Throughout its 4.5 billion year history, planet Earth has experienced several mass extinction events that have wiped out a significant proportion of life. These devastating events were caused by catastrophic changes in the environment, such as massive volcanic eruptions, asteroid impacts, climate shifts and more. Understanding the history of mass extinctions on Earth can provide insights into the past evolution of life and what may cause widespread extinctions in the future.
The Big Five Mass Extinctions
There have been five major mass extinction events identified by scientists in Earth’s history. These five events stand out as the most catastrophic, resulting in the extinction of over 50% of species on the planet. They are sometimes referred to as “The Big Five”:
- Ordovician-Silurian extinction – 439 million years ago, 86% of species extinct
- Late Devonian extinction – 375-359 million years ago, 75% species extinct
- Permian-Triassic extinction “The Great Dying” – 252 million years ago, 96% species extinct
- Triassic-Jurassic extinction – 201 million years ago, 80% of species extinct
- Cretaceous-Paleogene extinction – 66 million years ago, 76% of species extinct
The Permian-Triassic extinction event 252 million years ago was the most severe, with up to 96% of all species on Earth wiped out. This event is sometimes called “The Great Dying” and marked the end of the Permian period. The extinction of the dinosaurs 66 million years ago at the end of the Cretaceous period is also very well known.
Causes of Mass Extinctions
There are several leading theories as to the causes of these major extinction events:
- Asteroid or comet impacts – The impact of a large asteroid or comet could cause widespread destruction and disrupt the climate, leading to mass extinctions. There is strong evidence that an asteroid impact caused the Cretaceous-Paleogene (K-Pg) extinction 66 million years ago that killed off the dinosaurs.
- Large igneous province volcanic eruptions – The eruption of millions of cubic kilometers of lava over thousands of years can release vast amounts of gases and particulates that lead to climate change. The Permian-Triassic extinction is linked to massive volcanic eruptions in Siberia.
- Climate change – Rapid shifts in global climate such as warming or cooling can disrupt ecosystems. Some scientists propose climate change as a contributing factor in many extinctions.
- Sea level fluctuations – Rising or falling sea levels can connect or isolate landmasses, affecting species migration and distribution. Sea level changes have been proposed as a factor in some extinctions.
- Anoxia – When oceans lose oxygen, marine species die off en masse. Ocean anoxia has been linked to the Permian-Triassic and other extinctions.
In most cases, there were likely multiple contributing factors to these mass extinction events rather than just a single cause. The immense scale of the extinctions suggests a perfect storm of several catastrophic global changes converging at once.
Other Major Extinctions in Earth’s History
In addition to the Big Five worst extinctions, there have been many other lesser but still significant extinction events throughout Earth’s long history, including:
- End-Ordovician extinction – 450-440 million years ago, 86% of species lost
- Late Devonian extinction – 385-360 million years ago, 75% species lost
- End-Guadalupian extinction – 260-250 million years ago, up to 87% species lost
- End-Capitanian extinction – 262 million years ago, up to 70% of species lost
- Toarcian extinction – 183 million years ago, up to 76% of species extinct
- Aptian extinction – 125-113 million years ago, loss of many species
These ancient extinction events helped shape the rise and fall of different groups of organisms throughout the eons. While not considered among the “Big Five” largest mass extinctions, they had substantial impacts on biodiversity at the time.
Extinction Rates and Patterns Through Time
Scientists have found that extinction rates and the loss of biodiversity have varied significantly throughout Earth’s history. Here is a look at extinction intensities through different geological eras:
| Era | Extinction Rate | Description |
|---|---|---|
| Cambrian | Moderate | Rise of early complex life, fairly high extinctions |
| Ordovician | Intense | Two major extinction events, 86% species lost |
| Silurian | Low | Lower extinction rates, recovery from mass extinctions |
| Devonian | Intense | Two major extinctions, 75% species lost |
| Carboniferous | Low | Low extinction rates, expansion of forests |
| Permian | Catastrophic | “The Great Dying” – up to 96% species extinct |
| Triassic | High | Recovery from Permian extinction, fairly high rates |
| Jurassic | Low | Low extinction rates, dinosaurs dominant |
| Cretaceous | Moderate | Moderate extinctions, Cretaceous-Paleogene event at end |
| Paleogene | Moderate | Recovery after K-Pg extinction, moderate extinctions |
| Neogene | Mild Increase | Slight uptick in extinction rates |
This overview shows that extinction intensities have waxed and waned throughout Earth’s history, with periods of low extinction punctuated by major catastrophic extinction events. Even during times of low extinction, species were still going extinct at a low, “background” rate through natural selection and changes in the environment and climate. The current era starting in the Holocene has seen extinction rates begin to rise over this background level due to human activities altering environments and habitats.
