hierarchy to classify organisms
- Compare Western scientific taxonomy with Indigenous classification systems
- Explain why variation is essential for evolutionary change
Press your hand to your chest and feel your heart beat: right now, the bacteria in your gut, the eucalyptus outside your window and the blue whale 4,000 km away in the Southern Ocean all share the same core molecular machinery running their cells. Every living thing on Earth, from the bacteria in your gut to the whales in the Southern Ocean, shares a common biochemical toolkit. All use DNA as genetic material, RNA to transfer information, and proteins built from the same twenty amino acids. All read the genetic code in the same direction, using the same codons. This universality is powerful evidence that all life descended from a single common ancestor that lived roughly 3.5 to 4 billion years ago.
Evolutionary biologists reconstruct this history using the tree of life. By comparing DNA sequences, they can estimate how long ago species shared a common ancestor. The deepest split separates bacteria and archaea (both simple, single-celled organisms) from eukaryotes (organisms with complex cells, including all plants, animals and fungi). Within the eukaryote branch, animals diverged from fungi, then vertebrates from invertebrates, then mammals from reptiles, and so on.
Humans and chimpanzees share about 98-99% of their DNA. This does not mean we are 'almost chimpanzees', the 1-2% difference involves thousands of genes with significant effects. But it does mean our most recent common ancestor lived only about 6-8 million years ago. By contrast, humans and bananas share about 50% of their genes, reflecting a much older common ancestor roughly 1.5 billion years in the past.
Australian palaeontology: The Ediacara Hills in South Australia contain fossils of some of the earliest known complex multicellular organisms, dating back 560 million years. These soft-bodied creatures lived before the Cambrian explosion and represent a crucial transition in the tree of life from simple cells to complex animals. The site is so significant that the Ediacaran Period is named after it.
Students often think evolution is a ladder with bacteria at the bottom and humans at the top. This is wrong. Evolution is a branching tree, not a ladder. Bacteria are not 'primitive', they are exquisitely adapted to their niches and have been evolving for just as long as we have. Every extant species is equally 'evolved' because all have been adapting to their environments for the same 4 billion years.
Rank these events in the history of life from earliest to most recent.
- First cells appear
- Oxygen builds up in the atmosphere
- Dinosaurs dominate Earth
- First animals with backbones evolve
- Humans appear
Australia is one of 17 megadiverse nations on Earth. We harbour more than one million native species, and roughly 80% of our mammals, reptiles and flowering plants are found nowhere else, they are endemic. Our unique biodiversity is largely a legacy of Gondwana: when Australia separated from other continents around 50 million years ago, its flora and fauna evolved in isolation. This is why we have kangaroos instead of deer, eucalyptus instead of oak, and platypuses instead of beavers.
One of the most remarkable events in evolutionary history was the origin of eukaryotic cellscells with a nucleus and organelles. The leading theory, proposed by Lynn Margulis in the 1960s, is endosymbiosis. A large ancestral cell engulfed a smaller bacterium, but instead of digesting it, the two cells began to cooperate. Over millions of years, the engulfed bacterium became the mitochondrion, the cell's power plant. A similar event involving a photosynthetic bacterium gave rise to chloroplasts in plants and algae.
The evidence is compelling. Mitochondria and chloroplasts have their own DNA, which resembles bacterial DNA more than nuclear DNA. They reproduce independently within the cell by binary fission, like bacteria. And they are surrounded by double membranes, the inner one from the original bacterium, the outer one from the host cell's engulfing membrane. This partnership transformed life on Earth, enabling the evolution of all complex organisms.
Mitochondrial DNA is inherited only from the mother, because sperm mitochondria are destroyed after fertilisation. This unusual inheritance pattern allows geneticists to trace maternal lineages back through time. All humans alive today can trace their mitochondrial DNA to a woman who lived in Africa roughly 150,000-200,000 years ago, often called 'Mitochondrial Eve', not the first woman, but the most recent common ancestor of all maternal lines.
Australian research: Scientists at the University of Melbourne study the mitochondrial DNA of native Australian animals to reconstruct how species diverged as the continent's climate changed. By comparing mitochondrial genomes of kangaroos, wallabies and rat-kangaroos, they can estimate when these lineages split and how they adapted to Australia's drying landscape.
Tap each card to flip. Mark Got it when you can recall the answer without flipping.
