The 5 mass extinctions that have struck planet Earth. Here’s all you need to know

Mass extinction is the term scientists use to describe the end of more than 75 percent of the world's species in a relatively short span of time. This is as opposed to Earth's 'normal' background extinction rate, which is thought to be between 0.1 and 1 species per 10,000 species per 100 years.

Over the course of our planet's history, it is estimated that more than 99 percent of all species that have evolved on Earth have vanished. Since you're reading this, you're a member of the less than one percent of species still alive- at least for now. 

In total, our planet has experienced five mass extinctions in recorded history in the last 500 million years

Mass extinctions are those events that ultimately lead to an end of what geologists consider a period in geological time. In total, there have been at least five mass extinctions in the last 500 million years (and a number of smaller ones). The greatest of which occurred around 252 million years ago, known as the end-Permian mass extinction, eventually eliminating around 95 percent of all species, including most of the vertebrates.

Gradually, after each mass extinction, new species emerge to fill in the ecological niches left behind.

Mass extinctions were first categorized in 1982 at the University of Chicago by two quantitative paleontologists, Jack Sepkoski and David Raup.

Knowing that each mass extinction ultimately enabled Homo sapiens to thrive on this planet, let's go through all five. After all, it's the least we can do, right?

The Ordovician-Silurian mass extinction: 440 million years ago

Life on this planet began to spread and diversify about 3.7 billion years ago. During the Ordovician period (from about 485.4 million years ago to 443.8 Mya), the supercontinent Gondwana shifted towards the South Pole, resulting in much of it being submerged underwater. As such, this period is characterized by its diverse marine invertebrates.

Nevertheless, some 443 million years ago, the temperatures of the seas started to change. The result? Over 85 percent of all Ordovician species were eliminated. Still, it remains unknown as to what actually triggered the event.

One theory is that a cooling process was induced by the creation of the North American Appalachian Mountains. As the mountains' arose, their weathering removed carbon dioxide (CO₂) from the atmosphere, creating an "icehouse" effect- the reverse of a greenhouse effect.

However, not all scientists agree on this. According to an alternative theory, toxic metals could have dispersed into seawater during oxygen depletion, wiping out marine wildlife. Other scientists believe that a supernova's gamma-ray shockwave sawed a massive hole in the ozone layer, enabling lethal ultraviolet radiation to destroy life below water. Or, the mass extinction could have been triggered by volcanism.

Surprisingly, sponges survived this catastrophe.

At the end of the Ordovician period, glacier formation took place on such a vast scale that it locked away high percentages of the world's water, dramatically lowering the sea levels globally. This 'freezing over' alone destroyed many habitats of marine species, terminating food chains and reducing reproductive success.

Additionally, brachiopods--organisms that look like clams but aren't related--that had previously only been seen on other continents began repopulating Laurentian oceans after the extinction. In the fossil record, this is revealed by Silurian brachiopods being more widely distributed than their Ordovician predecessors.

The Late Devonian mass extinction: 374 million years ago

The Devonian period (from around 419.2 Mya to about 358.9 Mya), also known as the age of fish, saw the expansion and decline of several prehistoric marine organisms. In fact, this period saw three large extinction events, the last of which is generally considered a "mass" event.

Although animals had begun to evolve on land by this time, a large percentage of life remained in the oceans. That is, until the emergence of large numbers of vascular plants like trees and flowers may have caused mass extinction.

Plants' roots selectively changed the land they inhabited, converting rock and pebbles into soil. This soil, which was rich in nutrients, then flowed into the world's oceans, causing massive algae blooms. These blooms in turn created massive dead zones, which are areas where algae deplete the oxygen in the water, suffocating marine life and disrupting marine food chains. Organisms that were unable to adapt to reduced oxygen levels and a shortage of food perished.

Still, the validity of this theory is contested, and some scientists contend that volcanic eruptions were responsible for the drop in ocean oxygen levels. Nonetheless, collectively the extinction events in this period saw about 75 percent of species go extinct, and this marked the end of the Devonian period.

Dunkleosteus, a 10-meter-long armored fish, was one sea monster eradicated from the world's oceans at this time. Tetrapods, four-limbed animals transitioning from sea to land and eventually evolving into reptiles, amphibians, and mammals, were among the survivors.

The Permian-Triassic mass extinction: 252 million years ago

This mass extinction, also known as the "Great Dying," is considered the worst ever to affect Earth. It had a massive effect on the land-dwelling reptiles, insects, and amphibians, as well as the trees, wiping out approximately 90 percent of all species on the planet. This disastrous event is thought by some to have been caused by an extensive period of rapidly intensifying volcanism, others believe it was triggered by an asteroid impact.

