 Life on Earth has persisted for billions of years, but it's changed and evolved over long periods of time. And this happens slowly through mechanisms like natural selection over many generations of time. However, when there are rapid changes to Earth's environments, like with the tectonics or the climate or the ocean levels, then things change rapidly in the environment. And then sometimes those conditions are too quick for organisms and populations to adapt to or survive through. Now when things like this happen, it can lead to mass extinctions. And these are events where over 50% of species just die out and they go extinct. Now we can look at the fossil evidence and look at the life and look at how they might have lived and died. And then we can also look at the geochemistry of the rocks to look at what the environment was like. Now when we bring all these disciplines together, we can actually piece together all the different lines of evidence and find the bigger picture and figure out what was going on. So mass extinctions are driven by a complex interplay of climate, tectonics and geology. And whenever there are any changes in any one of them or together, then it leads on to massive flow-on effects that affect the whole globe. Now one such example is through massive volcanic eruptions. Scientists think that the volcanic eruptions were the main causes for at least three of the mass extinctions at the end of the Devonian, the Permian and the Triassic. So volcanic eruptions inject huge amounts of ash and dust into the atmosphere. And this can actually lead to the blockage of sunlight. And there's lots of organisms that rely on sunlight as their energy source such as plants and photosynthetic organisms. And so when they lose the sunlight, they lose their energy source and then they die. And then there's lots of other organisms that rely on those organisms as their food source. And you can see that those start dying and then eventually the whole food web can collapse. So another way that Earth's environments can change is through the tectonic plates moving. Now they move together over long periods of time and they can form super continents, which can then affect the albedo and the reflection of solar radiation on the Earth's surface. Now there was one such example at the end of the Ordovician. There was the supercontinent Gondwana around the South Pole. And so that meant that it led to the formation of massive ice sheets, which reflected a lot of solar radiation. And then this led to a massive cooling of the whole planet and sea level dropped because of the formation of all these ice sheets. And that meant that over 60% of marine species died because they just lost their habitat. Now the most well-known mass extinction event is the K-P-G mass extinction. You probably know it because that's the mass extinction when the dinosaurs weren't extinct. Now the fossil record does point to dinosaur population declining through the Cretaceous and that could have been actually because of another volcanic event at the Deccan Traps. But the main cause of their extinction was, they think, a meteorite impact. Scientists have brought together all the different lines of evidence and they've figured out that this meteorite was over 10 kilometers long. And it slammed into Earth's surface and it would have killed everything around it instantly. But then the long-term effects were similar to volcanic eruptions with that ash and dust blocking sunlight. So this mass extinction resulted in over 70% of species dying at this extinction event. These sorts of mass extinction events lead to over 50% of species dying out each time. But what happens is that it leaves habitats and ecosystems and ecological niches wide open. So then any surviving organisms that have traits that are advantageous to that new change setting, they can then fill these ecological niches and take advantages of what all the resources that's left behind. And then this can lead to the rapid diversification and the formation of new species. So in the K-P-G extinction, over 75% of organisms died out and they went extinct, including the dinosaurs, which were dominating the ecosystems at the time. But then with all those empty eat niches and ecosystems, it led to the rise of mammals. And in just 100,000 years, we already had possum-sized mammals. And then over a few more million years, they continue to get bigger and more diverse.