Extinctions - Online Tutor, Practice Problems & Exam Prep

Extinctions have significantly influenced the history of life on Earth, with background extinction representing the average rate of species loss due to factors like competition and environmental changes. In contrast, mass extinctions, such as the Permian and Cretaceous events, drastically reduced biodiversity, eliminating up to 96% of marine species and 75% of all species, respectively. Currently, scientists warn of a potential sixth extinction driven by human activities, with extinction rates 10 to 100 times higher than the historical background rate, primarily due to pollution, habitat loss, and climate change.

Extinctions

One force that has definitely shaped the history of life on Earth is extinctions, and extinctions, we're going to say here, they're events that metaphorically prune the tree of life. So, we're going to draw up here a phylogenetic tree, and you can see these speciation events happening, this diversification of the tree, these lineages splitting. But as you look, some of these branches, I'm just going to highlight them in yellow there, they end. Those species go extinct. Right?

And this is just the idea, not everything makes it. Now, as you look at this, you see that those extinctions kind of happen sporadically throughout this tree. That sort of sporadic extinction, we're going to call background extinction, or sometimes we call the background extinction rate. This is just that average low level of extinction that kind of happens throughout the history of life, you know, due to competition, a new species moves in, the species that was there before can't compete, it goes extinct. Environmental change.

Right? In some local area, the environment changes, the species there can't adapt fast enough. It goes extinct. Whatever it is, not everything makes it. But as we look at this tree, right?

I'm going to highlight this section here. What happened there? Something happened, right? That's a mass extinction where species across this entire phylogenetic tree, they're going extinct at the same time. So this had to have been some catastrophic environmental event.

Right? That simultaneously affects species across the tree of life. Now this is a big deal because in the history of life on Earth, we see several of these, and they wipe out sometimes 75% of all species or even more than that. Right? So just think about that.

Right? What makes it through that extinction event? That's going to have a huge effect on what life looks like going forward. And we also just want to note that after one of these mass extinctions, it can take 5 to 10 million years for similar levels of diversity to return to the Earth. Now the species that are gone, they're gone.

But to get to that similar level of diversity, 5 to 10 million years now, in the history of life on Earth, that's not that long. But for species living during that time, that's a long time for there just to be fewer species around. Alright. Now as we look at the history of life on Earth, scientists call out what we call the big five. The big five are the major extinctions in the Phanerozoic or the time since the start of the Cambrian.

So we're going to put down here our entire history of life on Earth that we've talked about. The Phanerozoic, that's just that very end part since the Cambrian; remember the Cambrian was this time when we get this rapid diversification, especially of animals. And it's from then on that we actually have a good enough fossil record to do this work to figure out when things are going extinct. So we're going to zoom in here, and we have the different eras and periods here highlighted. Now it's unlikely you're going to need to know all of these, but you're likely to see it drawn out like this, so that's why we have it.

We have the Paleozoic in blue there. The Paleozoic is the time from the Cambrian to the Permian. The Mesozoic, this is sort of the age of the dinosaurs here. And the Cenozoic, this is sort of the time since the dinosaurs, we can think of it as the age of mammals. I'm going to put here the extinction rate.

So we're drawing out the extinction rate. So when this line goes higher, more things are going extinct. When it's lower, fewer things are going extinct. All right. Now way over in that light blue, that's the Cambrian.

And the Cambrian, right, that's that time of rapid diversification. We can also see it's a time a lot of extinction. But going from there, we can call out these big five extinction events. Now it's unlikely that you need to know the details of all 5 of these. I'm going to at least say the names of all 5 so that you've heard them.

There's 2 that we're going to go through in more detail, but we'll do that in future videos. For now, I'll just put the names of those 2 on the screen. So as we go through this, the first real spike that we see after that Cambrian at least, this is the Ordovician extinction event. We go along, we then get the Late Devonian extinction event, and then well, we reach the big one. That's the Permian.

The end Permian extinction event, it's something like 95% of all marine species go extinct. That's a bad time to be alive, right? Alright. So from the Permian, we move forward. Now we're sort of well, the dinosaurs evolved in this next period, the Triassic.

