You’ve probably heard talk about whether we’re heading for a ‘sixth mass extinction.’ But what were the first five – and what might they teach us about what lies ahead?

Join us on a whistle-stop tour across our current geological eon, the Phanerozoic, which spans the past 538 million years.

There’s been life on Earth for much, much longer – over 3.7 billion years – so there were likely earlier mass extinctions, but they’re much harder to document because complex life only became abundant and visible in the fossil record at the start of the Phanerozoic.

So, in this explainer, we’ll take you through some of the biggest catastrophes in prehistory that we do know about, and then consider how today’s biodiversity crisis compares – and the implications for conservation and modern politics.

What is a mass extinction?

Extinction is a normal part of evolution. Most species that have ever lived are now gone, and background extinction rates tick along even in stable periods.

But a mass extinction is understood to be something different: a rapid event (well, in geological time, anyway!) in which a large proportion of the Earth’s species disappear across many groups and environments.

Mass extinctions are usually defined, rather arbitrarily, as events in which 75 percent or more of living species become extinct within a short geological window, usually of less than 2 million years.

Not all scientists agree on how many mass extinctions have occurred in this eon: while the established view refers to five, some estimates extend to more than 20, and others question whether they occurred at all. These differences depend largely on the data used to estimate past biodiversity and its loss.

Despite these debates, it’s still worth knowing a bit about each of the ‘Big Five’ mass extinctions – the Late Ordovician, the Late Devonian, the Permian–Triassic, the Triassic–Jurassic and the Cretaceous–Paleogene. Let’s take a look.

What are the ‘Big Five’ mass extinctions?

The Late Ordovician extinction

In this era, around 445 million years ago, most life was in the oceans. Plants had only just begun to colonize land, and most animal diversity existed in the shallow seas around the supercontinent of Gondwana.

Then, the Earth abruptly shifted into an ice age. Many species couldn’t handle the cold, and with much of the planet’s water drawn into giant ice sheets across Gondwana, sea levels dropped dramatically – draining the shallow seas where so many species had thrived.

Later, as global temperatures rose and the ice melted, the oceans were flooded with oxygen-poor, sulfur-rich water, killing many more of the species that had survived the first shock.

Overall, the Late Ordovician mass extinction wiped out an estimated 85 percent of marine species, triggering a significant ecological reset.

The Late Devonian extinction

This mass extinction, which took place around 372 to 359 million years ago, was less dramatic than its predecessor. It occurred as a series of extinction ‘pulses’ over tens of millions of years, in which about 70 to 85 percent of marine life died out.

The Devonian has been dubbed the ‘Age of Fish’ as this was the period in which fish species diversified spectacularly.

It was also a time when forests began to spread across land – and that process may have contributed to the crisis, as the explosion of land plants altered the global climate and increased nutrient runoff into the oceans.

The most popular theory suggests that oxygen levels in the planet’s waters dropped drastically over this period, which would have asphyxiated huge numbers of sea creatures.

Coral reef systems collapsed, and many ancient marine lineages disappeared; it’s estimated that undersea ecosystems took over 40 million years to recover.

The Permian–Triassic extinction (‘The Great Dying’)

Around 252 million years ago, the Earth went through the most severe biological crisis in its known history.

This mass extinction event seems to have been driven chiefly by vast volcanic eruptions, which released huge amounts of carbon dioxide and methane into the atmosphere.

This caused causing ozone depletion, rapid global warming, ocean acidification and severe oxygen loss – both in oceans and on land.

The impact was immense. Around 90 percent of marine species and 70 percent of terrestrial vertebrate species vanished. Tropical forests reached a tipping point and collapsed, further intensifying global warming.

Life seems to have taken at least 10 million years to recover from the Great Dying. From the devastation, however, emerged new types of animals, such as crabs, lobsters, marine reptiles – and the land reptiles that would later evolve into dinosaurs.

The Triassic–Jurassic extinction

We know a lot less about this mass extinction, which occurred around 201 million years ago at the end of the Triassic period, but scientists believe it most likely resulted from the breakup of the supercontinent Pangaea.

This seems to have triggered large-scale volcanic activity, causing rapid climate change, ocean acidification and ecological stress – just like the mass extinction that preceded it.

About 80 percent of species disappeared, with marine life, amphibians and reptiles hit particularly hard.

One group of reptiles, however, proved particularly resilient to the changed conditions – and then came to dominate terrestrial ecosystems for the next 135 million years. That group? The dinosaurs.

The Cretaceous–Paleogene extinction

You’ve probably heard of this mass extinction, though you might not know it by its name.

In the early to mid-Cretaceous, the Earth’s biodiversity began to increase at an unprecedented rate as flowering plants emerged, continental drift created diverse climates and habitats, and the Earth enjoyed relatively stable, warm conditions.

Then, 66 million years ago, an asteroid roughly 10 kilometers wide slammed into what is now southern Mexico about 20 times faster than a speeding bullet, releasing energy equivalent to about 100 million megatonnes of dynamite.

The impact triggered a broad swathe of disasters: vast wildfires, earthquakes, landslides, tsunamis and a ‘nuclear winter’ – compounded by ongoing volcanic activity – as dust and aerosols blocked the sunlight.

Photosynthesis stopped working, food webs unraveled and around 75 percent of species went extinct, including all non-avian dinosaurs.

Ultimately, though, the Cretaceous–Paleogene mass extinction turned out to be only a temporary setback to the explosion of biodiversity that continues to this day.

It cleared ecological space for mammals and birds, and within a few million years, the planet’s ecosystems had rebounded in new and increasingly complex forms.

Is this the sixth mass extinction?

In geological time, we exist in what many scientists believe is the ‘peak’ of biodiversity on Earth. Yet that biodiversity is now disappearing rapidly.

The average size of wildlife populations around the world has dropped by 73 percent over the past half-century, and the current rate of extinction is an estimated 1,000 to 10,000 times higher than it would be if humans were not around.

“In terms of rate, the current loss of biodiversity is perhaps the second fastest experienced by the biosphere in the last half billion years, after the end-Cretaceous mass extinction,” writes University of California, Berkeley paleobiologist Charles Marshall.

This time, rather than being caused by random events like asteroids and volcanic eruptions, it’s humanity – and the conscious decisions we make and actions we take as individuals, countries, companies and at global level – that is to blame.

Many scientists have said that this rapid decline in biodiversity could represent the start of the ‘Sixth Mass Extinction.’ Others disagree: there’s some spicy debate about whether or not humans could really wipe out 75 percent of all species.

Such arguments might be missing the point. For one thing, “we won’t know that it’s the sixth mass extinction until it’s actually happened,” says Robert Cowie, an ecologist at the University of Hawai’i – at which point we won’t be around to record it.

And regardless, Cowie argues, downplaying the impact or extent of human-caused extinction – ‘mass’ or otherwise – is a dangerous approach.

“There’s a ton of people who would like to think that there’s no biodiversity crisis because they don’t care,” he says.

More important, perhaps, is the fact that for the first time, there exists a species – us – that’s capable both of causing such a crisis and finding its way out of it.

If we fail, the Earth will eventually recover as it always has. But we’ll be long gone.

Our bodies, societies and food systems are closely adapted to and dependent on the current conditions in the biosphere, with its forests, pollinators, fisheries and climate stability.

The climate crisis is already showing us how tiny shifts in temperature can dramatically destabilize our systems and destroy our homes, health and livelihoods. We can’t wait 10 million years for a new equilibrium.

“The key is that every other animal except us doesn’t have choice like we do,” says Cowie. “We can say we will destroy the planet, or we can say: ‘No, we’re going to do everything we can to stop it.’ Other animals can’t do that.

“We have the responsibility, therefore, to make the right decision. And to my mind, the right decision is to save the planet.”