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In this unsettling episode of SciShow, Hank takes us on a trip through time to revisit the 5 major mass extinction events that have impacted species over the Earth's history. You'll be sure to leave this episode with some thoughts about what could possibly be the sixth event—the one caused by human activities. Let's go!

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Hello there, I may look strange to you but I like to be prepared at all times for whenever the next mass extinction might happen. But I do think it’s important to impart this information in a non-muffled way so I think I’m just going to go ahead and take this off.

It would suck to have to wear that all the time.

So you never know when the next mass extinction is going to happen, I mean there have been five of them in the last 450 million years alone. And it’s not like a mass extinction is some kind of blurry ill-defined term it’s when at least 50% of the species on the planet go extinct. One of them which has been aptly dubbed the great dying wiped out more than 95% of the species on the planet. It’s enough to make you walk around with a gas mask.


It’s possible that the gas mask is excessive; it’s also possible that it’s useless. I mean if we get hit by a big meteor it’s probably not going to help that much.

Mass extinction events tend to happen pretty fast on a geological scale but the actual, like, dying occurs on our scale quite slowly – tens of thousands of years. And of course the most important thing to look at here is what these five giant mass extinction events had in common, so that we can see them coming. 

Climate change: Usually caused by changing CO2 levels and relatively rapid global temperature changes. Do you see where I’m going with this? I don't mean to alarm anyone here but the idea that the earth is currently, as I sit here, going through a sixth mass extinction event is actually not that crazy. Scientifically it actually might make really good sense. 

Before we even look at climate change we already know that humans have been a strange and mysterious and new force for mass extinction. Current extinction rates are somewhere between 100 and 1000 times what they would be naturally due to humanity’s very enterprising nature. And that rate is similar to, or even greater than, the rates of some mass extinction events of the past.

So let’s forget about all of that depressing stuff for a moment and hop into the SciShow time machine. Our first stop: 450 million years in the past and we’re going to get a little bit wet because pretty much all life on earth at that time was in the ocean.

Welcome everyone to the Ordovician – Silurian mass extinction event. Now you’re not going to see any giant comets or meteors falling from the sky. But sometime in the next ten million years or so we’re going to see two really dramatic die-offs. Both of them involve an influx of gigantic glaciers and dramatic world wide falls in sea levels.

These two events will be separated by about 1 million years, that’s geologic time remember people, and by the end of the second one 27% of all marine families and 60% of all marine genera were extinct. And I can tell, from your blank stares, that we’re going to require a little bit of a refresher on the biological classification system. Which of course classifies all life based on common descent and which we will be talking a lot about as we get more into extinction events.

Starting with the most specific and then getting to the most general we have: Species, Genus, Family, Order, Class, Phylum, Kingdom, Domain, and good old life. Back to the Ordovician event; what scientists think happened here was that Gondwana, one of the two super continents at the time, moved into the polar region. This set off a huge drop in temperature, also a huge increase in glacial formation which dropped sea levels.

Marine invertebrates were the hardest hit. Two thirds of all the brachiopod and bryozoan families disappeared. It was also a tough time to be a bivalve or a trilobite. And because I find this fascinating, there’s also another theory about what happened to cause the Ordovician event. A gamma ray burst, from a hypernova, somewhere within 6,000 light-years of earth. A supernova; that’s when a star explodes, a hypernova is when a star like 100-300 times the size of the sun explodes. Even like ten seconds of a gamma ray burst from a hypernova would completely destroy the ozone layer and dose everything on earth with a nice, healthy amount of UV radiation. When I say healthy , I mean unhealthy.

Alright everybody, back in the time machine. We’re going ahead 100 million years to 360 million years ago to the Late Devonian extinctions. This is the beginning of a prolonged series of mass extinctions in which half of all genera and 70% of all species will go bye-bye.

Marine life will once again be hit very hard but so will the spiders and the scorpions and the proto-amphibians which are just taking their first steps out on the land. Narrowing down a time frame for this event has been difficult. Scientists can only say that over a period of 500,000 to 25 million years, that’s a big gap there, I know, there were a series of extinction pulses each lasting about 100,000 years. No Gamma ray bursts are suspected here this is once again being caused by a global drop off in temperatures. People aren’t sure what caused it, probably either an asteroid impact or a giant super-volcano.