And at the end of that period, we at the end Triassic extinction event. And then for this time, this is really sort of the age when the dinosaurs, the dominant land animals, until the Cretaceous extinction event. And if you've heard of one of these, it's probably the Cretaceous because this is the one that wiped out the dinosaurs and brings us into sort of the Cenozoic, that age of mammals. Alright. So again, we're going to look at the Permian and the Cretaceous extinction events in more detail coming up.

Before that, though, we got an example and practice. Check them out.

Extinctions Example 1

Our example tells us that below is a graph showing the number of marine families alive over time during the Phanerozoic. We need to use the graph to answer the questions below. Alright, so I'm going to scroll down a little bit, just so we can see all the questions here and see this graph nicely. We're looking at this graph, it says total marine families.

We see all the time periods and the eras of the Phanerozoic there. We have the Paleozoic, the Mesozoic, and the Cenozoic, and then all the different periods. We're not going to worry about those details right now though. And we see this line is jumping around. It goes from 0, and at the top on our scale here is up to a 1,000 different marine families.

Alright, so a just says draw arrows to where the 5 major extinction events occurred. Alright, the big five. So what would an extinction event look like on a graph like this?

And, well, where are the big five? Well, an extinction event, here, this is the total number of families. So when something goes extinct, it's going to cause a dip in the graph. So where are the 5 big dips? Well, the first one, here's a dip here.

That's a big extinction event, where the number of families, it was increasing a bunch during the Cambrian. It looks like the early Ordovician here. And then it goes down. A lot of things go extinct.

Alright, the next one, we're going along. Well, here's the next big one. Right there. That's the one in the Devonian.

We're going along looking for another big one. Here's a big one right there, right? That's a huge dive in the number of species, a lot of things had to have gone extinct at that point. Alright, we're going along and we have 3. The next two are a little bit harder to pick out, but as we go along, the next dip here is at the end of the Triassic, and that is the end Triassic extinction event that we definitely called one of the big five.

And then our final one is the biggest dip left on this graph right here at the end of the Cretaceous. That's the one that killed the dinosaurs. Alright, so the five major, we got 1, 2, 3, 4, 5. That means we've done a.

Let's see what b says. Are there other extinction events that you see in the data? If so, mark 2 with an asterisk. Alright. Do you see other extinctions?

What do you see? Well, if an extinction is anywhere that this line dips, well, I see other dips. Here's a dip. That looks like a mass extinction event. A lot of things went extinct there. Now not as big as the other ones, but still a lot of things went extinct. Here's another one.

Anywhere where you see that line take a little bit of a dive, that's going to be an extinction event. Alright, so now that we've done B, let's check out C. Circle which extinction event appears to have been the most consequential?

Which one sort of looks like the biggest? Well, the one I've labeled 3 here, that's that end Permian. Look at that sort of huge dive, just sort of falls off a cliff there. A lot of things going extinct there. Alright.

So we've done C. Crossed that one out. Next, we have D. Why do you think the data is based on marine families? What would you think?

Alright. Well, marine remember to do this, you need a really good fossil record because you're sorting through all sorts of organisms and figuring out when they're alive and when they're not. You need a good fossil record to do that, and marine organisms give you a really good fossil record because they live in an area where there's a lot of sedimentation. Alright. So I'm going to say here, we use marine families because it's good.

It's a good fossil record. Alright. Next, it says despite extinctions, what has happened to the total number of families over time? Take a look. Well, over time, the number of families goes up.

It seems to level, and then we have a big extinction, but then it just keeps going up over that. So, the general trend here is that the total number of families has increased over time, I'm going to say. And then finally, F here says the end Cretaceous extinction event is thought to have killed 75% or more of all species on Earth. The graph does not show a 75% reduction at this time, however. How could you explain the discrepancy?

Alright. So here's that end Cretaceous extinction event. We definitely see a dip here, but it doesn't go down by 75%. So why not? What I'm going to say here is that this is a graph of families. And in our sort of biological organization, it goes family, genus, species. So a family is a pretty big group. There are a lot of species in a family, sometimes dozens of species in a family. So for a family to go extinct, that means that all of those species in that family had to have gone extinct.

