For the last 3.5 billion or so years, about 99 percent of the estimated 4 billion species that ever evolved are no longer around. Extinction is a fact of life.
Many evolutionary family trees got the ax, so to speak, during a mass extinction. These events are defined as the loss of least 75 percent of species in the geological blink of an eye — which can range from thousands to millions of years. Researchers have enough data from the fossil record going back just over half a billion years to identify five such mass extinction events, and many scientists believe we’re in the middle of a sixth.
Defining ‘Mass Extinction’
Before learning more about these different mass extinction events, it is important to understand what can be classified as mass extinction and how these catastrophes shape the evolution of species that happen to survive them. A “mass extinction” can be defined as a time period in which a large percentage of all known living species go extinct. There are several causes for mass extinctions, such as climate change, geologic catastrophes (e.g. numerous volcanic eruptions), or even meteor strikes onto Earth’s surface. There is even evidence to suggest that microbes may have sped up or contributed to some of the mass extinctions known throughout the Geologic Time Scale.
Great die-offs result from a perfect storm of multiple calamities, such as ocean acidification coupled with a spike in land temperatures. While the catalysts of these events are sometimes unclear, large-scale volcanic activity, spread across an entire region, is a usual suspect. Theories that asteroid strikes initiate the massive die-offs remain largely speculative: Only one space rock has been conclusively linked to a mass extinction.
Throughout the 4.6 billion years of Earth’s history, there have been five major mass extinction events that each wiped out an overwhelming majority of species living at the time. These five mass extinctions include the Ordovician Mass Extinction, Devonian Mass Extinction, Permian Mass Extinction, Triassic-Jurassic Mass Extinction, and Cretaceous-Tertiary (or the K-T) Mass Extinction.
Mass Extinctions and Evolution
How do mass extinction events contribute to evolution? After a large mass extinction event, there is typically a rapid period of speciation among the few species that do survive; since so many species die off during these catastrophic events, there is more room for the surviving species to spread out, as well as many niches in the environments that need to be filled. 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.
As populations separate and move away over time, they adapt to new environmental conditions and are eventually reproductively isolated from their original populations. At that point, they can be considered a brand new species.
Each of these events varied in size and cause, but all of them completely devastated the biodiversity found on Earth at their times.
Each mass extinction ended a geologic period — that’s why researchers refer to them by names such as End-Cretaceous. But it’s not all bad news: Mass extinctions topple ecological hierarchies, and in that vacuum, surviving species often thrive, exploding in diversity and territory.
END-ORDOVICIAN: The 1-2 Punch
Trilobites got their start more than 520 million years ago, but faced their first decline during the End-Ordovician mass extinction.
When: About 443 million years ago
Why: In the first pulse of a double whammy, ice sheets advanced, radically changing ocean currents and creating a harsh climate in the equatorial and mid-latitude regions. Another possibility is continental drift and the related climate change.Many of the species that survived this first hit adapted to their new world, only to succumb to the second pulse: a sudden melt, another climate shift and another ocean circulation switch-up.
How bad: About 86 percent of species and 57 percent of genera — the next-higher taxonomic division, which may be a better gauge of biodiversity loss — went extinct.
What died: Animals that didn’t make it include most trilobite species, many corals and several brachiopods, a hard-shell marine invertebrate often mistaken for a clam today.
What thrived: Sea sponges did well in the aftermath of the End-Ordovician — a pattern repeated in subsequent mass extinctions. A 2017 Current Biology study suggests these humble marine animals may assist in the recovery of entire ecosystems. The sponges stabilize sediment, creating a favorable environment for brachiopods and other suspension feeders.
The first known major mass extinction event occurred during the Ordovician Period of the Paleozoic Era on the Geologic Time Scale. At this time in the history of Earth, life was in its early stages. The first known life forms appeared about 3.6 billion years ago, but by the Ordovician Period, larger aquatic life forms had come into existence. There were also even some land species at this time.
