Evolution Through Deep Time
In 1984, Chinese palaeontologist Hou Xianguang discovered the Chengjiang biota in Yunnan Province — more than 250 new animal species in a single Cambrian deposit dated to 538–520 million years ago. Within 20 million years, all major animal body plans appeared simultaneously in the fossil record: the Cambrian Explosion. Nothing comparable has occurred in the 520 million years since. The Chengjiang discovery filled a critical gap in the fossil record, showing that the rapid diversification predicted by evolutionary theory left a visible, dateable record in the rock — and that the stratigraphic tools from L18 can read it.
Birds look nothing like dinosaurs at first glance — yet scientists say birds are living dinosaurs. The fossil record is what changed their minds.
Before reading: what kind of fossil evidence do you think could show that one group of organisms gradually evolved into a very different group?
Know
- That the fossil record shows life changing over geological time
- What a transitional fossil is, with key examples
- The evolution of the horse lineage as a documented sequence
Understand
- How a sequence of fossils provides evidence of descent with modification
- Why transitional fossils are strong, testable evidence
- How environmental change drove the documented changes in a lineage
Can Do
- Analyse a transitional fossil's mix of ancestral and derived features
- Use a fossil sequence to argue for common ancestry
- Evaluate the claim that "gaps in the record disprove evolution"
Core Content
Reading life's history from oldest to youngest layers
In 1984, Hou Xianguang discovered the Chengjiang biota in Yunnan Province — over 250 animal species in a single Cambrian deposit from 538–520 million years ago. Every major animal body plan appeared in the rock record within a 20-million-year window: the Cambrian Explosion. When palaeontologists place the Chengjiang fossils in stratigraphic order and trace the lineages forward through 520 million years of subsequent rock layers, a directional pattern emerges: life changed steadily over time, with some lineages giving rise to fish, then amphibians, then reptiles, then birds. The rock record, read in stratigraphic order, is the strongest direct evidence that present-day organisms descended from very different ancestors.
Using the law of superposition and dating, fossils can be arranged in time. The record shows simple life forms appearing first, followed by progressively more complex and diverse organisms — and the appearance and disappearance of whole groups over millions of years.
This changing sequence is direct evidence of descent with modification: species arise, change, and give rise to new forms, while others go extinct — present-day organisms are the modified descendants of earlier ones.
Pause — copy the highlighted points into your book before the check below.
Darwin's idea that species change over generations from earlier forms is called descent with _____.
Fossils that capture evolution "in between"
We just saw that life changes through the record. That raises a question: can we actually catch the change happening between two very different groups? This card answers it → transitional fossils.
A transitional fossil shows a mix of ancestral and derived features — the intermediate stage evolutionary theory predicts between two groups.
A transitional fossil shows features of both an older ancestral group and a newer derived group, providing direct evidence of evolutionary change between them. Key examples:
Transitional fossils show the intermediate trait combinations predicted between ancestral and derived groups
Archaeopteryx had feathers and wings (bird) but teeth, clawed fingers and a bony tail (theropod dinosaur) — linking dinosaurs and birds. Tiktaalik linked fish and land tetrapods; Ambulocetus links land mammals and whales.
Add the definition and at least one example to your notes before the check below.
Archaeopteryx is a transitional fossil linking which two groups?
Analysing a Transitional Fossil
Pattern — Classify & Explain
Use Archaeopteryx for this task. In your book:
- Make a two-column table: "Features shared with dinosaurs/reptiles" and "Features shared with birds". Place at least three features in total.
- Explain why this combination of features makes Archaeopteryx a transitional fossil.
- State what this fossil suggests about the ancestry of modern birds.
- Explain why a fossil with only bird features, or only dinosaur features, would not be transitional.
A step-by-step fossil sequence linked to a changing environment
We just saw single transitional fossils. That raises a question: is there a whole sequence showing one lineage changing over time? This card answers it → the well-documented horse lineage.
The horse lineage is one of the most complete fossil sequences — and its changes track a changing environment, showing natural selection over deep time.
Over ~55 million years the horse lineage changed from Eohippus (small, dog-sized, several toes, low-crowned teeth, a forest browser) toward modern Equus (large, single hoof, high-crowned teeth, a grassland grazer):
| Feature | Early (Eohippus) | Modern (Equus) |
|---|---|---|
| Body size | Small (~0.4 m) | Large (~1.6 m) |
| Toes / foot | Several toes (padded) | Single hoof |
| Teeth | Low-crowned (soft leaves) | High-crowned (tough grasses) |
| Habitat | Forest browser | Open grassland grazer |
As climate dried and forests gave way to grasslands, natural selection favoured larger size and a single hoof (fast running on hard, open ground) and high-crowned teeth (grazing abrasive grasses). The horse fossil sequence shows evolutionary change driven by environmental change.
Add the horse trends (size, toes→hoof, teeth, habitat) and the link to environment to your notes.
