Extinction event
Based on Wikipedia: Extinction event
The Planet's Near-Death Experiences
Life on Earth has almost ended. Not once, not twice, but at least five times in the past half-billion years, the planet has experienced catastrophic die-offs so severe that they fundamentally reset the course of evolution. The most devastating of these killed more than 80 percent of all species alive at the time.
We're living through the sixth one right now.
But before we get to that unsettling present, let's talk about what extinction events actually are—and why understanding them matters for thinking about the future of humanity.
What Counts as a Mass Extinction?
Every year, some species go extinct. This is normal. It's called the background extinction rate, and it's been happening since life began. Species arise, species die out, and on average the two processes roughly balance each other over geological time.
A mass extinction is something different entirely. It's when the rate of species dying off suddenly spikes far above normal while the rate of new species appearing stays the same or drops. Imagine a bathtub where the drain suddenly opens wider while the faucet slows to a trickle. The water level—biodiversity, in this analogy—plummets.
Scientists measure these events by looking at how many taxonomic groups disappeared in a relatively short geological timeframe. Here's where it gets technical, but bear with me because the distinction matters.
Biologists organize life into a hierarchy: species, genera, families, orders, and so on up the ladder. A genus (plural: genera) contains multiple related species—like how the genus Canis includes wolves, dogs, and coyotes. A family contains multiple genera. When scientists say 50 percent of genera went extinct, that's actually worse than it sounds, because each genus contained multiple species, most of which also vanished.
The reason researchers often count families or genera rather than species is practical: the fossil record is incomplete. We'll never find fossils of every species that ever lived. But if an entire genus disappears from the rocks, we can be more confident something real happened rather than just a gap in our fossil collection.
The Big Five
In 1982, paleontologists Jack Sepkoski and David Raup published a landmark paper identifying five intervals in Earth's history with extraordinarily high extinction rates. These became known as the Big Five, and they've shaped how we think about catastrophic change ever since.
The term "Big Five" can be misleading, though. These events don't stand apart as clearly separate categories from dozens of smaller extinction pulses throughout Earth's history. They're more like the tallest peaks in a mountain range—impressive, yes, but part of a continuous landscape of rises and falls in biodiversity.
Still, the Big Five deserve their reputation. Each one fundamentally transformed life on Earth.
The End-Ordovician Extinction (445 Million Years Ago)
The first of the Big Five came in two pulses near the boundary between the Ordovician and Silurian periods. Together, these pulses killed 85 percent of all species, 57 percent of genera, and 27 percent of families. Many scientists rank it as the second-largest extinction in Earth's history.
For a long time, researchers blamed this extinction on cooling and glaciation—massive ice sheets forming and sea levels dropping. But more recent studies from 2020 pointed instead to global warming triggered by volcanic activity, which depleted oxygen in the oceans. The scientific debate continues, with some researchers now suggesting that volcanic ash drew carbon dioxide out of the atmosphere, triggering the glaciation after all.
What lived through this catastrophe would go on to populate the Silurian seas. What didn't was lost forever.
The Late Devonian Extinctions (375-359 Million Years Ago)
Unlike the sharp, sudden extinctions that bookend geological periods, the Late Devonian was a slow-motion disaster. High extinction rates persisted across millions of years, punctuated by two particularly severe events.
The Kellwasser Event, around 372 million years ago, devastated coral reefs and wiped out numerous animals that lived on the seafloor—creatures like brachiopods, trilobites, and jawless fish. Scientists believe nutrients washing into the oceans from land caused massive algal blooms. When the algae died and decomposed, bacteria consumed all the oxygen in the water, suffocating marine life. It's a process we still see today in much smaller "dead zones" caused by agricultural runoff.
The Hangenberg Event, about 359 million years ago, finished what the Kellwasser started. It eliminated the armored placoderm fish entirely and nearly killed off the newly evolved ammonoids—spiral-shelled relatives of squid that would later become one of the most successful groups in the oceans, only to finally meet their end alongside the dinosaurs hundreds of millions of years later.
Together, these events killed at least 70 percent of species.
The End-Permian Extinction: The Great Dying (252 Million Years Ago)
This is the big one. The worst mass extinction in the past half-billion years.
The numbers are staggering: 81 percent of all marine species, 70 percent of terrestrial vertebrate species, 84 percent of marine genera, 53 percent of marine families. The trilobites, which had survived for 300 million years through multiple previous extinctions, finally disappeared forever. Even insects—incredibly resilient creatures that can survive almost anything—experienced their largest known mass extinction.
The Great Dying took 30 million years to recover from. Thirty million years. For context, the dinosaurs went extinct 66 million years ago, and complex human civilization emerged maybe 10,000 years ago. The devastation at the end of the Permian was so complete that evolution needed an almost incomprehensible amount of time to rebuild comparable biodiversity.
But catastrophe creates opportunity. With so many ecological niches suddenly vacant, new groups could diversify into roles previously occupied by others. On land, the synapsids—our distant ancestors—lost their dominance, opening the door for archosaurs. These archosaurs would eventually give rise to dinosaurs, pterosaurs, and crocodiles.