It happened in two different waves. The first wave was an ice age that encompassed the entire Earth. Sea levels lowered and many land species could not adapt fast enough to survive the harsh, cold climates. The second wave was when the ice age finally ended—and it was not all good news. The episode ended so suddenly that the ocean levels rose too quickly to hold enough oxygen to maintain the species that had survived the first wave. Again, species were too slow to adapt before extinction took them out completely. It was then up to the few surviving aquatic autotrophs to increase the oxygen levels so new species could evolve.
END-DEVONIAN: The Long Road to Oblivion
The placoderm lineage of ferocious-looking armored fish, such as Dinichthys herzeri, ended during the End-Devonian mass extinction, a long downward spiral in biodiversity.
When: 359 million to 380 million years ago
Why: While the term mass extinction may suggest instant global catastrophe, these events can take millions of years. The End-Devonian, for example, consisted of a series of pulses in climate change over 20 million-plus years that led to periodic and sudden drops in biodiversity, including the Hangenberg Crisis, which some researchers consider a separate mass extinction event. The changes, possibly the result of significant volcanic activity in Siberia, reduced oxygen levels in the oceans and caused other environmental shifts.
How bad: About 75 percent of species and 35 percent of genera went extinct.
What died: Sadly, those awesome-looking armored fish known as placoderms were wiped out. Many species of coral and more trilobites bit the dust, too.
What thrived: Little guys fared best, particularly vertebrates less than a meter long (about 3.3 feet). Survivors included tetrapods, four-limbed animals that were transitioning from sea to land and would eventually evolve into reptiles, amphibians and mammals.
The second major mass extinction in the history of life on Earth happened during the Devonian Period of the Paleozoic Era. This mass extinction event actually followed the previous Ordovician Mass Extinction relatively quickly. Just as the climate stabilized and species adapted to new environments and life on Earth began to flourish again, almost 80% of all living species—both in the water and on land—were wiped out.
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 mass death in the oceans.
The plants’ quick move to land also had a major effect on the carbon dioxide available in the atmosphere. By removing so much of the greenhouse gas so quickly, temperatures plummeted. Land species had trouble adapting to these changes in climate and went extinct as a result.
The second wave of the Devonian mass extinction is more of a mystery. It could have included mass volcanic eruptions and some meteor strikes, but the exact cause is still considered unknown.
END-PERMIAN: The Big One
Widespread volcanic activity across Siberia likely caused the largest mass extinction documented in the fossil record, the End-Permian. Oceans acidified, and massive wildfires consumed entire ecosystems.
When: 251 million years ago
Why: Volcanic activity in Siberia is considered the main culprit for this mightiest of mass extinctions. Regionwide eruptions spewed toxic gas and acidified the oceans. The disruption may have even shredded the ozone layer, allowing in deadly ultraviolet radiation. A 2014 study suggested the apocalypse unfolded over a span of about 50,000 years.
How bad: About 96 percent of species and 56 percent of genera went extinct.
What died: So many forests were destroyed, apparently during wildfires, that afterward there was nothing left to burn; researchers have identified a “charcoal gap” in the geological record. Among the animals that met their doom: many amphibians and several synapsids, a diverse group of animals that included several apex predators of their day.
What thrived: Several species of fungus weathered the event well, possibly because they fed off all the decaying organisms that didn’t. The loss of many top-of-the-food-chain synapsids may have cleared the path for the earliest dinosaurs — and the first mammals, descended from one lowly synapsid branch that survived.
This is the largest of all known mass extinctions with a massive 96% of all species on Earth completely lost. It is no wonder, therefore, that this major mass extinction has been dubbed “The Great Dying.” Aquatic and terrestrial life forms alike perished relatively quickly as the event took place.
It is still much of a mystery what set off this greatest of the mass extinction events, and several hypotheses have been thrown around by scientists who study this time span of the Geologic Time Scale. Some believe there may have been a chain of events that led to so many species disappearing; this 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 quick change in the climate. 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 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.
END-TRIASSIC: History Nearly Repeats
Keichousaurus hui was a common marine reptile during much of the Triassic Period, but it didn’t live to see the Jurassic that followed.
When: 201 million years ago
Why: Volcanoes were at it again, but this time we can’t blame Siberia. Instead, massive eruptions in a hot spot at the center of what would eventually be the Atlantic Ocean created another bout of climate calamity. And it played like an echo of the End-Permian. Additional causes possibly were global climate change, and changing Ph and sea levels of the oceans.