Evolutionary biologists use two main approaches to date events in the history of life. The fossil record provides direct evidence of when organisms existed, but it is incomplete because fossils form only under special conditions. Soft-bodied organisms rarely fossilise, and some environments destroy bones rather than preserving them. The fossil record tells us the minimum age of a lineage, but not necessarily its true origin.
Molecular clocks provide a complementary approach. By comparing DNA sequences between species and estimating how fast mutations accumulate, scientists can calculate when two lineages last shared a common ancestor. When calibrated with fossil data, molecular clocks can estimate divergence times even for groups with no fossil record. The two methods often agree, and when they disagree, the discrepancy itself becomes a clue that drives new research.
For decades, the oldest known animal fossils were about 540 million years old, from the Cambrian period. Molecular clocks suggested animals originated earlier, but there was no fossil evidence. Then, in the 2010s, scientists discovered steroid molecules in 635-million-year-old rocks that are produced only by sponges. This 'molecular fossil' pushed back the origin of animals by nearly 100 million years, confirming the molecular clock prediction.
Australian molecular clocks: Researchers at ANU and the University of Sydney use molecular clock analyses to study the timing of Australia's unique biodiversity. Their work suggests that many Australian plant and animal lineages diverged around 30-40 million years ago, coinciding with the continent's northward drift into drier climatic zones. This link between geology and genetics helps explain why Australia's ecosystems are so distinctive.
COVID-19 Variants, Evolution in Real Time
The emergence of COVID-19 variants (Alpha, Delta, Omicron) was evolution happening before our eyes. The SARS-CoV-2 virus replicated billions of times inside human hosts. Random mutations occurred during replication. Variants with mutations that helped them spread faster outcompeted slower-spreading variants. This is natural selection operating on a virus, a powerful reminder that evolution is not just ancient history.
Wrong: "Evolution is just a theory, so scientists are not sure it is true."
Right: In science, a "theory" is a well-tested explanation supported by extensive evidence. Evolution is one of the most strongly supported theories in all of science.
Wrong: In science, a theory is a well-tested, evidence-based explanation, not a guess. Evolution is supported by fossils, DNA, anatomy, embryology and biogeography. It is one of the most strongly supported theories in all of science.
Right: Correct, a scientific theory is an evidence-based explanation backed by multiple independent lines of evidence. Evolution is as well supported as the theory of gravity.
Wrong: "Evolution happens to individual organisms during their lifetime."
Right: Evolution occurs in populations over generations, not in individuals during their lifetimes. An organism is born with its genes and does not evolve during its life.
Wrong: Evolution happens to populations over generations, not to individuals. An organism is born with its genes; it does not evolve during its life.
Right: Correct, evolution acts on populations over generations. Individual organisms do not change genetically during their lifetime; allele frequencies shift in the population's gene pool.
Classify These Organisms
1 Red kangaroo (Macropus rufus)
2 Golden wattle (Acacia pycnantha)
3 Bread mould (Rhizopus sp.)
4 Escherichia coli (gut bacteria)
5 Paramecium (a single-celled protist)
Western and Indigenous Classification
1 Choose an Australian organism (e.g., kangaroo, eucalyptus, emu, barramundi). Write its Western scientific name and classification.
2 Describe how an Aboriginal or Torres Strait Islander group might classify or categorise this same organism. What criteria do they use?
3 Explain why both classification systems are valuable. What does each system reveal that the other might not?
Copy Into Your Book
▼Core Definitions
- Biodiversity = variety of life (genetic, species, ecosystem)
- Classification = organising organisms into groups
- Taxonomy = science of naming and classifying
- Evolution = change in populations over generations
- Natural selection = mechanism of evolution
Classification Hierarchy
- Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species
- Broader groups contain more species
- Species = binomial name (Genus + species)
Evolution Unifies Biology
- Connects genetics, ecology, palaeontology
- Explains both unity (common ancestry) and diversity (branching)
- Supported by multiple independent lines of evidence
Indigenous Knowledge
- Aboriginal classification uses ecological, seasonal and relational criteria
- Not inferior to Western science, different and complementary
- Respect ICIP protocols when sharing knowledge
At the start of this lesson you were told that Australia is one of only 17 "megadiverse" countries on Earth, holding roughly 10% of the world's species on 0.5% of the planet's land, yet over 1,900 of those species are currently threatened. That context was meant to ground the abstract ideas of biodiversity and evolution in something urgent and real.
