While a meteor is presumed to have caused the last great extinction event in our planet’s history 66 million years ago, the Earth had already gone through many others in the thousands of millions of years that life existed before. Let’s look back through eons of time to explain the planet’s worst extinction events and why scientists think we may be in the middle of one right now.
What is an Extinction Event?
Also known as a mass extinction or biotic crisis, an extinction event is classified as some chain of events during which an inordinate amount of life on the planet ceased to exist. These events can take a long time and only affect a small proportion of planetary life, like the thawing of an ice age, or they can be cataclysmic and violent, caused by extraterrestrial impacts that blot out the sun. To date, scientists believe the Earth has experienced six major extinction events.
The Great Oxygenation Event, 2400 million years ago, is the Earth’s first known extinction event. It took place when algae had just developed photosynthesis. The atmosphere became saturated with so much oxygen that it killed off much of the life on Earth. This, eventually, had an enormously positive effect: it meant Earth’s atmosphere was flush with energy—energy that multi-cellular organisms could take advantage of through respiration in the eons to come.
Ordovician-Silurian Extinction Event
The first of Earth’s great extinction events was the Ordovician-Silurian Extinction Event, which took place between 450 and 440 million years ago.
This extinction came in waves, first caused by a gamma-ray burst coming from a hypernova. The burst destroyed the Earth’s ozone layer, depleting vast swaths of the ocean in the ensuing heat, which then caused the death of algae that had been providing oxygen to the planet. From there, parts of the ocean floor dissolved into the sea, introducing toxic metals to the waters, causing more death before planetary glaciation took over. The victims this time were primarily trilobites, corals, and bivalves.
Late Devonian Extinction
The late Denovian period was marked by the evolution of plants into trees. In this span of time, plants went from being one foot tall to ten feet high; this meant even more oxygen and the beginning of legged creatures on Earth. This extinction mainly affected the oceans, killing off 75% of species as a result of meteors, UV damage, the overgrowth of plants, and volcanic activity.
The Triple Threats
The next three extinction events were less cataclysmic and shared similar causes. In the Capthian Extinction Event, continental collisions and increased ocean salinity killed off many ammonoids and coral-builders. On land, cat-sized therapsids took the brunt of the damage.
The Permian-Triassic extinction event occurred just eight million years later and finally caught up with land-dwelling species, wiping out an estimated 70% of them. Similar volcanic activity in Siberia and climate change were the likely culprits, though some scientists think stores of fossil fuels may have ignited, resulting in planetary fires and colossal explosions.
The Triassic-Jurassic Extinction kicked off the age of the dinosaurs 200 million years ago. It was likely caused by an asteroid or comet hitting southern Canada, forming what can be observed today as the Manicouagan reservoir. Climate during this time also evened out as extreme habitat diversity around the planet became much more arid and temperate.
Cretaceous to Paleogene
Next was the event people are most familiar with: the Cretaceous-Paleogene extinction, which wiped out the dinosaurs. This event wiped out nearly every four-legged creature weighing over 50 pounds, only leaving modern-day dinos like crocodilians and turtles as a reminder of the era.
An asteroid or comet, estimated to have been nearly ten miles across, hit the Earth. Scientists believe they located the crater for this extraterrestrial object in the Yucatan, estimating it to be 112 miles across. The impact was so catastrophic it vaporized rock, not just boiling it into magma, but forcing enough energy into the rock to turn it into an aerosol gas.
The impact was unlike anything the planet had ever seen or has ever seen since, releasing 420 zeta joules of energy, more than a billion times the energy released by the atomic bombings of Japan. It hit so hard that fragments of Earth were ejected into space, then tore up the atmosphere on re-entry, not only causing the equivalent of nuclear winter but also disrupting the atmosphere enough for cosmic radiation to literally microwave organisms to death.
Over 75% of all species on Earth vanished as a result. It killed off dinosaurs, mammals, birds, insects, and even plankton. It even killed off the ammonites who had survived the previous four extinction events, thriving as the most prominent life-form on Earth for a time. They had been on planet Earth for almost 250 million years, throughout many, many perils, but even they succumbed to the Cretaceous extinction.
Mammals rose to prominence in the next period, and lately, the species Homo sapiens seems to have taken dominance. While that may seem like a good thing, scientists still believe we’re currently in the midst of another mass extinction event.
The culprit this time is given away in the name; scientists call it the Anthropocene extinction, anthropos being Greek for “human.” Since the dawn of humankind, extinction rates have risen by an order of a thousand. The first victims consisting of megafauna, mammoths, giant sloths, and massive armored armadillos have already gone, but extant species of rhino, whales, gorillas, elephants, and tigers seem to be on their way as well.
Humans don’t always overhunt an animal as they did with the dodo, they also out-compete other species for resources, space, and even climate. Humans aren’t just classified as apex predators, but also as global superpredators, with their appetites outmatching any other species per capita.
According to the International Union for the Conservation of Nature, at least 875 species have gone extinct since 1500 CE alone. With regional extinctions ranging from a 98% decline in ground insects in places like Puerto Rico to a 75% decline of flying insect species in Germany.
While the disappearance of lesser-known species like the pagan-reed warbler and Pinta Island tortoise may seem inconsequential, the dangers facing our ecosystems are apparent in the condition of our reefs and rainforests. Of course, some organizations are fighting to protect our endangered species. Ripley’s Aquariums, for example, are helping to restore and protect the African penguins. Other advancements have even worked with cloning to revitalize species.