When a species disappears, biologists say that the species has become extinct. By making room for new species, extinction helps drive the evolution of life. Over long periods of time, the number of species becoming extinct can remain fairly constant, meaning that an average number of species go extinct each year, century, or millennium. However, during the history of life on Earth, there have been periods of mass extinction, when large percentages of the planet’s species became extinct in a relatively short amount of time. These extinctions have had widely different causes. 

About 541 million years ago, a great expansion occurred in the diversity of multicellular organisms. Paleobiologists, scientists who study the fossils of plants and animals to learn how life evolved, call this event the Cambrian Explosion. Since the Cambrian Explosion, there have been five mass extinctions, each of which is named for the geological period in which it occurred, or for the periods that immediately preceded and followed it.

The first mass extinction is called the Ordovician-Silurian Extinction. It occurred about 440 million years ago, at the end of the period that paleontologists and geologists call the Ordovician, and followed by the start of the Silurian period. In this extinction event, many small organisms of the sea became extinct. The next mass extinction is called Devonian extinction, occurring 365 million years ago during the Devonian period. This extinction also saw the end of numerous sea organisms.

The largest extinction took place around 250 million years ago. Known as the Permian-Triassic extinction, or the Great Dying, this event saw the end of more than 90 percent of the Earth’s species. Although life on Earth was nearly wiped out, the Great Dying made room for new organisms, including the first dinosaurs. About 210 million years ago, between the Triassic and Jurassic periods, came another mass extinction. By eliminating many large animals, this extinction event cleared the way for dinosaurs to flourish. Finally, about 65.5 million years ago, at the end of the Cretaceous period came the fifth mass extinction. This is the famous extinction event that brought the age of the dinosaurs to an end.

In each of these cases, the mass extinction created niches or openings in the Earth’s ecosystems. Those niches allowed for new groups of organisms to thrive and diversify, which produced a range of new species. In the case of the Cretaceous extinction, the demise of the dinosaurs allowed mammals to thrive and grow larger.

Scientists refer to the current time as the Anthropocene period, meaning the period of humanity. They warn that, because of human activities such as pollution, overfishing, and the cutting down of forests, the Earth might be on the verge of—or already in—a sixth mass extinction. If that is true, what new life would rise up to fill the niche that we currently occupy?

 

Extinction

Many species have gone extinct throughout history and all that marks their presence on Earth are fossils, such as this one of a dinogorgon.

Anthropocene
Noun

period of time during which human activities have impacted the environment enough to constitute a distinct geological change.

biological
Adjective

having to do with the study of life and living organisms.

biologist
Noun

scientist who studies living organisms.

Cambrian Explosion of Life
Noun

rapid development of almost all major types (phyla) of organisms during the Cambrian time period.

Cretaceous period
Noun

145 million to 65 million years ago. The period ended with extinction of the dinosaurs and the rise of mammals.

Devonian
Adjective

geologic period between Silurian and Mississippian.

evolution
Noun

change in heritable traits of a population over time.

Noun

process of complete disappearance of a species from Earth.

Noun

remnant, impression, or trace of an ancient organism.

Jurassic
Adjective

having to do with the time period between 190 million and 140 million years ago, characterized by an abundance of dinosaurs and ammonites.

multicellular
Adjective

composed of more than one cell.

niche
Noun

role and space of a species within an ecosystem.

paleobiology
Noun

study of the biology of fossilized organisms.

Permian
Adjective

last geologic period of the Paleozoic Era.

Triassic
Adjective

start of the Mesozoic era when dinosaurs first emerged.