What does the fossil record show about evolution and the major changes in life through time?
Evolution and the fossil record: the evidence for evolution preserved in successive strata; modes of evolutionary change (gradualism and punctuated equilibrium); the use of evolutionary trends in lineages for dating; the major mass extinctions and their possible causes; and the broad pattern of the history of life through the geological time scale.
A focused answer to the Eduqas Geology statement on evolution. Covers the fossil evidence for evolution, gradualism versus punctuated equilibrium, evolutionary trends used in dating, the major mass extinctions and their causes, and the broad history of life through geological time.
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What this dot point is asking
Eduqas wants you to set out the fossil evidence for evolution in successive strata, to distinguish gradualism from punctuated equilibrium, to explain how evolutionary trends are used in dating, to describe the major mass extinctions and their possible causes, and to outline the broad history of life through the geological time scale. This builds on index fossils: evolution is why fossils change through time and so why they can date rocks.
The answer
The fossil evidence for evolution
The fossil record preserves a succession of different life forms through the strata, with simpler forms generally in older rocks and more complex and diverse forms higher up. The evidence for evolution includes:
- Progressive change in lineages through successive beds (for example trends in ammonites or in horse teeth).
- Transitional forms linking major groups (for example fossils with both reptile and bird features).
- The first appearances and extinctions of groups at consistent stratigraphic levels worldwide.
Because organisms change irreversibly through time, a given assemblage of species is unique to a particular interval, which is exactly what makes fossils useful for correlation and dating.
Gradualism and punctuated equilibrium
There are two models of the tempo of evolutionary change:
- Gradualism: slow, steady, continuous change, appearing in the record as a smooth series of transitional forms from ancestor to descendant.
- Punctuated equilibrium: long periods of little change (stasis) interrupted by short bursts of rapid change (often during speciation in small isolated populations), appearing as stasis then the abrupt appearance of new forms with few transitional fossils.
The real fossil record often shows stasis and sudden appearances, which supports punctuated equilibrium, though some of the apparent suddenness reflects gaps in the incomplete record. Both processes probably operate.
Evolutionary trends and dating
Within a lineage, features often change in a consistent direction over time (for example increasing size, increasing complexity of ammonite suture lines, or coiling changes). Such evolutionary trends let geologists place a fossil, and so its rock, within the lineage's history, refining the relative age. Rapidly evolving, widespread groups (ammonites, graptolites) give the finest biozones.
Mass extinctions
A mass extinction is a relatively sudden loss of a large fraction of species worldwide. Five major mass extinctions punctuate the record; the two most examined are:
- End-Permian (the largest, about 252 million years ago): up to about 90 percent of marine species lost; linked to massive Siberian flood-basalt volcanism, global warming and ocean anoxia.
- End-Cretaceous (about 66 million years ago): the loss of the non-avian dinosaurs and the ammonites; evidence includes an iridium-rich global clay layer, shocked quartz and tektites. The leading cause is a large meteorite impact (Chicxulub), with Deccan Traps flood-basalt volcanism contributing.
Mass extinctions reset ecosystems and are followed by the radiation of surviving groups into vacated niches (for example mammals after the dinosaurs).
The broad history of life
Through the time scale: simple prokaryotic life in the Precambrian, the Cambrian explosion of diverse marine animals at the start of the Palaeozoic, the colonisation of the land by plants and then animals, the age of reptiles (dinosaurs) in the Mesozoic, and the rise of mammals and flowering plants in the Cenozoic, with humans appearing very recently. The boundaries of the eras are marked by major extinctions.
Examples in context
Example 1. Mammals after the dinosaurs. The end-Cretaceous extinction removed the dominant large reptiles, and the surviving mammals radiated rapidly into the vacated niches, illustrating how extinctions reset and redirect the history of life.
Example 2. The iridium layer. The thin, worldwide iridium-enriched clay at the Cretaceous-Palaeogene boundary is the kind of single, datable, global marker that ties the extinction to an extraterrestrial impact.
Try this
Q1. State the difference between gradualism and punctuated equilibrium in one sentence each. [2 marks]
- Cue. Gradualism is slow continuous change with transitional forms; punctuated equilibrium is long stasis interrupted by short rapid bursts of change.
Q2. Name the two best-known mass extinctions and give one proposed cause of each. [2 marks]
- Cue. End-Permian (Siberian flood basalts, warming and anoxia) and end-Cretaceous (meteorite impact at Chicxulub, with Deccan volcanism).
Q3. Explain why an abrupt appearance of a new species in a section does not by itself prove punctuated equilibrium. [2 marks]
- Cue. The gap may be an artefact of the incomplete record (an unconformity, non-deposition or unsampled beds) that has removed the transitional forms, rather than genuinely rapid evolution.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20206 marksCompare the models of gradualism and punctuated equilibrium as descriptions of evolutionary change, and explain how each would appear in the fossil record.Show worked answer →
A levels-of-response answer; define each model and link it to the rock record.
- Gradualism
- Evolution proceeds by slow, steady, continuous change over long periods. In the fossil record this would appear as a gradual series of transitional forms passing smoothly from the ancestral to the descendant species through successive strata.
