Mass Extinctions

• Extinction is the permanent loss of all members of a species.
• Over the last billion years, many plant and animal species have evolved and then become extinct at a roughly uniform pace.
• Only a small fraction of the estimated four billion species that have ever lived are still present today; most have disappeared.
• Well‑known extinct organisms include dinosaurs, sabre‑toothed tigers and dodo.

Mass extinction is an event in which a large number of species become extinct over a relatively short span of geological time due to extreme environmental stress or drastic changes.

Key Features:

• Involves many species across different groups, not just one or two.
• Occurs over a short geological period compared to normal, slow background extinction.
• Environmental stress becomes so extreme that even well‑adapted species are wiped out because their existing adaptations no longer help them survive.

At least five major mass extinction events are commonly recognised in Earth’s history.

Historical Sequence:

1. Life diversified across geological time.
2. Sudden environmental crises occurred.
3. Many species disappeared in short spans of geological time.
4. Surviving groups adapted and expanded.

Natural Causes

• Asteroid or meteorite impact.
• Large-scale climate change or global cooling.
• Rapid environmental disturbance affecting habitats and food chains.

Human-Linked Causes in Present Times

• Habitat destruction.
• Biodiversity loss caused by human activity.
• Disturbance of ecological balance leads to faster extinction rates.

One major extinction event led to the disappearance of dinosaurs around 60 million years ago, according to the study material on the page. A widely discussed explanation is the impact of a comet or meteorite, supported by the presence of an iridium-rich layer in geological records.

Fossils help scientists understand species that no longer exist and provide clues about past biodiversity and extinction.

Important Fossil Points

• Some organisms are preserved in amber.
• Some remains may be found in asphalt or oil-soaked ground.
• Fossil patterns across geological eras help trace the rise and disappearance of groups.
• Archaeopteryx is an important fossil in evolutionary study.

• Extinction is the permanent loss of a species, whereas mass extinction is the rapid, global disappearance of multiple species due to extreme environmental stress.
• These events occur over very short geological timeframes, making existing survival adaptations completely useless.
• Earth has experienced at least five major mass extinctions, including the famous Cretaceous event that eliminated the dinosaurs.
• Historical triggers include asteroid impacts and severe climate shifts, while modern extinction threats are primarily driven by human activities like habitat destruction.
• Fossils – whether preserved in rocks, amber, or asphalt – provide the key physical evidence for tracking the rise and fall of past species.