In the seas, the proportion of animals that could move around increased significantly compared to stationary filter-feeders. The ocean ecosystem that emerged from the Great Dying was fundamentally different from what came before.
Interestingly, a 2025 study in China found evidence of a thriving ecosystem just 75,000 years after the extinction—remarkably quick recovery for at least some communities. And some scientists now argue that the extinction's impact on land may have been less severe than previously thought, with fossil pollen and spores showing minimal disruption to certain plant communities.
The End-Triassic Extinction (201 Million Years Ago)
About 70 to 75 percent of all species vanished at the end of the Triassic period. In the oceans, entire groups like the conodonts and certain types of ammonoids disappeared. Reef ecosystems collapsed. On land, most large amphibians and many reptile lineages were eliminated.
This extinction cleared the stage for the age of dinosaurs. With their competitors gone, dinosaurs diversified rapidly and would dominate terrestrial ecosystems for the next 135 million years. A few temnospondyl amphibians—survivors of a once-great lineage—hung on in isolated pockets. The species Koolasuchus, a crocodile-sized amphibian, survived in Australia until well into the Cretaceous, a living fossil from a world that had otherwise moved on.
The End-Cretaceous Extinction (66 Million Years Ago)
This is the famous one—the extinction that killed the non-avian dinosaurs.
For most of the 20th century, scientists knew this extinction happened but had no satisfying explanation for why. Then in 1980, physicist Luis Alvarez and his geologist son Walter published a stunning hypothesis: they had found unusually high concentrations of iridium in rock layers from exactly this time period. Iridium is rare on Earth's surface but common in asteroids. Their conclusion? A massive asteroid impact.
The Alvarez hypothesis was initially controversial but is now widely accepted. The impact crater—the Chicxulub crater, buried beneath Mexico's Yucatan Peninsula—has been found. It's about 180 kilometers across.
About 75 percent of all species went extinct, including all non-avian dinosaurs, all ammonoids (after 350 million years of success), the marine reptiles like mosasaurs and plesiosaurs, and many other groups. The percentage of sessile—meaning immobile, anchored in place—marine animals dropped to about a third of what it had been.
Two groups emerged from this catastrophe to inherit the Earth: mammals and birds. Mammals, our ancestors, had spent over 150 million years as small, mostly nocturnal creatures living in the shadow of dinosaurs. Birds, which are technically dinosaurs themselves—the one lineage of theropods that survived—diversified into the thousands of species we see today. Every sparrow, every eagle, every penguin is a living dinosaur, the last survivors of a dynasty that otherwise ended 66 million years ago.
Before the Big Five
The Big Five all occurred within what geologists call the Phanerozoic Eon—the last 540 million years, characterized by abundant multicellular animal life. But life is older than that, and so are mass extinctions.
About 2.4 billion years ago, early photosynthetic bacteria began pumping oxygen into Earth's atmosphere. Oxygen was toxic to most life at the time—a waste product as dangerous to ancient microbes as nerve gas would be to us. This event, called the Great Oxidation Event (or, more dramatically, the Oxygen Catastrophe), may have been the most devastating mass extinction in Earth's history. We'll never know how many microbial lineages vanished because they left no fossils.
Another mysterious extinction occurred at the end of the Ediacaran period, just before the Cambrian explosion that produced the ancestors of all modern animal groups. We don't know how severe this extinction was, but the strange, quilted creatures of the Ediacaran—organisms that don't fit neatly into any modern category—largely disappeared, making way for something new.
The Sixth Extinction
Here's where the story becomes personal.
Research published after Sepkoski and Raup's foundational 1982 paper has led to a disturbing conclusion: a sixth mass extinction is currently underway, and we're causing it.
Since 1900, extinctions have been occurring at more than 1,000 times the natural background rate. The 2019 Global Assessment Report on Biodiversity and Ecosystem Services, produced by the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES), estimated that one million plant and animal species face extinction, many within decades. In late 2021, the World Wildlife Fund warned that over a million species could vanish within a decade—potentially the largest mass extinction since the dinosaurs disappeared.
A 2023 study in the Proceedings of the National Academy of Sciences found that at least 73 genera of animals have gone extinct since 1500. Under natural conditions, that many genera would take 18,000 years to disappear.
The causes are familiar: habitat destruction, pollution, climate change, overexploitation, and invasive species—all driven by human population growth, economic expansion, and overconsumption of natural resources. Scientists have a term for this: ecocide.
Unlike previous mass extinctions, this one has a culprit capable of understanding what's happening. That knowledge creates both responsibility and, possibly, hope.
The Limits of the Fossil Record
Before drawing too many conclusions from these patterns, it's worth understanding how difficult this science actually is.
The fossil record is not a complete archive of ancient life. It's more like a few scattered pages from a vast library, some charred, some water-damaged, most simply lost. Older fossils are harder to find because they're buried deeper. Dating them is more difficult. Certain environments and time periods have been studied intensively while others remain almost unknown.