A transitional fossil shows a combination of features from an ancestral group and a derived group.
Changes in the horse lineage (single hoof, high-crowned teeth) are linked to a shift from forest to grassland.
A sequence of fossils showing gradual change is evidence that species never share common ancestors.
Interpreting the Horse Fossil Sequence
Pattern — Analyse a Trend
Using the horse data in Card 3 and your knowledge of natural selection, answer in your book:
- Describe two clear trends in the horse lineage over time.
- For each trend, explain the selective advantage as the environment changed from forest to grassland.
- Explain how this fossil sequence supports "descent with modification" rather than separate, unrelated species.
- A student says the horse "decided to grow a single hoof to run faster". Identify and correct the misunderstanding using the language of natural selection.
Fossil record & change
- Ordered fossils show life changing over deep time.
- Evidence of descent with modification — present-day organisms descend from earlier forms.
Transitional fossils
- Mix of ancestral + derived features.
- Archaeopteryx (dinosaur↔bird), Tiktaalik (fish↔tetrapod), Ambulocetus (mammal↔whale).
- Strong evidence: show the intermediate forms evolution predicts.
Horse lineage
- Eohippus → Equus: bigger, toes→single hoof, low→high-crowned teeth.
- Driven by forest→grassland change (natural selection).
A fresh set drawn from this lesson's question bank — feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence — that tells the system what to drill next.
UnderstandBand 3(3 marks) 1. Define a transitional fossil and explain, using one named example, how it provides evidence for evolution.
1 mark: definition (mix of ancestral + derived features) · 1 mark: named example with features · 1 mark: how it links two groups as evidence of change
ApplyBand 4(4 marks) 2. Using the horse lineage, explain how the fossil record provides evidence of evolutionary change driven by a changing environment.
1 mark: describes the sequence/trend · 1 mark: names ≥2 changes (size, toes→hoof, teeth) · 1 mark: environmental change (forest→grassland) · 1 mark: links via natural selection
EvaluateBand 5(4 marks) 3. "Gaps in the fossil record show that evolution did not happen." Evaluate this statement, referring to the nature of the fossil record and the evidence that does exist.
up to 2 marks: why gaps exist (incomplete/biased fossilisation) · up to 2 marks: evidence that supports evolution (sequences, transitional forms) + judgement
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Short Answer Model Answers
Q1 (3 marks): A transitional fossil is a fossil that shows a combination of features of an older ancestral group and a newer derived group. For example, Archaeopteryx had feathers and wings (features of birds) but also teeth, clawed fingers and a long bony tail (features of theropod dinosaurs). Because it possesses the intermediate combination of traits expected between dinosaurs and birds, it provides direct evidence that birds evolved from dinosaur ancestors — i.e. evidence of evolutionary change linking the two groups.
Q2 (4 marks): The horse lineage is preserved as a graded fossil sequence spanning roughly 55 million years. Over this time the lineage changed from the small, several-toed, low-crowned-toothed forest browser Eohippus toward the large, single-hooved, high-crowned-toothed grassland grazer Equus — clear trends in body size, foot structure (toes to a single hoof) and teeth. These changes coincided with a drying climate in which forests were replaced by open grasslands. Natural selection favoured individuals with variations suited to the new environment: larger size and a single hoof for fast running on hard, open ground (escaping predators), and high-crowned teeth for grazing abrasive grasses. The fossil record therefore documents evolutionary change driven by environmental change.
Q3 (4 marks): Gaps in the fossil record exist because fossilisation is rare and biased: it requires specific conditions (rapid burial, hard body parts), so many organisms and environments are under-represented, and many fossils have been eroded, destroyed or not yet found. However, gaps reflect the incompleteness of preservation, not the absence of evolution. The fossils that do exist provide strong positive evidence: ordered sequences show life changing through time, and transitional fossils (e.g. Archaeopteryx, Tiktaalik) show exactly the intermediate trait combinations that evolution predicts — and newly discovered fossils continue to fill the gaps. Therefore the statement should be rejected: gaps are expected and do not disprove evolution, while the available evidence strongly supports it.
Timed questions on the fossil record, transitional forms and the horse lineage. Beat the boss to bank a tier — gold (perfect + fast), silver (80%+), or bronze (cleared).
⚔ Enter the arenaHou Xianguang's 1984 Chengjiang discovery revealed that all major animal body plans appeared within 20 million years at the start of the Cambrian (538–520 Ma). From those first body plans, 520 million years of fossil record shows lineages diverging and changing — the horse lineage from dog-sized forest browsers to large open-plain grazers; the dinosaur-bird lineage captured in transitional fossils like Archaeopteryx with feathers and claws simultaneously. These sequences and transitional forms provide the strongest evidence that present-day species descended with modification from very different ancestors.
Return to your Think First response. Could you now identify the two types of fossil evidence for evolution — graded sequences and transitional fossils — and explain why each is strong, testable evidence?