Mass Extinctions

Mass extinctions are catastrophic global events in Earth’s history during which a large proportion of species across various ecosystems disappear in a relatively short geological period. These episodes have dramatically altered the course of evolution, reshaping biodiversity and paving the way for new species to emerge. Scientists identify five major mass extinctions in the fossil record, each associated with significant climatic, geological, or extraterrestrial changes.

Definition and Concept

A mass extinction is defined as an event in which at least 75% of all species on Earth become extinct within a geologically brief timeframe, typically less than two million years. Unlike background extinctions—which occur continuously at a slow, natural rate—mass extinctions are abrupt, global, and often caused by a combination of environmental and biological factors.
These events disrupt ecological balance, leading to the collapse of food chains, loss of habitats, and transformation of entire ecosystems. Following each mass extinction, the recovery of biodiversity can take millions of years, during which surviving species diversify and occupy newly available ecological niches.

Causes of Mass Extinctions

The causes of mass extinctions are complex and multifaceted. Scientists identify several major drivers:

• Climate Change: Rapid shifts in global temperature, whether due to greenhouse gas accumulation or glaciation, can alter habitats beyond the capacity of species to adapt.
• Volcanic Activity: Large-scale volcanic eruptions release vast amounts of carbon dioxide, sulphur dioxide, and particulates, leading to global warming or cooling, acid rain, and ocean acidification.
• Asteroid and Comet Impacts: Extraterrestrial collisions can cause shockwaves, tsunamis, wildfires, and dust clouds that block sunlight, triggering rapid climatic disruption.
• Ocean Anoxia: Depletion of oxygen in oceans due to warming or nutrient influx can lead to mass marine die-offs.
• Sea-Level Fluctuations: Changes in sea level affect shallow marine environments, which host much of Earth’s biodiversity.
• Biological Factors: Competition, disease, and changes in plant or animal populations can also contribute to widespread extinction.

The Five Major Mass Extinctions

Scientists classify Earth’s five major mass extinction events as follows:

1. Ordovician–Silurian Extinction (around 443 million years ago)

This was the first major extinction event, eliminating about 85% of marine species. The cause is believed to be a short, intense ice age triggered by the movement of Gondwana over the South Pole, leading to global cooling, falling sea levels, and habitat loss in shallow seas. As marine ecosystems dominated the Earth at the time, this event primarily affected trilobites, brachiopods, and reef-building organisms.

2. Late Devonian Extinction (around 372–359 million years ago)

This extinction unfolded over millions of years rather than a single catastrophic event. It eliminated around 75% of species, particularly affecting marine life such as corals and jawless fishes. Probable causes include extensive volcanic activity, global cooling, oceanic anoxia, and nutrient run-off from land plants leading to eutrophication. The event drastically altered marine ecosystems and paved the way for modern fish species to diversify.

3. Permian–Triassic Extinction (around 252 million years ago)

Known as the “Great Dying,” this was the most severe extinction in Earth’s history, eradicating about 96% of marine species and 70% of terrestrial species. Massive volcanic eruptions in the Siberian Traps released vast quantities of greenhouse gases, leading to global warming, ocean acidification, and anoxia. The event destroyed entire ecosystems and reset life on Earth, allowing reptiles and early dinosaurs to dominate the subsequent Triassic period.

4. Triassic–Jurassic Extinction (around 201 million years ago)

This event eliminated about 80% of species, including many amphibians and large reptiles, but left room for the dinosaurs to become the dominant terrestrial animals. The causes are linked to extensive volcanic activity in the Central Atlantic Magmatic Province (CAMP), resulting in rapid climate change, increased carbon dioxide levels, and habitat disruption.

5. Cretaceous–Palaeogene (K–Pg) Extinction (around 66 million years ago)

One of the best-known mass extinctions, it marked the end of the dinosaurs (except for birds). Approximately 75% of all species vanished, including ammonites and many marine reptiles. The most widely accepted cause is the impact of a 10-kilometre-wide asteroid at Chicxulub, Mexico, which generated global fires, blocked sunlight, and caused a “nuclear winter” effect. Secondary effects from volcanic activity in the Deccan Traps may have worsened the crisis. This event led to the rise of mammals and ultimately, humans.

Other Significant Extinctions

In addition to these five, several smaller but significant extinction events have occurred throughout Earth’s history, such as the End-Capitanian event (260 million years ago) and the Eocene–Oligocene extinction (34 million years ago), which also reshaped global ecosystems.

Consequences of Mass Extinctions

Mass extinctions profoundly affect the evolution of life on Earth. While they cause immense destruction, they also act as evolutionary catalysts, enabling the rise of new species and life forms adapted to changed environments. Key consequences include:

• Loss of Biodiversity: Sharp decline in genetic diversity and ecosystem complexity.
• Ecological Vacancies: Elimination of dominant groups allows new species to evolve and occupy empty niches.
• Evolutionary Innovation: Post-extinction periods often witness bursts of adaptive radiation, as seen with mammals after the K–Pg event.
• Geological and Climatic Reset: Changes in atmospheric composition and ocean chemistry often follow large-scale die-offs.

The Sixth Mass Extinction: The Anthropocene Crisis

Scientists have warned that Earth is currently undergoing a sixth mass extinction, driven largely by human activity. This ongoing event, sometimes referred to as the Anthropocene extinction, is characterised by the rapid decline of species due to:

• Habitat destruction through deforestation, urbanisation, and agriculture.
• Climate change caused by greenhouse gas emissions.
• Pollution of land, air, and water ecosystems.
• Overexploitation through hunting, fishing, and resource extraction.
• Invasive species displacing native organisms.