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Mass Extinction: Causes & Theories

Instructor: Artem Cheprasov
This lesson is going to go over the largest mass extinctions to have ever occurred. You'll learn what they're called, when they happened, how many animals died, and what may have caused them.

Mass Extinctions

What percentage of organisms that have ever lived on Earth are extinct?

A. More than 90%

B. About 50%-75%

C. About 25%-50%

D. Less than 10%

If you guess more than 90 percent, then you are correct! In fact, there have been five mass-extinction events, events where more than 50% of all the species on Earth died off in a relatively short space of time (geologically speaking).

Let's go over some of the proposed theories and causes of these mass extinctions.

The First Three Mass Extinctions

Around 443 million years ago, something known as Ordovician-Silurian mass extinction occurred. Right before this happened, most of the life on Earth was in the sea. It is believed that an ice age was the cause of the mass extinction. The ice age led to a fall in sea levels and an alteration in the chemistry of the seas as well, which led to the loss of about 85% of the life at sea.

There was also the Late Devonian mass extinction. This occurred about 359 million years ago. It led to the extinction of 75% of all the world's species (mainly sea-life). It appears a wide variety of factors may have led to this mass extinction, including sea level changes and even asteroid impacts.

One of the most famous mass extinctions was the Permian mass extinction, which occurred roughly 248 million years ago. An amazing 90-96% of the world's species died, and that's why this mass extinction has been called The Great Dying as a result. There have been numerous proposed theories as to the cause of this mass extinction, including asteroid impacts, sea level fluctuations, a release of a humongous amount of methane gas, volcanic activity, or drops in oxygen levels on Earth. Some of these may be connected, such as an asteroid impact triggering volcanic activity.

The Next 2-3 Mass Extinctions

The Triassic-Jurassic mass extinction occurred about 200 million years ago. Everything from asteroids to climate change to volcanic activity has been proposed as the cause of this mass extinction. Consequently, about 50% of all the species of animals on Earth were killed off.

An asteroid may have killed (or finished off) the dinosaurs.
An asteroid may have killed (or finished off) the dinosaurs.

Perhaps the most famous mass extinction event was the Cretaceous-Tertiary mass extinction. This was the one that killed off the dinosaurs, and it occurred about 65 million years ago. While a huge asteroid impact is known to have been a very large contributor to this mass extinction, many scientists believe it was only part of a much greater picture. Things like falls in sea levels, global warming, and volcanic eruptions have been theorized as possible causes as well. About half of all the world's species died off during this mass extinction event.

And, we'll end on a somber note. It appears that human activity is driving the sixth mass extinction event. Scientists estimate that human activity, such as pollution and overfishing (among others), may lead to the extinction of over 50% of the world's species in the near future.

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