4.5 Major Events in Earth's History

Mass Extinction Events and Causes

Earth's history includes five major mass extinctions, each of which wiped out a huge percentage of life on the planet. These events reset the course of evolution, clearing the way for new groups of organisms to rise. Understanding what caused them helps explain why life looks the way it does today.

The Big Five Mass Extinctions

Ordovician-Silurian Extinction (about 440 million years ago) Global cooling drove sea levels down, destroying shallow marine habitats where most life existed at the time. Roughly 85% of marine species went extinct.

Late Devonian Extinction (about 375 million years ago) This one played out over millions of years rather than in a single pulse. Ocean anoxia (widespread loss of dissolved oxygen in seawater) and the Kellwasser Event likely drove the crisis. About 75% of species were lost.

Permian-Triassic Extinction (about 252 million years ago) Known as "The Great Dying," this is the worst mass extinction in Earth's history. An estimated 96% of marine species and 70% of terrestrial vertebrate species went extinct. The leading cause was massive volcanism from the Siberian Traps, which triggered ocean acidification, plummeting oxygen levels, and extreme global warming.

Triassic-Jurassic Extinction (about 201 million years ago) Volcanism from the Central Atlantic Magmatic Province released enormous amounts of carbon dioxide, causing rapid climate change and ocean acidification. About 80% of species disappeared, clearing ecological space for dinosaurs to dominate.

Cretaceous-Paleogene Extinction (about 66 million years ago) The most famous extinction event. A massive asteroid struck what is now Mexico's Yucatán Peninsula, forming the Chicxulub crater. The Deccan Traps volcanism in India was already stressing ecosystems. Together, these events wiped out roughly 76% of species, including all non-avian dinosaurs like Tyrannosaurus rex and Triceratops.

Common Causes Across Extinctions

A few recurring triggers show up across these events:

• Volcanism releases greenhouse gases and toxic compounds, causing rapid climate shifts and ocean acidification (Permian-Triassic, Triassic-Jurassic, Cretaceous-Paleogene)
• Global cooling and sea-level drops destroy shallow marine habitats (Ordovician-Silurian)
• Ocean anoxia suffocates marine life when oxygen levels in seawater plummet (Late Devonian, Permian-Triassic)
• Asteroid impacts throw dust and debris into the atmosphere, blocking sunlight and collapsing food chains (Cretaceous-Paleogene)

Most mass extinctions involve multiple causes acting together, not just a single trigger.

Oxygenation of Earth's Atmosphere

For roughly the first two billion years of Earth's history, the atmosphere contained almost no free oxygen. The rise of oxygen transformed the planet's chemistry, its climate, and the kinds of life that could exist.

The Great Oxygenation Event (GOE)

Around 2.4 to 2.1 billion years ago, cyanobacteria (photosynthetic microorganisms in the oceans) began producing oxygen as a byproduct of photosynthesis. Over hundreds of millions of years, oxygen accumulated in the atmosphere.

This had several major consequences:

• Ozone layer formation. Atmospheric oxygen reacted to form ozone ($O_3$), which shields Earth's surface from harmful ultraviolet radiation. Without this shield, life on land would have been impossible.
• Aerobic respiration became possible. Oxygen-based metabolism is far more efficient at extracting energy from food than anaerobic (oxygen-free) metabolism. This energy boost was a prerequisite for larger, more complex organisms.
• Mass die-off of anaerobic life. Oxygen is toxic to many anaerobic organisms. The GOE caused a massive extinction of these early life forms, but it opened ecological space for oxygen-tolerant species to diversify.

The Neoproterozoic Oxygenation Event (NOE)

A second major rise in oxygen occurred between about 800 and 540 million years ago. This pushed atmospheric oxygen to levels high enough to support large, active animals.

The NOE set the stage for the Cambrian Explosion (starting around 541 million years ago), when most major animal groups (arthropods, chordates, mollusks) appeared in the fossil record over a geologically short span of time. Without sufficient oxygen, the energy demands of these complex body plans couldn't have been met.

