The strongest evidence for evolution does not come from bones or fossils alone. It comes from the molecules inside every living cell — and from the patterns of where species live on Earth. Together, these lines of evidence paint an undeniable picture of shared ancestry.
Humans share about 98.8% of our DNA with chimpanzees, about 85% with mice, and about 50% with bananas.
Write down your answers before reading on:
If you want to know how closely related two species are, compare their DNA. The molecule that stores genetic instructions is a historical document — and it records the evolutionary past with remarkable fidelity.
All living things use DNA as their genetic material, and the genetic code (which codons specify which amino acids) is nearly universal. This universality itself is strong evidence for common ancestry. But the details are even more revealing:
Scientists also compare specific proteins. Cytochrome c, a protein involved in cellular respiration, has been sequenced in hundreds of species. The number of amino acid differences between species correlates strongly with how long ago they diverged. Humans and chimpanzees have identical cytochrome c. Humans and yeast differ by 45 amino acids.
Humans share about 50% of our DNA with bananas. This does not mean we are "half banana." It means that about half of the genes in a banana cell perform basic cellular functions — making energy, building proteins, responding to stimuli — that are also essential in human cells. These core genes have been conserved for over a billion years because they work so well. Evolution rarely fixes what is not broken.
DNA accumulates changes over time at a roughly predictable rate. This allows scientists to use DNA as a kind of clock — not to tell the exact hour, but to estimate how long ago two species shared a common ancestor.
The basic idea is simple:
For example, if two species differ by 10 mutations in a gene that typically accumulates 1 mutation per million years, scientists estimate they diverged roughly 5 million years ago (because each lineage accumulated mutations independently). This is a conceptual tool, not a precise stopwatch — mutation rates vary between genes and lineages. But molecular clocks consistently match the timing suggested by fossils and geology.
At Stage 5, you do not need to perform molecular clock calculations. You need to understand the concept: more DNA differences = longer time since divergence = more distant common ancestor.
The geographical distribution of species provides powerful evidence for evolutionary history. If species were independently created, there would be no reason to expect their distribution to match the movement of continents. But they do.
Biogeography is the study of where species live and why. Key patterns include:
Marsupial evolution in Australia is one of the great biogeographical stories. When Australia separated from Gondwana around 50 million years ago, it carried marsupial mammals with it. Isolated from placental mammals that dominated other continents, Australian marsupials diversified into an astonishing array of forms: carnivorous Tasmanian devils, burrowing wombats, gliding possums, hopping kangaroos and koalas that climb eucalypts. This adaptive radiation occurred because marsupials filled ecological niches that placental mammals filled elsewhere — a pattern predicted by evolution and explained by Australia's long isolation.
Perhaps the most medically urgent example of evolution is antibiotic resistance. When antibiotics are used, they create a powerful selection pressure on bacterial populations:
MRSA (methicillin-resistant Staphylococcus aureus) is a major problem in Australian hospitals. It evolved through natural selection acting on bacterial variation in response to antibiotic use. Understanding evolution is not just academic — it is essential for modern medicine.
The Australian Commission on Safety and Quality in Health Care tracks antibiotic resistance nationwide. In 2023, MRSA and resistant strains of E. coli and Klebsiella caused thousands of serious infections. The World Health Organization has declared antimicrobial resistance one of the top ten global public health threats. Combating it requires not just new drugs, but also public understanding of evolution: finishing prescribed courses, avoiding unnecessary antibiotic use, and recognising that bacteria evolve in response to our medical practices.
1 Based on DNA similarity, which species is most closely related to humans? Explain your reasoning.
2 Why do humans and bananas share 50% of their DNA despite looking completely different?
3 A scientist sequences cytochrome c from an unknown mammal and finds it differs from human cytochrome c by 12 amino acids. From chimpanzee cytochrome c, it differs by 11 amino acids. What can you conclude?
4 Explain how molecular evidence supports the theory of common ancestry independently from fossil evidence.
5 If molecular clock estimates suggest humans and mice diverged 90 million years ago, but a new fossil discovery pushes that date back to 100 million years, which estimate is likely more reliable? Why?
1 Explain why Australia has so many marsupials while other continents have mostly placental mammals.
2 Describe how natural selection and Australia's unique environments contributed to the evolution of kangaroos, koalas and Tasmanian devils from a common marsupial ancestor.
3 Explain how antibiotic resistance in bacteria demonstrates all five principles of natural selection. Use MRSA as your example.
1. Why do closely related species have more similar DNA?
2. What is a molecular clock?
3. Biogeography supports evolution because...
4. Antibiotic resistance in bacteria is an example of...
5. Which statement best integrates the evidence for evolution?
6. Explain how DNA similarities between species provide evidence for common ancestry. 3 MARKS
7. What is a molecular clock, and why is it useful for understanding evolutionary relationships? 4 MARKS
8. Using antibiotic resistance as an example, explain how evolution by natural selection can be observed directly. Refer to variation, selection pressure and heritability in your answer. 5 MARKS
Go back to your Think First responses at the top of the lesson.
1. Most closely related: Chimpanzees share ~98.8% of DNA with humans, the highest similarity shown [1 mark]. This indicates the most recent common ancestor [0.5 mark].
2. Humans and bananas: Both are eukaryotes that need the same core genes for basic cellular functions (cellular respiration, DNA replication, protein synthesis) [1 mark]. These genes have been conserved for over a billion years because they are essential for survival [1 mark].
3. Unknown mammal conclusion: The unknown mammal is very closely related to both humans and chimpanzees, likely another great ape [1 mark]. The small difference (11–12 amino acids) suggests divergence very recently in evolutionary terms [0.5 mark].
4. Independent support: Molecular evidence is independent of fossils because it comes from living organisms, not rocks [1 mark]. When DNA trees match fossil timelines, the conclusion is much stronger [0.5 mark].
5. More reliable estimate: Both estimates should be considered together. Molecular clocks provide relative timing, but fossils anchor estimates to absolute geological time [1 mark]. A conflict suggests the molecular rate estimate may need revision [0.5 mark].
2. Marsupial radiation: After Gondwana separation, Australian marsupials were isolated from placental competitors [1 mark]. Different environments (arid plains, forests, grasslands) created different selection pressures [1 mark]. Variation in the ancestral population meant some individuals were better suited to each environment [1 mark]. Natural selection favoured different traits in different niches, leading to adaptive radiation [1 mark].
3. MRSA and natural selection: Variation exists — some bacteria have random mutations conferring resistance [1 mark]. Antibiotics create selection pressure, killing susceptible bacteria [1 mark]. Resistant bacteria survive and reproduce more [1 mark]. Resistance is heritable (encoded in DNA), so the trait spreads [1 mark]. The result is a population shift from susceptible to resistant — evolution observed in real time [1 mark].
1. B — Closely related species share more DNA because they diverged more recently from a common ancestor. Option A confuses ecology with genetics. Option C reverses cause and effect. Option D is false.
2. C — A molecular clock uses mutation rates to estimate divergence times. Options A, B and D describe laboratory equipment, not the conceptual tool.
3. A — Species distribution matches continental drift and fossils. Options B, C and D are false.
4. C — Antibiotic resistance is evolution by natural selection in real time. Option A describes Lamarckism. Option B describes a social issue, not a biological mechanism. Option D anthropomorphises bacteria.
5. D — Multiple independent lines of evidence converge on common ancestry. Options A, B and C are false or present false conflicts.
Q6 (3 marks): DNA similarities indicate common ancestry because all living things inherited their genetic code from shared ancestors [1 mark]. The more similar the DNA, the more recently the species shared a common ancestor — for example, humans and chimps share ~98.8% of DNA, reflecting divergence only 6–7 million years ago [1 mark]. Even distantly related species like humans and bananas share ~50% of DNA because core cellular genes have been conserved for over a billion years [1 mark].
Q7 (4 marks): A molecular clock is a conceptual tool that uses the roughly steady rate of DNA mutations to estimate when two species diverged from a common ancestor [1 mark]. Scientists count the number of DNA differences between species and use known mutation rates to calculate time [1 mark]. It is useful because it provides independent estimates of divergence times that can be compared with fossil and geological evidence [1 mark]. While not exact, molecular clocks consistently support the evolutionary relationships predicted by anatomy and fossils, strengthening the overall evidence [1 mark].
Q8 (5 marks): Antibiotic resistance is a direct observation of evolution by natural selection. Variation exists within bacterial populations — some bacteria carry random mutations that make them less susceptible to antibiotics [1 mark]. When antibiotics are used, they create a strong selection pressure that kills susceptible bacteria while resistant ones survive [1 mark]. This is differential survival — the resistant bacteria reproduce more because they are alive [1 mark]. Resistance is heritable because it is encoded in bacterial DNA and passed to offspring during reproduction [1 mark]. Over many generations, the bacterial population shifts from mostly susceptible to mostly resistant — the population has evolved [1 mark]. MRSA in Australian hospitals is a direct result of this process.
Test your knowledge of DNA evidence, molecular clocks and biogeography in this fast-paced quiz battle. Correct answers power your attacks!
Climb platforms using your knowledge of DNA, molecular clocks and biogeography. Pool: Lesson 14.
Tick when you have finished all activities and checked your answers.