What Caused the 5 Mass Extinctions of the Earth?

Life on Earth began under mysterious circumstances billions of years ago. The oldest dated microbial fossils suggest that life is at least 3.5 billion years old, during which time it experienced monumental setbacks. Although species come and go naturally, several mass extinctions have occurred, leading to the disappearance of many or most species on Earth.

The prevailing conditions at each of these moments tell us about the progress of life. Some of these events led Earth’s geological journey into a new path. What do we know about the five great mass extinctions?

Late Ordovician (443 million years ago)

The first recorded mass extinction separates the Ordovician period from the subsequent Silurian period. At this stage of history, almost all life was still in the sea. Mollusks and various simple hard-shelled creatures such as trilobites were dominant. The first jawed fish appeared, destined to be the ancestors of almost all modern vertebrates. The first plant fossils on land appear to date from this period, indicating what was to come.

The extinction of Ordovician wiped out about 85% of all marine species. Nearly all of the land mass was in Earth’s southern hemisphere at the time, and the current leading hypothesis is that the formation and subsequent recession of glaciers in this hemisphere caused its extinction. As the glaciers grew, some species died, while others adapted to colder, drier conditions. As the ice melted, more survivors and adapters drowned, overheated, or couldn’t handle the changing atmospheric composition. The cause of the ice age is debated. Studies have suggested everyday causes like rock weathering, or more exotic triggers like a shower of chondrite meteors or a gamma-ray burst. The problem is that this event happened so long ago that the seafloor and continents have all moved and regenerated significantly, obscuring the evidence. (Very little of the current seafloor is more than 150 million years old.)

Surprisingly, this extinction didn’t push Earth’s dominant species in a new direction. Most extant forms – including obviously our vertebrate ancestors – persisted in smaller numbers. They recovered to roughly their previous patterns within a few million years.

Late Devonian (372 million-359 million years ago)

During the Devonian period, the colonization of the land grew as plants and insects moved to solid ground. Plants developed seeds and internal vascular systems to transport and store water. They have not yet experienced substantial competition from terrestrial herbivorous animals, and the explosive growth of plants could have lowered atmospheric carbon dioxide levels and caused global cooling. After the extinction of the Devonian, tetrapods – ancestors of the first amphibians and later of reptiles, birds and eventually mammals – began to dominate the land.

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The extinction that ended the Devonian seems to have started with the Kellwasser event, continued slowly for a few million years, and ended with the Hangenberg event.

As cool as those names sound, we don’t know what the events actually were. Around this time, a 32-mile-wide crater formed in Europe, possibly indicating a meteorite impact. Two other strikes appear to have taken place within a few million years of this period. A group of scientists propose that a nearby supernova reduced ozone in the atmosphere. But the evidence is circumstantial and speculative, making it difficult to draw firm conclusions. A group of researchers argue that this was not actually an extinction event. Rather, it was a period in which a somewhat greater natural die-off coincided with a relatively slower evolution of new species. Lots of theories, but no clear answer will remain a theme of these mass extinctions, with one notable exception.

Permian-Triassic (252 million years ago)

The most brutal mass extinction occurred about 250 million years ago, and it wiped out the majority of species on the planet. Some scientists believe that as much as 90% to 96% of all marine species have disappeared, while others argue that it was probably closer to 80% to 85%. At least 70% of terrestrial vertebrates also became extinct. Several species of animals are currently completely disappearing from the fossil record. The supercontinent Pangea began to disintegrate at that time and the land was dominated by amphibians, early reptiles and giant flying insects whose rule of the sky had not yet been challenged by birds and flying reptiles.

The causes of this event are not well known – they were buried too deeply and scattered by continental drift. The event appears to be relatively short according to geologic timescales, possibly concentrated over a million years or less. As with the other extinction events, researchers have found numerous geological patterns that are shifting dramatically around this time, but they can’t pinpoint a single precise cause. Atmospheric carbon isotopes shifted and giant volcanic eruptions occurred in modern China and Siberia. Coal beds may have burned and microbes may have flourished, altering the atmosphere with their metabolic processes. A number of current schools of thought speculate that a combination of these factors together has warmed the climate. In any case, this extinction has changed the course of life. It took land creatures millions of years to recover, and they did so with new forms.

Triassic Jurassic (201 million years ago)

The Triassic period ended with dieback, and it was much less severe than its predecessor. Large crocodile-like reptiles called archosaurs dominated the land during the Triassic. The extinction of the Triassic-Jurassic era wiped out most archosaurs and paved the way for the emergence of an evolved subgroup of archosaurs that became dinosaurs and birds. These would come to dominate the country during the Jurassic period. Early mammals survived the event and continued to evolve slowly, though they may have been relegated to eating insects after dark, while cold-blooded reptiles ruled the daylight hours.

The most commonly believed causal factor is disruptions in atmospheric composition from volcanic activity that occurred in the Central Atlantic Magmatic Province around this time. Magma welled up in modern North America, South America, and Africa as these masses began to disintegrate. As they drifted away, these continental masses each carried a piece of the original field across what would become the Atlantic Ocean. Other theories about cosmic impact causes have fallen out of favor. As with the Devonian extinction, it is possible that no particular disaster occurred and that life simply went through a period of dying that went a little faster than it grew.

Cretaceous-Paleogene (66 million years ago)

This is the one you probably know: the end of the dinosaurs and the beginning of the modern (Cenozoic) era. Unlike the others, the cause of this extinction has become very clear to almost everyone. Geologic sediment layers around the world show a layer of rock with greatly elevated levels of the element iridium, a heavy metal that is extremely rare in the planet’s crust. Iridium is much more common in asteroids. The layer depth corresponds to the time of extinction. A 2016 drilling experiment at Chicxulub Crater in Mexico removed cores from the impact structure. Under analysis, these revealed iridium anomalies and other elemental features connecting the crater to the global iridium-rich layer.

The speculative and reconstructive nature of geological history makes the events of previous eras a vague mystery. We can designate limit values ​​in time, when fossil remains of certain species disappear. We can roughly examine the data of changes in the Earth’s atmosphere, analyze deposits of material from volcanic activity and bolide attacks, and try to explain various geological trace records. But to definitively attribute mass extinctions to this circumstantial evidence of events is fraught. Perhaps only the famous Cretaceous event, so clearly delineated by its global iridium layer, can be conclusively explained. Regardless of why these events took place, their mastery of dominant life forms is the plotline that has put us here today.