How bad: About 80 percent of species and 47 percent of genera went extinct.
What died: This was not a good time to be a conodont. Already in decline, the last of these ribbonlike fish, as well as many reptiles, were among the hardest hit.
What thrived: Dinosaurs really hit their stride after the End-Triassic, exploding in diversity, as did their archosaur relatives, the crocodylomorphs, descendants of which include modern crocodiles.
The fourth major mass extinction 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 the time perished. The causes of these individual small extinctions can, for the most part, be attributed to volcanic activity with basalt flooding. 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.
END-CRETACEOUS: It Really Made an Impact
The End-Cretaceous mass extinction event claimed nearly all the dinosaurs, including the iconic T. rex. Only one lineage, now known as birds, survived.
When: 65.5 million years ago
Why: The cause of the End-Cretaceous extinction remains hotly debated. No one disputes that a chunk of space rock slammed into the planet near Mexico’s Yucatan Peninsula at the time. But researchers disagree on whether the asteroid strike caused or merely contributed to the die-off. Large-scale volcanic activity in India’s Deccan Traps was already underway before the impact, and a Science Advances study published in February suggests both the asteroid hit and Deccan Traps activity coincided with a general uptick in planetwide volcanism. Whatever the catalyst, it appears a spike in carbon dioxide and a drop in ocean oxygen levels were followed by a rapid post-asteroid cooling.
How bad: About 76 percent of species and 40 percent of genera went extinct.
What died: The sun finally set on the Long Day of the Dinosaur, though many other organisms were also wiped out, such as ammonites, mollusks with distinctive spiral shells.
What thrived: Mammals quickly adapted to exploit newly vacated ecological niches, as did the sole surviving branch of dinosaurs, better known today as birds.
The fifth major mass extinction event is perhaps the best-known, despite it not being the biggest. 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. It is also the event that wiped out the dinosaurs. The dinosaurs were not the only species to go extinct, however—up to 75% of all known living species died during this mass extinction event.
It is well-documented that the cause of this mass extinction was a major asteroid impact. The huge space rocks hit Earth and sent debris into the air, effectively producing an “impact winter” that drastically changed the climate across the entire planet. Scientists have studied the large craters left by the asteroids and can date them back to this time.
SIXTH EXTINCTION: Present and Future?
In the 1990s, paleontologist and famed conservationist Richard Leakey warned that human activity was causing a “sixth extinction.” In the decades since Leakey’s observation, with piles of new supporting evidence, many more researchers have signed on to the idea.
Across time and around the planet, extinctions of one or another individual species are always occurring. Known as the “background rate” and documented both historically and in the fossil record, these extinctions are like low-volume static compared with the sudden cymbal crash of a mass die-off. Determining extinction rates as they are unfolding is difficult, but a 2015 Science Advances study, using a range of conservative estimates, placed the current pace at up to 100 times the normal background rate.
Human activities are to blame, including population growth, increased resource consumption and climate change spurred by fossil fuel burning and the release of greenhouse gases.
In the journal PNAS in 2017, scientists concluded that focusing on species extinction may actually underrepresent the severity of what one team called “biological annihilation.” The global extinction of a species is, after all, just the final nail in the coffin.
The downward spiral begins with the destruction and fragmentation of habitat, and the introduction of invasive species and pathogens. The killing of individual members of a species through overhunting or poaching also takes its toll. Just ask any rhino, if you can find one. All of these activities can result, over time, in local extinctions known as extirpations. Even before global extinction of a species occurs, these extirpations reduce biodiversity and can destabilize ecosystems, leading to more extinctions.
As Leakey observed in his landmark 1995 book on the topic: “Homo sapiens might not only be the agent of the sixth extinction, but also risks being one of its victims.”
Resources
discovermagazine.com, “The Five Mass Extinctions That Have Swept Our Planet: It wasn’t all bad news.” By Gemma Tarlach; thoughtco.com, “The 5 Major Mass Extinctions.” By Heather Scoville;