About 3.8 billion years ago, life formed in the oceans as simple soft-bodied microorganisms. There were no other types of life forms on the planet at this time. Life only existed in the oceans. For more than 3 billion years, microorganisms ruled the planet. Then, about 670 million years ago, a mass extinction killed nearly all life. This mass extinction is not very well known. Scientists hypothesize that a change in the ocean level could have affected the habitat of the microorganisms in numerous ways.
After this first mass extinction, millions of years passed. It was as though life was gathering its breath. Then, about 570 million years ago, it seemed as if a gigantic water balloon suddenly burst open. Life exploded across the world. Animals developed hard parts, like shells and skeletons. The first vertebrates appeared at this time. They were the earliest ancestors of all the major groups of animals, including the human animal. Scientists call this the Cambrian Explosion of Life.
Like a gigantic wheel, this cycle of life and extinction continues throughout the history of the Earth. Let us briefly examine the five biggest mass extinctions.
Ordovician Mass Extinction—440 million years ago
The Ordovician mass extinction wiped out about 50% of some groups of marine animals. Some scientists think the most likely cause was an ice age. The Earth has experienced many ice ages over its history. An ice age is when most of the water on Earth freezes into thick sheets of ice. This would have destroyed marine habitats.
Devonian Mass Extinction—365 million years ago
The Devonian Period was known as the Age of the Fishes. The first sharks appeared, as did many kinds of primitive fishes. The Devonian mass extinction wiped out about 70% of tropical animals living in the ocean. Plants and animals on land were less affected. This mass extinction may have been caused by a global climate change, such as an ice age. This would have cooled the warm tropical waters, killing most of the animals that lived there.
The Great Dying: Permian Mass Extinction— 250 million years ago
Life flourished for more than 100 million years after the Devonian mass extinction. Reptiles appeared. These were the early ancestors of the dinosaurs, yet they were not the strongest creatures on the planet. During this time, mammal-like reptiles called gorgons were the most powerful reptiles on the planet. These ferocious creatures looked half-lion and half-dragon.
Then, the largest extinction in Earth’s history took place. It happened at the end of the Permian Period around 250 million years ago and lasted millions of years. It was far more devastating than the Cretaceous mass extinction that wiped out the dinosaurs. It is estimated that as much as 96% of all marine species were lost during the Permian mass extinction. On land, more than 75% of all animals died out.
Not all land animals became extinct at this time. This is lucky for human beings. The mammal-like reptiles did not die off completely during this mass extinction. Scientists have theorized that mammals (including Homo sapiens) eventually evolved from these animals.
Some scientists believe an asteroid hit the planet and caused what is often called the Great Dying. The most recent evidence suggests that a huge volcanic explosion in Siberia may have caused massive climate change, including extreme temperatures and lack of oxygen. We will look at some of these theories in greater detail later.
Triassic Mass Extinction—208 million years ago
The Triassic mass extinction took place about 208 million years ago. About 35% of life, including the mammal-like reptiles, died at this time. Like mammals, dinosaurs evolved from the mammal- like reptiles. The Age of the Dinosaurs began in the Triassic Period, as dinosaurs began to take over the planet. True mammals appeared near the end of the Triassic. They were only as big as a shrew, with a skull several inches long. Scientists think a combination of extremely hot temperatures and a lack of oxygen were responsible for this mass extinction, similar to the causes of the Permian mass extinction.
Cretaceous (K-T) Mass Extinction—65 million years ago
Mass extinction is known by the boundary between one geologic time period and the next. The Cretaceous mass extinction, better known as the K-T mass extinction, happened at the boundary between the Cretaceous Period and the Tertiary Period. The K stands for Kreide, which is the German word for chalk. It describes the chalky texture of the clay found in sedimentary rocks from that time. This clay layer is also known as the K-T boundary.
The Cretaceous mass extinction marked the end of the Age of the Dinosaurs. But the dinosaurs were not the only victims. Around 75% of all species were destroyed. All land animals over 55 pounds (25 kilograms) became extinct.
Some groups of animals escaped this mass extinction. There are many theories to explain why. Crocodiles, turtles, lizards, mammals, and birds were affected. However, they survived with most of their species intact. Many plants either died out or suffered heavy losses. However, the roots of many plants managed to survive and eventually grew again.
Mammals were no larger than a cat at this time. Most mammals lived underground, as if waiting for their turn to dominate the planet. They waited about 140 million years! After the dinosaurs became extinct, the Age of Mammals began. It still took nearly 60 million more years for the first humanlike ancestor to walk the Earth.
The extinction of the dinosaurs has been studied extensively. For the past 25 years, most scientists have thought that a very large asteroid hit the planet. Other scientists think that a massive volcanic eruption in India caused this extinction.
Are extraterrestrial asteroids responsible for the mass extinction of the dinosaurs? Could asteroids have caused any other mass extinction?