The 5 Major Mass Extinctions

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A History of Mass Extinctions

 



ROGER HARRIS/SCIENCE PHOTO LIBRARY / Getty Images

Throughout the 4.6 billion years of history the Earth has been around, there have been five known major mass extinctions that wiped out an overwhelming majority of all species living at that time. These five major mass extinction events include the Ordovician Mass Extinction, Devonian Mass Extinction, Permian Mass Extinction, Triassic-Jurassic Mass Extinction, and Cretaceous-Tertiary (or the K-T) Mass Extinction. All of these major mass extinction events varied in size and causes, but all of them completely devastated the biodiversity found on Earth at the times that they happened.

Defining Mass Extinctions

MARK GARLICK/SCIENCE PHOTO LIBRARY / Getty Images

Before diving into these different mass extinction events in depth, it is important to understand what can be classified as a mass extinction event and how mass extinction shape the evolution of species that happen to survive these extreme catastrophes. A “mass extinction” can be defined as a time period in which a large percentage of all known species living at the time goes extinct, or is completely wiped out. There are several causes for mass extinctions such as climate change, geologic catastrophes (such as large amounts of volcanic eruptions), or even meteor strikes on the Earth’s surface. There is even evidence to suggest that microbes may have sped up or contributed to some of the mass extinctions that are known throughout the Geologic Time Scale.

Mass Extinctions and Evolution

The tardigrade (water bear) has survived all 5 major mass extinction.
SEBASTIAN KAULITZKI/SCIENCE PHOTO LIBRARY / Getty Images

So how do mass extinction events contribute to evolution? Usually, after a very large mass extinction event, there is a very rapid period of speciation among the few species that do survive. Since so many species die off during these catastrophic events, there is so much more room for the surviving species to spread out and so many niches in the environments that need to be filled. As populations separate and move away, they adapt over time to the new environmental conditions and eventually are reproductively isolated from the original populations of the species. At that point, they can be considered a brand new species and biodiversity expands rather quickly. The rate of evolution is significantly increased because of all of the roles and spaces that need to be filled by individuals that managed to survive. There is less competition for food, resources, shelter, and even mates, allowing the “leftover” species from the mass extinction event to thrive and reproduce rapidly. More offspring and more generations tend to favor an increased rate of evolution.

The First Major Mass Extinction - The Ordovician Mass Extinction

TRILOBITE (ISOTELUS GIGAS). ORDOVICIAN, OH. H. Getty/Schafer & Hill

When: The Ordovician Period of the Paleozoic Era (about 440 million years ago)

Size of the Extinction: Up to 85% of all living species at the time were eliminated

Suspected Cause or Causes: Continental Drift and subsequent climate change

The mass extinction event that happened during the Ordovician Period of the Paleozoic Era on the Geologic Time Scale is the first known major mass extinction. At this time in the history of life on Earth, really, life was in its early stages. The first known life forms appeared about 3.6 billion years ago. By the Ordovician Period, however, larger aquatic life forms had come into existence. There were also even some land species at this time. The cause is thought to be due to the shift in the continents and the drastic climate change. It happened in two different waves. The first wave was an ice age that encompassed the entire Earth. Sea levels were lowered and many land species could not adapt fast enough to survive the harsh, cold climates. It was not all good news, however, when the ice age ended. It ended so suddenly that the ocean levels rose too fast to keep enough oxygen in them to keep the species that had survived the first wave alive. Again, species were too slow to adapt before extinction took them out completely. It was then up to the few aquatic autotrophs that had survived to increase the oxygen levels so new species could evolve.

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The Second Major Mass Extinction - The Devonian Mass Extinction

Doryaspis, an extinct genus of primitive jawless fish that lived in the ocean during the Devonian Period. Getty/Corey Ford/Stocktrek Images

When: The Devonian Period of the Paleozoic Era (about 375 million years ago)

Size of the Extinction: Nearly 80% of all living species at the time were wiped out

Suspected Cause or Causes: Lack of oxygen in the oceans, quick cooling of air temperatures, possibly volcanic eruptions and/or meteor strikes

The second major mass extinction in the history of life on Earth happened during the Devonian Period of the Paleozoic Era. This major mass extinction event actually followed the previous Ordovician Mass Extinction event relatively quickly. Just as life on Earth began to rebound and flourish as the climate stabilized and species adapted to the new environments, almost 80% of all living species, in both the water and on land, were wiped out.