The region of the world we now refer to as Siberia experienced explosive volcanic eruptions at the end of the Permian period. This resulted in a large amount of CO₂ being released into the atmosphere, resulting in a greenhouse effect that heated the planet. Weather patterns changed, sea levels rose, and acid rain pelted the Earth.

Vast amounts of the greenhouse gas dissolved into the seas too, poisoning marine life by depriving them of oxygen-rich water.

Some scientists believe that Pangaea, a single supercontinent that covered the entire planet at around this time, contributed to the oceans' lack of circulation. As a result, there was a global pool of stagnant water that pretty much served to sustain carbon dioxide accumulation.

Around ninety-six percent of marine life and 70 percent of terrestrial life went extinct at the end of the Permian period. Corals, being one of the marine life forms, were among the worst affected. In fact, it took over 14 million years for the ocean reefs to recover to their former splendor.

The Triassic-Jurassic mass extinction 201 million years ago

Dinosaurs started to emerge during the Triassic period, which saw the emergence of new and diverse life forms. Unfortunately, many volcanoes erupted at the same time. Although scientists are unsure why this fourth mass extinction happened, they believe massive volcanic eruptions took place in a region of the world presently covered by the Atlantic Ocean.

Volcanoes released massive amounts of carbon dioxide, similar to what may have happened in the Permian extinction, causing climate change and destroying ecosystems. In short, the ice melted, global temperatures rose, sea levels rose, and the oceans became more acidic. Consequently, many marine and terrestrial species went extinct, including many families of gastropods, bivalves, marine reptiles, and many vertebrates - although pterosaurs, crocodiles, mammals and fish were affected far less severely. Some researchers argue that this extinction event opened up ecological niches on land that were quickly filled by dinosaurs. 

Alternative explanations for this mass extinction contend that releasing trapped methane from permafrost due to rising carbon dioxide levels would have caused a similar chain of occurrences.

The Manicouagan reservoir in Quebec, a massive crater formed by a Late Triassic impact, was previously thought to be a likely candidate. However, radiometric dating determined that it is about 13 million years older than the Triassic–Jurassic boundary.

Eighty percent of the species went extinct during the Triassic –Jurassic mass extinction event. It would have been terrible to be a conodont--an extinct group of agnathan (jawless) vertebrates resembling eels--at this time too. The last of these ribbon-like fish, as well as many reptiles, were already in decline, and they were among the most severely affected.

The Cretaceous-Paleogene mass extinction: 66 million years ago

The general consensus is that an asteroid about 13 kilometers broad smashed into what is now Yucatan, Mexico, traveling at a speed of about 72,000 kilometers per hour (km/h). This caused the 180 km wide and 19 km deep Chicxulub crater to be formed. The impact, which would have scorched the whole area within a 1,450 km radius, signaled the beginning of the end of the dinosaurs' 180 million-year reign over Earth.

Debris and dust released into the atmosphere after impact resulted in months of blackened skies. Because plants could not absorb sunlight, they died in vast numbers, disrupting the dinosaurs' food chains.

Additionally, the world entered a prolonged cold winter due to the sharp decline in global temperatures. According to scientists, most extinction at the time would have happened within a few months of the impact.

Nevertheless, many species that could fly, burrow, or dive to the bottom of the oceans survived. For example, modern-day birds are the only direct descendants of dinosaurs living today — over 10,000 species are considered to have descended from impact-surviving victims.

The Cretaceous-Paleogene extinction, sometimes referred to as 'the day the dinosaurs died', is the most well-known of all mass extinction events. You'd think geologists would call this extinction event 'C-Pg' instead of 'K-Pg', right? They've had to do this since the letter 'C' is shorthand for a previous geological period known as the Cambrian.

The event is also occasionally referred to as the 'K-T extinction.' The letter 'K' is derived from the word 'Kreide'- a German word meaning 'chalk' that references the chalky sediment of the Cretaceous period. 'T' is for the term 'tertiary,' traditionally used for the time spanning the Paleogene and Neogene periods.

The sixth mass extinction is inevitable and will be the first to be caused by non-natural events

Many believe that the sixth mass extinction is already well underway — triggered by human activity. This is mainly due to (though not solely) the unsustainably high use of land, water, and energy, as well as the ongoing climate crisis. However, the sixth mass extinction differs from past extinction events because natural impacts solely caused the latter.

But of course, natural or unnatural, mass extinctions are one of many ways Earth expresses itself as a dynamic planet. Therefore, a sixth is inevitable. 

Could Earth be experiencing a 7th - not a 6th - mass extinction event?

A recent Interesting Engineering (IE) article highlighted 80 percent of species existing at the end of the Ediacaran Period (nearly 550 million years ago) became extinct due to a drop in oxygen availability worldwide.

Should this be the case, then the next mass extinction event will be the seventh, not the sixth. This because six would have already preceded it- not five.