Right? You know, say there's 20 species in a family. If 19 of them go extinct, but one survives, that's not going to register on this graph. Right? So 75% of all species went extinct, but that doesn't mean that all those families went extinct.

A lot of families just lost a lot of diversity, but the family didn't go completely extinct because some species were still around. Alright. With that, more practice after this. Give it a try.

How do mass extinctions differ from background extinctions?

• A. Mass extinctions affect larger numbers of diverse species simultaneously.
• B. Mass extinctions tend to affect larger organisms instead of smaller organisms.
• C. Mass extinctions can be caused by a change in the environment, while background extinctions cannot.
• D. Mass extinctions are typically caused by an increase in competition, while background extinctions are not.

What could be one reason why paleontologists mostly study extinctions in the Phanerozoic (since the Cambrian explosion)?

• A. Mass extinctions were rare before the Phanerozoic but have become more common since.
• B. Plants and animals did not live on land until the Phanerozoic.
• C. More hard-bodied organisms evolved in the Phanerozoic.
• D. The Phanerozoic was a time of more active volcanic activity, leading to more easily dateable fossils.

Permian Extinction

We've talked about the big five extinction events, but of those 5, the big one was the Permian extinction. So let's talk about that now. The Permian extinction, sometimes people refer to it as the great dying. Now I just include that for illustrative purposes. Right?

If you're alive at a time called the great dying, it's a bad time to be alive. Now this happens about 252,000,000 years ago at the end of the Permian. Now the Permian is the end of the Paleozoic Era. The Paleozoic goes from the Cambrian to the Permian, and after the Permian starts the Mesozoic. You can sort of think of that as the time of dinosaurs.

Alright. So what was life like on earth at this time? Well, we had reptiles, sort of proper reptiles that you would recognize as reptiles running around. There weren't dinosaurs yet and there weren't birds yet, but there were reptiles. Now there were no mammals, but there were mammal ancestors, the ancestors of mammals sort of had their own branch on the tree of life at this point.

And, well, there were fish, sort of proper bony fish that you would recognize as relatively modern looking fish swimming the oceans. There were land plants. Now there weren't flowering plants yet, but there were ferns and conifers living everywhere, and there are all sorts of insects running around. Now right now the earth just has one super continent, and that continent we call Pangaea. So all life on land lives on Pangaea, and really all life in the ocean most life in the ocean lives in the shallows.

So most of that life is living sort of around the coasts of Pangaea. Alright. So this Permian extinction, we're gonna say that about 96% of marine species were wiped out. That's a lot. Right?

96%, almost all of them, and about 70% of terrestrial species. Right. So what happened? Well, again, sort of illustrative name here, the world went to hell hypothesis. Well, this is the idea that there was just this massive volcanic activity.

And this volcanic activity, which we've illustrated right here, right, it's gonna have lava flows in what we now call Siberia that were hundreds of meters thick. So at least that part of the world's pretty inhospitable life. But from these volcanoes, it's just releasing so much gas that it really changes the chemistry of the Earth. It's gonna increase CO₂ levels so much that you're gonna get a 6 degrees Celsius rise in atmospheric temperature, which we've sort of illustrated up there. Right?

This massive global warming, 6 degrees Celsius is about 11 degrees Fahrenheit. That's massive global warming, and if that happens relatively quickly, it would be very hard for organisms to adapt to that sort of temperature change. Now we're gonna have acid rain, and acid rain is gonna kill the plants. And if the plants are dying, well, the things that eat the plants, they're also gonna have a hard time. Right?

Now from the CO₂, other gases, you're also getting acidification of the oceans that we see there. And from all these things dying, you're having all these bacteria growing up as decomposers, and those are gonna lower oxygen levels. Oxygen levels in the ocean are gonna drop. So in the oceans, if the oceans acidify and you're losing lots of oxygen, it's hard to stay alive, and that's why most things didn't. Right?

