Six (Mass) Extinctions in 440 Million Years
All things must pass. But the idea that a species could go extinct is a relatively new one, first proposed by anatomist Georges Cuvier in a presentation in Paris in 1796 in a lecture on the extinction of the mastodon, then thought by some to still be roaming the ill-explored western reaches of North America.
Cuvier’s suggestion that life on Earth was not static, and that species could disappear, was groundbreaking. Studying the collections of the National Museum of Natural History in Paris and records from other collections around the world, he soon identified several species whose like we would never see again, including the mosasaur, the cave bear, and the Irish elk.
Buoyed by the research of scientists like Charles Lyell and Charles Darwin, the idea that species developed gradually, over time, gained acceptance in the scientific community. For generations, it was dogma that extinctions happened slowly, too. The idea that species could be wiped out in a fell swoop, even one with catastrophic consequences, wasn’t given much credence.
That changed in the late 1980s and early 90s, when the Alvarez hypothesis, which stated that a huge comet or asteroid impact was responsible for the sudden disappearance of non-avian dinosaurs and many other forms of life 66 million years ago. Proposed by physicist Luis Alvarez and his son, geologist Walter Alvarez, the hypothesis took time to gain acceptance, but buoyed by evidence like the crater pictured below, an impact is now the most widely accepted explanation for the Cretaceous-Paleogene (K-Pg) extinction.
That acceptance also opened the door for further study of geological and fossil records, which led researchers to a surprising conclusion: While the K-Pg extinction event was a very bad day for life on Earth, it was by no means the only one on record. Researchers now think that the K-Pg was just the latest of five major extinction events—and that we’re currently in the middle of a sixth mass extinction, one caused not by a volcano or asteroid impact, but by humans.
Each event had a different impetus. Some took place over the span of millions of years while others were extremely sudden. What they have in common, though, is that they reshaped the face of life on Earth by wiping out a significant portion of it.
The earliest known mass extinction, the Ordovician Extinction, took place at a time when most of the life on Earth lived in its seas. Its major casualties were marine invertebrates including brachiopods, trilobites, bivalves and corals; many species from each of these groups went extinct during this time. The cause of this extinction? It’s thought that the main catalyst was the movement of the supercontinent Gondwana into Earth’s southern hemisphere, which caused sea levels to rise and fall repeatedly over a period of millions of years, eliminating habitats and species. The onset of a late Ordovician ice age and changes in water chemistry may also have been factors in this extinction.
Towards the end of the Devonian period around 370 million years ago, a pair of major events known as the Kellwasser Event and the Hangenberg Event combined to cause an enormous loss in biodiversity.
Given that it took place over a huge span of time—estimates range from 500,000 to 25 million years—it isn’t possible to point to a single cause for the Devonian extinction, though some suggest that the amazing spread of plant life on land during this time may have changed the environment in ways that made life harder, and eventually impossible, for the species that died out.
The brunt of this extinction was borne by marine invertebrates. As in the Ordovician Extinction, many species of corals, trilobites, and brachiopods vanished. Corals in particular were so hard hit that they were nearly wiped out, and didn’t recover until the Mesozoic Era, nearly 120 million years later. Not all vertebrate species were spared, however; the early bony fishes known as placoderms met their end in this extinction.
The Permian-Triassic extinction killed off so much of life on Earth that it is also known as the Great Dying. Marine invertebrates were particularly hard hit by this extinction, especially trilobites, which were finally killed off entirely. But you don’t get a nickname like the Great Dying for playing favorites; almost no form of life was spared by this extinction, which caused the disappearance of more than 95 percent of marine species and upward of 70 percent of land-dwelling vertebrates.
So many species were wiped out by this mass extinction it took more than 10 million years to recover from the huge blow to global biodiversity. This extinction is thought to be the result of a gradual change in climate, followed by a sudden catastrophe. Causes including volcanic eruptions, asteroid impacts, and a sudden release of greenhouse gasses from the seafloor have been proposed, but the mechanism behind the Great Dying remains a mystery.
This extinction occurred just a few millennia before the breakup of the supercontinent of Pangaea. While its causes are not definitively understood—researchers have suggested climate change, an asteroid impact, or a spate of enormous volcanic eruptions as possible culprits—its effects are indisputable.
More than a third of marine species vanished, as did most large amphibians of the time, as well as many species related to crocodiles and dinosaurs.
The most recent mass extinction event is also likely the best understood of the Big Five.
In addition to its most famous victims, the non-avian dinosaurs, the K-Pg event caused the extinction of pterosaurs and extinguished many species of early mammals and a host of amphibians, birds, reptiles, and insects. Life in the seas was also badly disrupted, with damage to the oceans causing the extinction of marine reptiles like mosasaurs and plesiosaurs, as well as of ammonites, then one of the most diverse families of animals on the planet.
In all, scientists estimate that 75 percent of species living at the time of the K-Pg extinction were wiped out.
The Holocene Extinction hasn’t been defined by a dramatic event like a meteor impact. Instead, it is made up of the nearly constant string of extinctions that have shaped the last 10,000 years or so as a single species—modern humans—came to dominate the Earth. Some have even suggested that the Holocene Extinction would be more aptly named the Anthropocene Extinction, after the role humans have played in this ongoing loss of biodiversity around the world.
“Many of the past mass extinction events are mysterious in some ways because we really don’t know the cause,” says Michael Novacek, the Museum’s provost of science and a curator in the Division of Paleontology. "But we have a good idea of what the cause of the current changes are, this century and the centuries before: it’s human activity.”
Humans have contributed to factors like climate change and the introduction of invasive species, which are leading to even more extinctions as animal habitats disappear or are disrupted by new species. “Some biologists think that the current rate of species loss is probably a thousand times what the normal rate is,” says Novacek.
Many of the species going extinct are doing so before they are even identified. In light of this, researching new species for a fuller understanding of the world’s biodiversity grows ever more urgent for institutions like the American Museum of Natural History. These records, Novacek says, are vital to our knowledge of the world around us.
“The collections in the Museum here and other museums are really a record of life," Dr. Novacek says. “They’re very important for not only telling us what went extinct, but what survived.”