There are several hypotheses as to why this second mass extinction occurred at that time in Geologic History. The first wave, which dealt a major blow to aquatic life, may have actually been caused by the quick colonization of land. Many aquatic plants adapted to live on land, leaving fewer autotrophs to create oxygen for all of the sea life. This led to a mass death in the oceans. The quick moving on to the land of plants also had a major effect on the carbon dioxide available in the atmosphere. By removing so much of the greenhouse gas relatively quickly, the temperatures plummeted. Land species had trouble adapting to these changes in climate and also went extinct. The second wave is more of a mystery. It could have included mass volcanic eruptions and some meteor strikes, but the exact cause of the second wave is still considered unknown.

The Third Major Mass Extinction - The Permian Mass Extinction

Dimetrodon skeleton from the Permian Period. Getty/Stephen J Krasemann

When: The Permian Period of the Paleozoic Era (about 250 million years ago)

Size of the Extinction: An Estimated 96% of all species living on Earth at the time

Suspected Cause or Causes: Unknown - Possibly asteroid strikes, volcanic activity, climate change, and microbes.

The third major mass extinction was during the last period of the Paleozoic Era called the Permian Period. This is the largest of all known mass extinctions with a whopping 96% of all species on Earth completely lost. It is no wonder this major mass extinction has been dubbed “The Great Dying”. It seems as if nothing was safe from this massive extinction event. Aquatic and terrestrial life forms alike perished relatively quickly as the event took place.

It is still pretty much a mystery as to what set off this greatest of the mass extinction events. Several hypotheses have been thrown around by scientists who study this time span of the Geologic Time Scale. Some believe it may be a chain of events that led to so many species disappearing. It could have been massive volcanic activity paired with asteroid impacts that sent deadly methane and basalt into the air and across the surface of the Earth. These could have caused a decrease in oxygen that suffocated life and brought about a very quick climate change. Newer research points to a microbe from the Archaea domain that flourishes when methane is high. These extremophiles may have “taken over” and choked out the life in the oceans, as well. Whatever the cause, this biggest of the major mass extinctions ended the Paleozoic Era and ushered in the Mesozoic Era.

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The Fourth Major Mass Extinction - The Triassic-Jurassic Mass Extinction

Pseudopalatus fossil from the Triassic Period. National Parks Service

When: At the end of the Triassic Period of the Mesozoic Era (about 200 million years ago)

Size of the Extinction: More than half of all known species living at the time

Suspected Cause or Causes: Major volcanic activity with basalt flooding, global climate change, and changing pH and sea levels of the oceans.

The fourth major mass extinction event was actually a combination of many, smaller extinction events that happened over the last 18 million years of the Triassic Period during the Mesozoic Era. Over this long time span, about half of all known species on Earth at that time perished. The causes of these individual small extinctions can be attributed to volcanic activity with basalt flooding for the most part. The gases spewed into the atmosphere from the volcanoes also created climate change issues that changed sea levels and possibly even pH levels in the oceans.

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The Fifth Major Mass Extinction - The K-T Mass Extinction

Extinction of the dinosaurs, artwork. Getty/KARSTEN SCHNEIDER

When: At the end of the Cretaceous Period of the Mesozoic Era (about 65 million years ago)

Size of the Extinction: Nearly 75% of all known species living at the time

Suspected Cause or Causes: Extreme asteroid or meteor impact

The fourth major mass extinction is perhaps the most well-known mass extinction event. The Cretaceous-Tertiary Mass Extinction (or K-T Extinction) became the dividing line between the final period of the Mesozoic Era, the Cretaceous Period, and the Tertiary Period of the Cenozoic Era. This one, even though it is not the biggest, is the most well known because it is the mass extinction when the dinosaurs died off. Not only the dinosaurs went extinct, however, up to 75% of all known living species died during this major mass extinction event. It is pretty well documented that the cause of this mass extinction was a major asteroid impact. The huge space rocks hit the Earth and sent debris into the air, effectively producing an “impact winter” that drastically changed the climate all over the Earth. Scientists study the large craters left by the asteroids and can date them back to this time.

The Sixth Major Mass Extinction - Happening Now?

Lion Hunters. Getty/A. Bayley-Worthington

Is it possible we are in the midst of the sixth major mass extinction? Many scientists believe we are. Many known species have been lost since humans have evolved. Since these mass extinction events can take millions of years, it is possible we are witnessing the sixth major mass extinction event. Will humans survive? That is yet to be determined.