Each of those things would have released mass amounts of dust or ash into the atmosphere, drastically changing the climate. Glaciers probably once again inundated Gondwana. What we also know is that ocean levels fell and lost most of their oxygen in the process in what’s called ocean anoxia. Some believe that temperatures in the steaming seas of the Devonian fell from 93 to about 78 degrees. Coral reefs were hit so hard that it would be 100 million years before they recovered.

But there’s another interesting theory here and that’s that plants, which were for the first time really taking hold on land, were absorbing so much carbon dioxide from the atmosphere that they actually caused global cooling. So in fact, plants were responsible for that mass extinction event. How could you?

And now, we will travel to the big one. Also known as the Permian-Triassic extinction event. Also known as the Great Dying. It is now about 250 million years ago and a combination of catastrophic events has totally altered life on planet Earth. So destructive is this event that it remains the only known mass extinction of insects.

This may have been triggered by a comet or asteroid impact but what scientists feel fairly certain about is that a subsequent volcanic eruption in an area known as the Siberian Traps was responsible for an increase in global temperatures of 5-10 degrees. These volcanic eruptions covered an area roughly the size of Western Europe and they lasted for a million years.

But then, it got worse. The sudden release of gasses from methane hydrate reservoirs below the sea floor possibly caused by underwater volcanic activity decimated the seas leading to periods of too much oxygen, called hyperoxia, and too little oxygen – the return of anoxia. And marine life does not fare well in either of these conditions. These two events at least, possibly others, lead to a shift in ocean currents and general inhospitable environments for most of the earths species.

When the die off was finally finished 53% of marine families, 84% of marine genera, 70% of land species and 95% of marine species, were extinct. The Permian extinction had an enormous evolutionary effect. Taking place just as mammal like reptiles like pelycosaurs and therapsids were in their prime. It would be more than 30 million years before these vertebrates and the earth in general would recover.

But it did recover. Until 200 million years ago, when the Triassic-Jurassic extinction killed off many early land animals. This includes the death of most of the therapsids, large amphibians, and the archosaurs. Paving the way for the age of the dinosaurs. Scientists probably know less about this mass extinction event than any other but what is known is that 23% of families and 48% of genera went extinct. But what was left fairly untouched were plants and dinosaurs, which now had a lot less competition.

This was roughly the time when the massive supercontinent known as Pangaea was breaking apart due to volcanic eruptions. And the massive release of carbon dioxide from those eruptions may have caused some pretty intense global warming. Again, species don’t fare well when temperatures increase quickly like this. And it wouldn’t be an extinction event without scientists wondering about whether or not there was a massive impact of some kind. So far, nobody’s found any likely crater candidates.

Do you know where there is evidence of an impact crater? Right off the coast of Mexico’s Yucatan Peninsula, and you’ve heard of this one; it’s where a giant asteroid or comet struck 65 million years ago. And that kicked off the Cretaceous Tertiary extinction. You know this one, it’s the one that killed off all the non-avian dinosaurs and ushered in the glorious age of mammals and birds. Mammals and Birds!

The K- T extinction, as it’s also known, killed off half of all genera and 75% of species. And while the giant asteroid impact winter theory is the leading one it’s by no means the only one. Also happening at the same time is a giant volcanic eruption in India that may have caused massive global warming. And that final extinction event brings us back around to today where 99% of the species that have ever existed on this planet are extinct. But we aren’t.

We are the one percent! Now, lucky for us, in the history of humanity we haven’t had to deal with any giant comet strikes or super-volcanoes but we do have to deal with a possibly equally destructive force: Each other. Some scientists think that we are already in the middle of an event that’s moving faster than the K-T extinction. But instead of being driven by continents or volcanic eruptions these extinctions are being caused by rising CO2 levels, habitat destruction, invasive species, pollution, and over harvesting.

We already know what changes in temperature can do to a planet. Click over here for information on that in our climate change video. Normal extinction rate for our planet is about 10-25 species per year and we’re already doing about 100 times that. So clearly, the 7 billion of us here on this planet are having a pretty big impact. I guess the question that remains for us to answer is: What are we going to call this sixth mass extinction event? I would love for you to leave ideas for that in the comments.

Thank you for watching, of course, there are sources for all of our information down in the description because we are scientists here, we do… argh it’s hard to breathe. We are around if you have questions or suggestions on Facebook, Twitter and of course the comments below. And if you want to continue to be smarter and know more about the world go to and subscribe. Goodbye.