Now that you have explored how scientists classify life's diversity and trace it through evolutionary time, explain how those threatened species fit into the bigger picture. How does understanding evolutionary thinking change the way you think about conservation in Australia?
Q1. Define biodiversity and explain why Australia is called a megadiverse country. 3 MARKS
Q2. Describe one way Aboriginal and Torres Strait Islander Peoples classify organisms, and explain how this reflects deep ecological knowledge. 4 MARKS
Q3. Explain why evolution is described as the 'unifying theory' of biology. Refer to at least two branches of biology in your answer. 5 MARKS
Revisit Your Initial Thinking
Go back to your Think First responses at the top of the lesson.
- Did you correctly identify that life's diversity is the product of billions of years of evolutionary change?
- Did you recognise that classification systems (both Western and Indigenous) help us organise and understand this diversity?
- Write one sentence summarising the most important new concept you learned about how evolution unifies biology.
Model answers (click to reveal)
Comprehensive Answers
▼Activity 1, Classify These Organisms
1. Red kangaroo: Animaliamulticellular, heterotrophic, motile, lacks cell walls.
2. Golden wattle: Plantaemulticellular, autotrophic (photosynthetic), has cell walls containing cellulose.
3. Bread mould: Fungiheterotrophic, absorbs nutrients, has chitin cell walls, reproduces by spores.
4. E. coli: Bacteria (Monera)prokaryotic, single-celled, no nucleus, unicellular.
5. Paramecium: Protistaeukaryotic, mostly unicellular, does not fit neatly into plant, animal or fungi kingdoms.
Activity 2, Western and Indigenous Classification
3. Both systems are valuable: Western taxonomy reveals evolutionary relationships and shared ancestry [1 mark]. Indigenous classification reveals ecological relationships, seasonal cycles and practical uses that Western taxonomy may overlook [1 mark]. Both are evidence-based systems accumulated over long periods, Western science over centuries, Indigenous knowledge over tens of thousands of years [1 mark]. Together they give a richer, more complete understanding of biodiversity [1 mark].
Multiple Choice
1. CBiodiversity is defined as the variety of life at genetic, species and ecosystem levels. Options A, B and D are too narrow.
2. ADomain → Kingdom → Phylum → Class → Order → Family → Genus → Species is the correct hierarchy. Option B reverses it. Option C swaps Domain and Kingdom. Option D swaps Phylum and Class.
3. CEvolution is a well-supported scientific theory. Option A confuses "theory" with "guess." Option B describes Lamarckism or individual change, which is wrong. Option D is false, evolution applies to all life.
4. BIndigenous classification uses habitat, season, use and relationships. Option A describes Western taxonomy. Option C is disrespectful and false. Option D is too narrow.
5. BEvolution unifies genetics, ecology, palaeontology and anatomy. Option A is about consensus, not explanatory power. Option C is historically false. Option D confuses evolution with geology/cosmology.
Short Answer Model Answers
Q6 (3 marks): Biodiversity is the variety of life on Earth at three levels: genetic, species and ecosystem [1 mark]. Australia is called megadiverse because it harbours more than one million native species and is home to approximately 80% endemic species, found nowhere else on Earth [1 mark]. This high endemism is a result of Australia's long isolation from other continents since the breakup of Gondwana, allowing unique evolutionary pathways [1 mark].
Q7 (4 marks): Aboriginal and Torres Strait Islander Peoples often classify organisms by ecological relationships and seasonal behaviour [1 mark]. For example, plants may be grouped by the season in which they flower or produce edible fruit, or by which animals depend on them [1 mark]. This reflects deep ecological knowledge because it is based on careful observation of species interactions over tens of thousands of years [1 mark]. It is not inferior to Western taxonomy, it is complementary, revealing practical and relational knowledge that Linnaean classification does not capture [1 mark].
Q8 (5 marks): Evolution is called the unifying theory because it provides a single explanatory framework that connects all branches of biology [1 mark]. In genetics, evolution explains why DNA is conserved across species and why mutations create the variation that natural selection acts upon [1 mark]. In palaeontology, evolution explains why simpler fossils appear in older rocks and why transitional fossils like Tiktaalik show intermediate forms [1 mark]. In ecology, evolution explains why species are adapted to their environments and why Australia has such unique fauna [1 mark]. Because evolution makes testable predictions across multiple independent fields, it unifies biology into a coherent scientific framework [1 mark].
Jump Through Evolution!
Climb platforms using your knowledge of biodiversity, classification and evolution. Pool: Lesson 11.