- Punctuated equilibrium
- Species remain largely unchanged (in stasis) for long periods, then change rapidly in short bursts, often associated with speciation in small isolated populations. In the fossil record this appears as long intervals with little change, separated by abrupt appearances of new forms with few transitional fossils preserved.
- Reconciling with the record
- The fossil record often shows stasis and sudden appearances, which fits punctuated equilibrium, but this can also partly reflect the incompleteness of the record (gaps where transitional forms are not preserved). Both processes probably operate.
Top-band answers define each model clearly, describe how each looks in successive strata (smooth transitions versus stasis then abrupt change), and note that record incompleteness complicates the distinction.
Eduqas 20185 marksDescribe the evidence for a major mass extinction at the end of the Cretaceous, and outline two hypotheses proposed to explain it.Show worked answer →
Give the evidence in the rocks, then two causal hypotheses.
- The evidence
- A sharp, widespread loss of species at the Cretaceous-Palaeogene boundary, including the non-avian dinosaurs and the ammonites, recorded as the abrupt disappearance of their fossils. A worldwide clay layer at the boundary is enriched in iridium (rare on Earth but common in meteorites), and shocked quartz and tektites are found.
- Hypothesis 1: meteorite impact
- A large asteroid impact (linked to the Chicxulub crater) threw dust and aerosols into the atmosphere, blocking sunlight, collapsing photosynthesis and food chains, and causing rapid global cooling then disruption. The iridium and shocked quartz support this.
- Hypothesis 2: massive volcanism
- Huge, prolonged eruptions (the Deccan Traps flood basalts) released volcanic gases and ash, causing climate change, acid rain and ocean acidification over a longer period.
- Conclusion
- Many workers see the impact as the trigger, with Deccan volcanism contributing stress; the iridium layer is the key impact evidence.
Markers reward the abrupt fossil loss and the iridium-rich boundary clay as evidence, plus the impact and flood-basalt hypotheses with their effects on climate.
Related dot points
- Fossils, preservation and index fossils: the modes of fossil preservation (unaltered hard parts, recrystallisation, replacement, moulds and casts, carbonisation, trace fossils); the conditions that favour preservation; and the characteristics that make a good index (zone) fossil for biostratigraphic correlation.
A focused answer to the Eduqas Geology statement on fossils. Covers the modes of fossil preservation, the conditions that favour fossilisation, the difference between body and trace fossils, and the characteristics of a good index (zone) fossil for biostratigraphic correlation.
- Relative dating and stratigraphic principles: the principles of superposition, original horizontality, lateral continuity, cross-cutting relationships and included fragments; way-up (younging) indicators; and the use of these principles to reconstruct the sequence of geological events from a section or map.
A focused answer to the Eduqas Geology statement on relative dating. Covers the principles of superposition, original horizontality, lateral continuity, cross-cutting relationships and included fragments, way-up indicators, and how to reconstruct a sequence of geological events from a section or map.
- Radiometric dating and half-life: radioactive decay and the concept of half-life; the use of parent-to-daughter ratios to calculate absolute ages; the main dating methods and their suitable age ranges (for example uranium-lead, potassium-argon, rubidium-strontium and carbon-14); the assumptions and limitations of radiometric dating; and the construction of the absolute geological time scale.
A focused answer to the Eduqas Geology statement on radiometric dating. Covers radioactive decay and half-life, calculating absolute ages from parent-to-daughter ratios, the main dating methods and their ranges, the assumptions and limitations, and how the absolute time scale is built.
- Palaeoenvironments and palaeoclimate proxies: the use of fossils, sedimentary structures and lithology to reconstruct past environments; palaeoclimate proxies (for example coal, evaporites, tillites, reef limestones, oxygen isotopes and fossil leaf shape); the use of facies and Walther's law; and the evidence for past climate change recorded in the rocks.
A focused answer to the Eduqas Geology statement on palaeoenvironments. Covers reconstructing past environments from fossils, sedimentary structures and lithology, palaeoclimate proxies such as coal, evaporites and tillites, facies and Walther's law, and the rock evidence for past climate change.
- Sedimentary rocks and depositional environments: the classification of clastic rocks by grain size (conglomerate and breccia, sandstone including arkose, greywacke and orthoquartzite, siltstone, mudstone and shale) and of chemical and biogenic rocks (limestone including oolitic, micritic and fossiliferous, chalk, the evaporites rock salt and gypsum, and coal); sedimentary structures (cross-bedding, graded bedding, ripple marks, desiccation cracks) as way-up and environment indicators; depositional environments (fluvial, deltaic, shallow marine, deep marine, desert); and diagenesis and lithification.
A focused answer to the Eduqas Geology statement on sedimentary rocks. Covers clastic classification (conglomerate to mudstone, with arkose, greywacke and orthoquartzite), chemical and biogenic rocks (limestones, chalk, evaporites, coal), sedimentary structures as way-up and environment indicators, depositional environments, and diagenesis and lithification.
Sources & how we know this
- Eduqas A Level Geology Specification (A220QS) — Eduqas (2017)