EVSC10001 · The Global Environment
Life, Mass Extinctions & Human Origins
This chapter reads four billion years of biology out of dated rock — from the earliest stromatolites and the Great Oxygenation Event, through the Cambrian Explosion, the Big Five mass extinctions, and the mechanism of natural selection, to human origins out of Africa. It maps directly onto the deep-time block of EVSC10001, and because the theory exam is a hurdle worth 60% with Part-A short-answers that each require a labelled diagram, knowing the milestones, the two best-understood extinctions, and the selection recipe is exam-critical.
What this chapter covers
- 01History of life & the fossil record (faunal succession, index fossils)
- 02Stromatolites, the Great Oxygenation Event & banded iron formations
- 03Eukaryotes, endosymbiosis & the Ediacaran–Cambrian Explosion
- 04The Big Five mass extinctions & their recurring causes
- 05End-Permian (Siberian Traps) vs K–Pg (Chicxulub bolide + iridium)
- 06Natural selection: variation + heritability + differential survival
- 07Adaptive radiation & extinction in deep time
- 08Human origins: bipedalism, brain expansion, Homo out of Africa
WE — Part A: the K–Pg extinction, cause + diagnostic evidence + labelled diagram
- +1Define the term: a mass extinction is a short interval in which a large fraction of species die out far above the normal background rate; there are five in the Phanerozoic.
- +1Name the cause: the K–Pg (~66 Ma) is attributed to a large asteroid/bolide impact at Chicxulub (Yucatán), which threw dust and soot into the atmosphere and triggered a global 'impact winter' that collapsed photosynthesis.
- +1Give the diagnostic evidence: a thin, worldwide boundary clay enriched in iridium — an element rare in Earth's crust but abundant in meteorites — together with shocked quartz and tektites (glassy impact droplets).
- +1State the biological outcome: the non-avian dinosaurs and ammonites go extinct, opening niches into which mammals subsequently radiate (adaptive radiation).
- +1Draw the labelled diagram (the marked component): a vertical stratigraphic column with a lower 'Cretaceous' bed (dinosaur/ammonite fossils labelled below) and an upper 'Paleogene' bed (mammals diversifying above), separated by a thin 'boundary clay' layer; annotate that clay with arrows reading 'iridium spike + shocked quartz' and place a small inset deep-time axis ticking the K–Pg at 66 Ma.
- +1Label fully and correctly — axes/units (depth or time), the Cretaceous–Paleogene boundary, and the iridium anomaly — because Part-A diagram marks are lost for unlabelled sketches.
Key terms
- Faunal succession
- The principle (William Smith) that fossil assemblages occur in a fixed, recognisable order through the rock record, so the fossils in a rock pin its relative age and allow correlation between distant sites (biostratigraphy).
- Great Oxygenation Event (GOE)
- The rise of free atmospheric oxygen produced by cyanobacterial photosynthesis; oxygen first oxidised dissolved iron in the oceans, recorded as banded iron formations, before accumulating in the air.
- Cambrian Explosion
- The geologically rapid appearance and diversification of most modern animal phyla and the first hard skeletons at the base of the Cambrian (~541 Ma), preceded by the soft-bodied Ediacaran fauna.
- Mass extinction
- A short interval in which a large fraction of species die out at a rate far above the background rate; the Phanerozoic has five, with the end-Permian the largest and the K–Pg the most clearly impact-linked.
- Iridium anomaly
- A worldwide thin clay layer at the K–Pg boundary enriched in iridium — an element scarce in Earth's crust but common in meteorites — taken as the key evidence for a large bolide impact.
- Natural selection
- The mechanism of evolution requiring heritable variation, over-production of offspring, and differential survival/reproduction of better-suited variants, so favourable traits accumulate across generations (descent with modification).
Life, Mass Extinctions & Human Origins FAQ
What is the difference between the K–Pg and end-Permian mass extinctions?
The end-Permian (~252 Ma) is the largest extinction and is attributed to Siberian Traps flood-basalt volcanism that drove prolonged greenhouse warming and ocean anoxia. The K–Pg (~66 Ma) is attributed to a near-instantaneous Chicxulub bolide impact, identified by a global iridium anomaly. Cite the specific signature for each — trap basalts versus iridium — rather than blaming every extinction on 'a meteorite'.
What are the three ingredients natural selection needs?
Heritable variation among individuals, over-production of offspring (a struggle for existence because resources are limited), and differential survival and reproduction of the better-suited variants. Over many generations the favourable traits accumulate, populations adapt, and — given isolation — new species form.
Why does the Cambrian Explosion matter and what is the Burgess Shale?
The Cambrian Explosion (~541 Ma) is the rapid appearance of nearly all modern animal phyla and the first hard skeletons. The Burgess Shale matters because its exceptional soft-body preservation gives a rare window onto early animal diversity and body plans, not just the shelly parts that normally survive.
What are the three big trends in human origins?
Bipedalism first (upright walking predates the large brain and freed the hands), a marked increase in brain size along the Homo lineage, and dispersal of Homo out of Africa in successive waves — the same evolutionary mechanism read from dated fossils on the youngest sliver of the time scale.
Why does every EVSC Part-A answer need a labelled diagram?
Part A is answer 7 of 9 short-answers (~10 min each), and a labelled diagram is required and separately scored on every one. For this block, rehearse the geological time scale with milestones marked, an extinction timeline, and a natural-selection loop; always label axes, boundaries and the diagnostic signature, because unlabelled sketches lose marks.
Exam move
Memorise the deep-time order as one spine — oldest life ~3.7 Ga (stromatolites) → GOE/banded iron → eukaryotes → Ediacaran → Cambrian Explosion 541 Ma → end-Permian 252 Ma (largest) → K–Pg 66 Ma — and hang every fact off it. Drill the cause-plus-signature pairing for the two exam-favourite extinctions (Siberian Traps for end-Permian volcanism; Chicxulub iridium anomaly for the K–Pg impact) and the three-part selection recipe (variation + heritability + differential survival → adaptation), then practise sketching three labelled diagrams cold: the time-scale column with milestones, an extinction timeline, and a natural-selection loop. Since the theory exam is closed-book and a hurdle, write answers as 'define → cause → evidence → outcome → labelled diagram' and always label axes, boundaries and the diagnostic signature so you bank the diagram marks.