Marine animals fossilize better than land animals, but even marine fossils show gaps. Soft-bodied creatures rarely leave traces. Environmental catastrophes can disrupt the deposition of sediments that would otherwise preserve fossils.
Some researchers have even suggested that apparent patterns in biodiversity might partly reflect patterns in rock availability—more rock from a given era means more fossils to find, making that era look more diverse. Statistical analysis suggests this sampling bias can explain about half the observed variation, but not all of it. Other evidence, like sudden spikes in fungal spores (fungi thrive on dead matter, so rapid increases suggest mass death), confirms that most recognized extinction events were real biological catastrophes, not artifacts of incomplete data.
The Hunt for Patterns
Scientists love patterns. Patterns suggest underlying causes, and causes can be understood and perhaps even predicted.
In 1984, Raup and Sepkoski proposed something provocative: mass extinctions might follow a 26-million-year cycle. Two teams of astronomers quickly connected this to a hypothetical companion star to our Sun—a dim brown dwarf orbiting far out in the solar system that would periodically disturb the Oort cloud of comets, sending some hurtling into the inner solar system. They called this hypothetical star Nemesis.
The Nemesis hypothesis captured public imagination but hasn't held up well scientifically. No such star has been found despite increasingly thorough surveys of the sky. The 26-million-year periodicity itself is disputed. Most paleontologists now view mass extinctions as driven by various causes—volcanic eruptions, asteroid impacts, changes in ocean chemistry, climate shifts—rather than a single periodic mechanism.
The Asteroid That Changed Everything
For most of the 20th century, mass extinctions were scientific orphans—acknowledged but unexplained. The prevailing view of Earth's history was gradualistic: change happened slowly, species evolved and went extinct at relatively steady rates, and dramatic events were exceptional aberrations rather than major drivers of evolutionary history.
The Alvarez hypothesis changed that. By providing a concrete, physical mechanism for the end-Cretaceous extinction—a rock from space, impact energy equivalent to billions of nuclear weapons, global darkness from debris in the atmosphere, collapse of photosynthesis—it made catastrophism scientifically respectable again.
This didn't mean every extinction was caused by asteroids. The search for impact craters corresponding to other mass extinctions has produced mixed results. But the Alvarez discovery opened minds to the possibility that Earth's history has been punctuated by catastrophic events with profound evolutionary consequences.
Survivors and Victims
Why do some groups survive mass extinctions while others perish?
It's not always the fittest that survive, at least not in any simple sense. The traits that make a species successful in normal times may become liabilities during catastrophes. Large body size, specialized diets, long generation times, limited geographic range—all can become death sentences when conditions change rapidly.
Small, generalist species with rapid reproduction often fare better. So do species that can enter dormant states or survive on detritus when primary food sources collapse. Geographic distribution matters: species spread across multiple continents are less likely to be completely wiped out than those restricted to a single region.
But there's also luck. Being in the wrong place at the wrong time—near a volcanic eruption, in the path of a tsunami, in an ocean basin that becomes anoxic—can doom even well-adapted species. Some survivors may have simply gotten lucky.
Recovery and Opportunity
Mass extinctions aren't just endings. They're also beginnings.
When the Great Dying cleared away the synapsid-dominated ecosystems of the Permian, it created opportunities for archosaurs to diversify. When the end-Cretaceous extinction eliminated the dinosaurs, mammals finally had their chance after 150 million years of waiting in the wings.
Recovery takes time—millions of years to rebuild complex ecosystems with diverse predators, prey, and symbiotic relationships. The first species to colonize devastated environments are often weeds and opportunists, generalists that can thrive in disturbed conditions but don't build complex ecological communities. Full recovery, with specialized niches and intricate food webs, requires patience on geological timescales.
This pattern has implications for our current extinction crisis. Even if we stopped all human-caused extinctions today, rebuilding the biodiversity we've lost would take millions of years. The decisions we make in the next few decades will echo through geological time.
What It Means for the Future of Humanity
The Substack article that prompted this exploration focuses on the future of humanity. Mass extinctions offer some sobering lessons on that topic.
First: Earth doesn't care about us any more than it cared about the trilobites or the dinosaurs. The planet will continue regardless of whether humans survive or thrive. Life has recovered from worse than we can likely inflict.
Second: we are both unprecedented and precedented. No species has ever before had the knowledge to understand mass extinctions or the power to cause and potentially prevent one. But species have gone extinct by the billions before. Dominance is temporary.
Third: the timescales involved are almost impossible for human minds to grasp. We struggle to think in centuries, let alone millions of years. Yet the consequences of current extinction rates will persist for geological ages.
Finally: catastrophes create opportunities. The asteroid that killed the dinosaurs was the best thing that ever happened to mammals. Whatever comes after humanity—whether our descendants, our successors, or simply a world without intelligent life—will inherit both our destruction and the ecological openings we create.
The question isn't whether there will be a future. There will be. The question is what role, if any, humanity will play in it.