10-12-2025 Mains Question Answer
What is mass extinction? Explain previous mass extinctions?
When a species goes extinct, its role in the ecosystem is usually filled by new species, or other existing ones. Earth’s ‘normal‘ extinction rate is often thought to be somewhere between 0.1 and 1 species per 10,000 species per 100 years. This is known as the background rate of extinction.
A mass extinction event is when species vanish much faster than they are replaced. This is usually defined as about 75% of the world’s species being lost in a short period of geological time – less than 2.8 million years.
It is characterized by:
- A significant loss of species across different taxonomic groups.
- Occurring over a relatively short geological time frame.
- Resulting in a drastic reduction in the abundance and diversity of life.
Previous mass extinctions (The “Big Five”)
Throughout Earth’s history, there have been five major mass extinction events, often referred to as the “Big Five.” Each event had unique causes and profound impacts on life.
| Extinction Event | Approximate Time Period | Key Characteristics and Causes | Impact |
| 1. Ordovician- Silurian Extinction | ~443 million years ago (Ma) | ● Second largest in terms of percentage of life lost. ● Associated with a period of intense glaciation and subsequent global warming. ● Sea levels dropped significantly, destroying coastal habitats. | ● ~85% of marine species perished. ● Affected brachiopods, trilobites, bryozoans, and graptolites. |
| 2. Late Devonian Extinction (Frasnian-Famennian) | ~372 Ma | ● A prolonged series of extinction pulses over several million years. ● Possible causes: global cooling, oceanic anoxia (lack of oxygen), asteroid impacts, massive volcanic eruptions. | ● ~75% of all species, primarily marine life, went extinct. ● Coral reefs were severely impacted. |
| 3. Permian- Triassic Extinction (The Great Dying) | ~252 Ma | ● Earth’s most severe known extinction event. ● Caused by massive volcanic activity (Siberian Traps) leading to extreme global warming, ocean acidification, and widespread anoxia. | ● ~96% of all marine species and 70% of terrestrial vertebrate species died out. ● Paved the way for the rise of dinosaurs. |
| 4. Triassic- Jurassic Extinction | ~201 Ma | ● Likely caused by massive volcanic eruptions (Central Atlantic Magmatic Province – CAMP) releasing CO2 and sulfur dioxide. ● Led to global warming, ocean acidification, and sea-level changes. | ● ~70-75% of all species, including many large amphibians and reptiles, disappeared. ● Allowed dinosaurs to become the dominant terrestrial vertebrates. |
| 5. Cretaceous- Paleogene Extinction (K-Pg Extinction) | ~66 Ma | ● Most famous for the extinction of non-avian dinosaurs. ● Primary cause: a large asteroid impact (Chicxulub impactor) in present-day Mexico. ● Secondary causes: massive volcanic activity (Deccan Traps) and climate change. | ● ~75% of all plant and animal species went extinct. ● Opened ecological niches for the diversification of mammals and birds. |
Understanding these past events helps us contextualize the current biodiversity crisis, often referred to as the “Sixth Mass Extinction.”