Mass Extinctions

Mass Extinction Events & Terraforming

EPSC186 – Lecture 23
Date: April 1, 2025

Extinction: A Common Fate

• Of all species that have ever existed, 99.9% are now extinct.

• Extinction is the most likely fate of any species.

• There is a normal background rate of extinction, occasionally punctuated by mass extinctions.

Understanding Extinction Rates

• Scientists use the fossil record to estimate background extinction rates.

• Method:

  • Count species in a time/place.

  • Identify which went extinct.

Limitations of the Fossil Record:

• Incomplete representation of past biodiversity.

• Fossil formation is rare in many environments.

  • Shallow seas = most conducive to fossil formation.

  • Other habitats are underrepresented.

• Rare fossilization methods: freezing, drying, encasement (tar, resin).

• Most fossils form in watery environments, buried in mud/silt.

• Soft-bodied organisms (e.g., jellyfish, worms) do not fossilize well.

• Fossils of land animals are rarer than plant fossils.

• Species classification is often inaccurate, especially from small fragments.

  • Morphology-based ID can be misleading — genetically distinct species may look identical.

Extinction Rates Are Usually Measured at the Family Level

• Not at species or genus level.

• Hard to distinguish species in the fossil record.

Causes of Extinction

Biotic Mechanisms (Biological Causes):

• Competition from closely related species.

• Prey develops defenses.

• New predators expand territory.

• Diseases wipe out populations.

Abiotic Mechanisms (Environmental Causes):

• Habitat no longer supports life due to:

  • Climate change

  • Sea-level fluctuations

  • Meteor impacts

  • Volcanism

Mass Extinctions

• Extinction rate greatly exceeds the background rate.

• Affects many unrelated species.

• Occurs over a geologically short time.

• Global impact on biodiversity.

The "Big Five" Mass Extinctions

1. Late Ordovician (Phanerozoic – Paleozoic)

2. Late Devonian (Phanerozoic – Paleozoic)

3. Late Permian (Phanerozoic – Late Paleozoic)

4. Late Triassic (Phanerozoic – Early Mesozoic)

5. Late Cretaceous (Phanerozoic – Late Mesozoic)

Effect of Mass Extinctions on Evolution

• Rapid speciation among survivors.

• New niches become available.

• Isolated populations adapt and evolve — biodiversity rebounds quickly.

Key Mass Extinctions in Detail

Late Ordovician (440 Mya)

• Most life was marine.

• Affected nearly all taxonomic groups:

  • 49–60% of marine genera

  • ~85% of marine species

• Cause: Global cooling & sea-level fall.

  • Shallow, warm marine habitats disappeared.

  • Glaciation due to continental drift to the South Pole.

  • Water locked in glaciers = ocean levels dropped = shallow seas drained.

Late Permian – “The Great Dying” (250 Mya)

• Biggest known extinction event.

• 70% of land vertebrates, 96% of marine species went extinct.

• All current life descended from the survivors.

Causes:

• Massive volcanic eruptions (likely Siberian Traps).

  • Enough lava to cover the U.S. in km-deep magma.

  • Released CO₂ = greenhouse effect, ocean acidification.

• CO₂ + water = carbonic acid → acidic oceans → marine life collapses.

• Aerosols blocked sunlight → disrupted photosynthesis on land and sea.

• Acid rain damaged ecosystems.

Late Cretaceous (66 Mya)

• 75% of species went extinct.

• No tetrapods >25 kg survived (except turtles and crocodiles).

• End of dinosaurs → rise of mammals.

Impact Hypothesis:

• 10–15 km-wide asteroid impact:

  • Dust & ash blocked sunlight.

  • Triggered global winter.

  • Photosynthesis halted, causing ecosystem collapse.

• Evidence:

  • 150 km-wide crater dated to 66 Mya.

  • Global iridium spike (160x normal) — rare on Earth, common in asteroids.

  • Shocked quartz near impact crater — only forms under extreme pressure.

Volcanism Hypothesis:

• Deccan Traps (India):

  • Huge volcanic eruptions.

  • CO₂, SO₂ emissions and aerosols may have worsened extinction.

  • Possibly triggered by or coinciding with the asteroid impact.

The Sixth Mass Extinction? (Anthropocene)

• Some scientists believe we are in an ongoing mass extinction.

• Background rate: 0.1 species/million/year.

• Current rate: 100 species/million/year.

  • That's 100 to 1000x the natural rate.

Human-Driven Causes:

• Habitat destruction

• Invasive species

• Overhunting

• Pollution, climate change (indirect)

Terraforming: A Future Beyond Earth?

Why Leave Earth?

• Population growth

• Resource depletion

• Backup plan for climate disasters

• Exploration and technological ambition

Terraforming Mars

• Ideal Candidate:

  • Close to Earth

  • Similar day length and seasons

  • Resources: water, metals

  • High potential for modification

Challenges on Mars:

• Cold (-63ºC average)

• Thin, toxic atmosphere (95% CO₂)

• Frozen water

• High radiation

• Low gravity

• Psychological challenges from isolation

Terraforming Goals

1. Temperature

• Target: from -63ºC → 0–30ºC.

• Methods:

  • Introduce greenhouse gases (CO₂, CH₄).

  • Use methanogenic microbes to release gases.

  • Orbital mirrors (Mylar disks ~155 miles wide) to focus sunlight.

  • Factories to generate greenhouse gases (CFCs, CH₄, CO₂).

2. Oxygen

• Mars' atmosphere: 95% CO₂.

• Must introduce photosynthetic organisms (e.g. cyanobacteria, algae).

  • Cyanobacteria have been grown using Martian soil simulants.

• Potential for genetically engineered plants to survive Martian soil (rich in iron).

3. Atmospheric Pressure

• Current: 8 millibars (~1% of Earth's sea level).

• Human lungs need ~250 millibars → would burst in Mars' pressure.

• Can increase pressure by:

  • Thickening atmosphere (via greenhouse effect).

  • Need nitrogen for Earth-like atmosphere.

• Alternatively: Build pressurized glass domes for life support.

Terraforming Timeline

• Phase 1: Warming & CO₂ atmosphere — relatively fast (~100 years).

• Phase 2: Oxygenation for human breathing — much slower (100,000+ years), unless tech breakthroughs occur.

Final Thought: