Major extinction events have happened multiple times throughout Earth's history. The one we most commonly hear of is what killed off the dinosaurs (or at least the non-avian ones), known as the K-Pg mass extinction. Well, despite happening around 66 million years ago, the end cretaceous mass extinction is actually the most recent...and not even the worst...
So, there have been many extinction events in the past, but only a few have been severe enough to make it into what is known as the 'Big 5'. It is these big 5 extinction events that we'll be covering here. Also, whilst these will be in chronological order, if your struggling to keep up with the story being told with these extinction events (since I may be skipping a few parts of Earth's history) you can find a full in depth guide to all of Earth's chapters here.
Ordovician-Silurian
The first of the Big 5 extinction events was the end-Ordovician extinction event. After life had began really starting to run during the GOBE, life on planet Earth saw its second worst extinction event, in which 80% of life went extinct. The Ordovician extinction event actually happened in two pulses.
The first of these pulses was due to an extreme drop in global temperatures, leading to heavy and rapid glaciation across the globe. Whilst the cause of this glaciation is heavily debated, it is most likely that it was due to the combined efforts of volcanism, which would have released a high amount of sulphur aerosols which had a cooling effect, as well as the first appearance and rapid spread of terrestrial plants, which removed the many greenhouse gases that kept the Earth warm. Just as sea levels rise with global warming, sea levels will decrease with global cooling. The only animals seen at this time were all marine invertebrates, so the many species that called the shallower parts of the ocean their home lost their environment. They also couldn't deal with such a rapid change in temperature in the ocean, since temperature dependency is much more delicate with marine life.
The second of these pulses occurred as the glaciation receded as quickly as it came and temperatures became warmer again. The pulse of Ordovician extinction event was thanks to what is known as anoxia. Anoxia is essentially what happens when the ocean become extremely depleted of oxygen. Oxygen in the water is absolutely vital for life, so when it becomes too low (or maybe even hits zero), few things can survive. The anoxia is only made worse when the dead bodies of organisms begin to decay and produce more oxygen depleting chemicals such as phosphate. We know anoxic conditions occur when a marine sedimentary rock shows very dark colours, as well as producing anoxic minerals such as pyrite, AKA fool's gold.
Again, just what caused this is heavily debated, but it could be simply the domino effect of what had caused the first pulse.
Late Devonian
This is another one of the Big 5 extinction events that seemed to have occurred in pulses. It is also another victim of that pesky anoxia, though the only effects seen were under the sea, where trilobites and brachiopods saw extreme decreases, most reef building organism being nearly wiped out and placoderms (the group that includes the fearsome Dunkleosteus) went went extinct altogether.
But what caused it?...well...we know less about that than we do the previous extinction event. What makes this one particularly difficult is that the geological record isn't clear about how long it lasted (anywhere between 500,000 years and 25 million years), so pinning down an exact cause is very tricky. The leading theories are that the Devonian mass extinction was caused by extreme global cooling (therefore decreasing the sea levels) from either: asteroid impacts, which caused a dusty winter (more on that one below!), the advanced evolution of plants reaching massive sizes, therefore not only increasing the consumption of greenhouse gases, but also causing heavy weathering on rocks from the ever increasing root system and changing the chemical balances of the sea, or underwater volcanic activity, in which vast amounts of CO2 and SO2 were released into the ocean, increasing anoxia and also spilling out into the atmosphere.
End Permian
Now, this one was a biggie. In fact, out of all of the Big 5 mass extinction events, this one was the worst. A staggering 98% of all life on the planet was wiped out! Safe to say the life on our planet has never received such a devastating blow.
Pretty much all groups of flora and fauna took heavy hits, changing what life looked like drastically when it re-diversified in the millennia to come. For a long time, this was another event that caused some controversy like those that came before it (most surface rocks are subducted and recycled back into the Earth every 200 million years or so, making the record become less and less clear the further we look back). Now, however, it is mostly agreed upon that this mass extinction event was caused by massive spikes in methane and carbon dioxide. This was in fact caused by Earth's biggest volcanic event seen in the Siberian traps. During this time, billions of tonnes of greenhouse gases were released when pure lava covered around 1.2 million square miles...and this was going on for 2 million years.
We've seen the devastating effects that greenhouse gases like this can have with our own eyes in the modern day, so with this magnitude, it's easy to understand why life had such a rough time. What is even scarier about the Permian mass extinction than how close life came to being completely wiped out, is how many scary parallels it draws with today's climate.
If you'd like to know more about the third of the Big 5 mass extinction events and why we should be talking more about it, we have just the post for you here.
Triassic/Jurassic
It wasn't all doom and gloom though. Whilst it may have taken a long time, life soon bounced back. Certain groups were doing particularly well on land, namely the new kids on the block, mammals, dinosaurs and pterosaurs. Disaster for many, however, would strike again in the fourth of the Big 5 mass extinction events. Even more climate change occurred from either gradual processes or volcanic eruptions in the Central Atlantic Magmatic Province, increasing ocean acidification yet again and atmosphere changes. Whilst many groups on land went extinct (for example, all archosaurs that weren't crocodilians, pterosaurs or dinosaurs), this event mostly affected life under the sea.
In fact, the main groups that were completely unaffected by this event were plants, pterosaurs, mammals and dinosaurs, marking the latest and biggest opportunity for pterosaurs and dinosaurs to become the top dogs of the land...for now.
End Cretaceous
Now, HERE is one that we all know. The last of the big 5 mass extinction events is the end-Cretaceous extinction, often abbreviated to the K-Pg (Cretaceous-Paleogene) extinction event. You may also see the term K-T extinction event, with the T standing for Tertiary (a term now considered 'informal'). If you're confused and think that paleontologists can't spell, the 'K' actually stands for Kreidezeit, the German word for Cretaceous.
Spelling out of the way, let's get into the meat of it. The end-Cretaceous extinction event was caused when a huge asteroid, around 10 to 15 km (6 to 9 mi) wide, struck in the the Gulf of Mexico 66 million years ago. The impact of such a huge asteroid would have many devastating effects across the globe. First is the initial impact felt. This was the same as several million nuclear bombs going off simultaneously, completely vaporising anything within 50-100 miles almost instantly. As our friend Phil Swift would say...
Then there is the next outer circle from this impact. Much of the world would have seen horrendous tsunamis, earthquakes and volcanic eruptions from the impact shaking things up so much. If an animal was lucky enough to survive all of that, it then had the final nail in the coffin to deal with. Like throwing a rock into water or sand, a lot of debris is going to be thrown up by the impact.
Since we're talking about such a huge object entering Earth's atmosphere, that debris would have also been vaporised within its immediate vicinity, creating sulphuric gases that blocked out the sun (alongside the solid debris it was carrying) globally. As well as a global dust cloud that created an instant impact winter, acid rain from the sulphur would have rained constantly around the world. This acidification spelled doom for marine organisms and the lack of sunlight spelled doom for many of the plants, in turn the animals that eat the plants which then spells further doom for the animals that eat those animals. Add this to such sudden freezing temperatures that lasted anywhere between 3-10 years meant that, with exception of the odd turtle or crocodilian, no animal weighing more than 55 pounds (25 kilograms) survived. Even those that weighed under, such as small non-avian dinosaurs didn't have much luck, as they had become too specialised (in other words, fussy what they eat) to be able to adapt to the food scarcity quick enough. Roughly 75% of life on Earth went extinct.
The only hope an animal would have would be if it was small...unspecialised...able to bury underground and live on scraps...maybe like those little mammal fellows?...
Today?
So what about today? This might seem like a silly question, after all, a look outside your window probably doesn't seem to show much disaster in terms of the natural world...right?
Well, remember that these things rarely happen in a day. The K-Pg mass extinction event was stretched out to a decade and that is pretty much instantaneous in geological time. The rest of the Big 5 mass extinction events occurred over thousands, perhaps even millions, of years or so. Simply read back on the Permian-Triassic extinction event, where only 2% of life on the planet was left over the course of approximately 2 million years!
When we compare that today, where around 500 species of animals alone have gone extinct in the last 100 years, we realise that species are indeed dropping at an alarming rate. In just 200 years, human beings have had more effect on the atmosphere than the Earth could manage on its own in 1000 years. Though these effects can take a while to be seen (at least in terms of human lifespan), it doesn't mean that something disastrous couldn't happen to all of Earth's ecosystems later down the line, even if all human activity stops this very second. One thing is consistent with the Big 5 mass extinctions and that is that climate change is always the culprit of such widespread death, it is only the cause of it that changes. We can only hope that Humans aren't added to those causes along with the Siberian traps and massive asteroids and that the Big 5...doesn't become the Big 6.