The Current Extinction Event
Many scientists argue that we have now entered a sixth mass extinction event due to the activities of modern human civilization. Sometimes called the Anthropocene or Holocene extinction, this event is marked by:
- A rapid rise in extinctions since the Industrial Revolution and surge in human population growth
- Habitat destruction, overhunting, overfishing, invasive species, and other human impacts
- Extinction rates estimated to be 100 to 1000 times higher than the natural background rate
- Potential loss of 75% of species diversity over next centuries if extinctions continue
While definitions vary, most estimates suggest that current extinction rates are higher than any known previous mass extinction event, with dozens of species going extinct every day. However, the full extinction magnitude cannot be quantified until well into the future when the final scale is known. There is still intense debate around classifying the current biodiversity crisis as a true “mass extinction event” comparable to the Big Five of the past. Regardless, the rapid loss of species poses a catastrophic threat to global ecosystems.
Which Species Are Going Extinct Today?
The species most threatened in the modern extinction crisis are those with populations already vulnerable and facing pressure from human activities such as:
- Large mammals – elephants, rhinos, big cats, bears, great apes
- Birds – especially endemic island birds vulnerable to invasive predators
- Amphibians – 32% of amphibians threatened with extinction
- Reptiles – turtles, crocodiles, lizards threatened by habitat loss
- Sharks and rays – 37% of sharks and rays endangered from overfishing
- Coral species – coral reef ecosystems threatened by warming oceans
- Plants – high extinction risk for many plant groups
- Insects – 40% of insect species declining in population
- Trees – increased logging threatening many forest and tree species
These examples highlight that the modern mass extinction threat impacts species across many types of ecosystems. Our actions have created a biodiversity crisis affecting a broad spectrum of plant and animal life.
Looking to the Future
The current and future course of the Anthropocene extinction event depends heavily on how human civilization progresses over the coming decades and centuries. Some key questions looking ahead are:
- Will habitat destruction and degradation continue at current rates or accelerate? Rainforests, wetlands and other high biodiversity ecosystems face extreme pressures.
- Can overhunting and overfishing be curtailed through regulation, education and conservation? Billions of animals are harvested unsustainably each year.
- Will climate change continue to disrupt ecosystems through warming, acidification and extreme weather? Average global temperatures are projected to rise substantially.
- Can invasive species spread be contained as global trade and transportation increases? Invasives are a leading threat to island ecosystems.
- Will toxins, plastic waste and other pollution threatening wildlife decline or increase over time?
- Can the illegal wildlife trade driving rare species declines be stopped?
Efforts to promote biodiversity conservation and sustainable development could help reduce pressures driving the modern mass extinction crisis. However, the coming decades will prove critical in determining just how severe this event will be compared to Earth’s past extinction episodes.
Conclusion
Earth has experienced five major mass extinction events over its long history, where over 50% of species disappeared in a geological blink of an eye. These past catastrophes were caused by asteroid impacts, volcanic eruptions, climate shifts and other upheavals on a global scale. Currently, the planet is in the midst of a sixth extinction crisis caused by human activities and destruction of natural habitats and ecosystems. While definitions vary, today’s extinction rates are clearly exceptionally high compared to the natural background rate, resulting in possibly 75% species loss over the next centuries. How severe this modern mass extinction becomes depends on steps taken to change humanity’s relationship with nature in the critical period ahead.