Evidence for Climate Change

Earth's climate has swung between extremes over its 4.6-billion-year history, from planet-wide glaciations to hothouse conditions with no polar ice at all. Scientists reconstruct these past climates using physical and chemical records preserved in natural materials.

Paleoclimatic Proxies

A paleoclimatic proxy is any preserved natural record that indirectly tells us about past climate conditions. The most important ones include:

• Ice cores drilled from glaciers in Greenland and Antarctica trap tiny bubbles of ancient atmosphere. Oxygen isotope ratios ($\frac{^{18}O}{^{16}O}$) in the ice indicate past global temperatures: higher ratios of the heavier isotope generally correspond to colder conditions.
• Tree rings record annual growth patterns. Wider rings typically indicate warmer, wetter years; narrower rings suggest drought or cold.
• Marine sediments accumulate on the ocean floor over millions of years. They contain fossils of tiny organisms called foraminifera, whose shell chemistry records ocean temperature and carbon dioxide levels.

Major Climate Events in Earth's History

Huronian Glaciation (2.4 to 2.1 billion years ago) One of the earliest known ice ages. It may have been triggered by the Great Oxygenation Event, which removed methane (a potent greenhouse gas) from the atmosphere, weakening the greenhouse effect and plunging temperatures.

Cryogenian Period (720 to 635 million years ago) Two severe glaciations occurred during this period: the Sturtian and the Marinoan. The "Snowball Earth" hypothesis proposes that ice sheets may have extended all the way to the equator, covering nearly the entire planet. Volcanic carbon dioxide emissions eventually built up enough to trigger a greenhouse warming that melted the ice.

Paleocene-Eocene Thermal Maximum (PETM, about 56 million years ago) A rapid release of carbon dioxide into the atmosphere caused global temperatures to spike by 5–8°C over a few thousand years. Oceans acidified, and both marine and terrestrial ecosystems shifted dramatically. The PETM is often studied as an analog for modern carbon dioxide-driven warming.

Pleistocene Glaciation (2.6 million to 11,700 years ago) Repeated glacial-interglacial cycles defined this period. Ice sheets advanced and retreated across North America and Europe, driven by cyclical changes in Earth's orbit (called Milankovitch cycles) combined with shifts in atmospheric greenhouse gas concentrations.

Consequences of Climate Change

• Species either adapt to new conditions, migrate to more suitable habitats, or go extinct.
• Ecosystem structure shifts as species compositions change and food webs reorganize.
• For human societies, climate change affects agricultural productivity, freshwater availability, and sea levels.

Supercontinent Formation and Breakup

Over billions of years, Earth's continents have repeatedly drifted together into giant landmasses called supercontinents, then rifted apart again. This cycle profoundly affects climate, ocean circulation, and the evolution of life.

The Supercontinent Cycle and the Wilson Cycle

Plate tectonics drives continents together through continental collisions and pulls them apart through rifting. The Wilson Cycle describes this process: ocean basins open as continents rift apart, then close as plates converge and continents collide.

The cycle affects global climate in two key ways:

• During formation: Large landmasses increase continental weathering and erosion. Chemical weathering pulls $CO_2$ out of the atmosphere, which can lower global temperatures and potentially trigger glaciations.
• During breakup: Rifting produces increased volcanic activity along new plate boundaries, releasing greenhouse gases that warm the planet.

Influence on Earth's Systems

Supercontinents reshape how the planet works:

• Ocean circulation changes depending on how continents are arranged. A single giant landmass blocks or redirects ocean currents, altering how heat and nutrients are distributed. This affects marine productivity and biodiversity.
• Climate shifts as weathering rates and volcanic activity change atmospheric composition over millions of years.
• Life evolves differently depending on whether continents are joined or separated. Connected landmasses let species migrate freely, while breakup isolates populations on separate continents. This isolation drives allopatric speciation, where separated populations evolve into distinct species over time.

Examples of Supercontinents