So, again, 96% of marine species going extinct, 70% of terrestrial species. That means if a species is alive at this time, it's likely it wasn't afterwards. And what is able to survive, that's gonna have a huge impact on, well, how life develops going forward. Alright. We're going to look at more extinction events coming up.

I'd say it's going to be fun, but there will be lots of death.

What is thought to have been the major cause of the end-Permian extinction event?

• A. An asteroid impact that caused global winter.
• B. The diversification of the dinosaurs forced other species into extinction.
• C. Major volcanic activity disrupted the global atmosphere and ocean chemistry.
• D. Continental drift shifted environments suddenly and drastically.

Cretaceous Extinction

We now want to talk about the Cretaceous extinction event. The Cretaceous extinction is the last of those big five extinction events that we mentioned, and there's a good chance you're going to be responsible for knowing about it, and it's also one that there's a good chance you already know at least something about. So this one happened about 66,000,000 years ago, and it killed the dinosaurs. Alright? And we want to say we also sometimes refer to it as the KT extinction event.

So, just if you see KT extinction, that's what we're talking about. That k, that's just how you spell Cretaceous in German, and the t comes from the tertiary, which comes after this. Alright. So what was life like at this time? Well, there were dinosaurs.

Now it says we had dinosaurs. Right? So after this, the dinosaurs are gone. But at this time, there were T-Rex and Velociraptors running around. Now there are flowering plants.

Flowering plants really diversified during the Cretaceous. There were mammals. A lot of people don't realize this, but there were actually a lot of mammals at this time, probably hundreds of species. Now a lot of them go extinct, but most of the mammals at this time were small. Really all the mammals were small, probably raccoon size and mostly smaller than that.

Now we have lots of insects and specifically bees and ants, the social insects have evolved at this time. Alright. So what happens was this Cretaceous extinction event, we're going to say wipes out about 75% of all species, and most notably those non-avian dinosaurs. Alright. What do we mean by non-avian?

Well, avian refers to birds. So you may realize that birds or you may know that birds are descended from dinosaurs. So one way to think about it is birds are the only dinosaurs that didn't go extinct. Now to be clear, there were a lot of birds alive at this time, and most of them went extinct. But the few species that didn't, they survived and they, you know, evolved into all the species of birds that we see today.

Alright. Well, what happened? We call this the impact hypothesis. My own personal name for it is just a very, very bad day on planet Earth. Alright.

So what happened was a meteorite struck Earth near the Yucatan Peninsula in the Gulf of Mexico. Right? This meteorite was like 10 kilometers wide, over 6 miles wide, and it slammed into Earth with what we think is about the energy of a 1,000,000,000 atomic bombs. So imagine, right, here's our image. We have basically, like, a 1,000,000,000 atomic bombs going off in the Gulf of Mexico there.

What's going to happen? Well, first off, we get these mega tsunamis. Right? These tidal waves that wash deep inland on different continents. Well, we're also going to have worldwide firestorms.

So right. Basically, the forests all over Earth are going to catch fire as well. All the debris that gets kicked up, it starts raining down all over the planet, basically as fireballs. And how big these firestorms were is a little bit debatable, but some people think basically all forests on earth caught fire that day. Alright.

So that's relatively short term. We're talking about, you know, the hours and days after this impact. Now as we go a little bit long term, we're going to go into a global winter. Right? From all the debris that has been kicked up, it sort of hangs out in the atmosphere.

We think for about 3 years, blocking out the sun, and well, for 3 years, we have winter on Earth. That can be tough to survive if all of a sudden without warning, you go into 3 years of winter. Now after those 3 years and up to that, debris comes down, and it goes back to a little bit normal. But we have this horrible acid rain and ocean acidification. Alright.

Acid rain and ocean acidification. The acid rain is going to kill all sorts of plants, the things that eat plants. Ocean acidification makes it hard for all sorts of different organisms to survive. And from all the stuff that's kicked up and from all the burning, etcetera, we think that we get about a 100,000 years of global warming after this. Right?

So after this, we get a 100,000 years of major climate change. Alright. All that together, it's tough to survive. Right? So why did the mammals survive?

Well, again, most mammals went extinct. Right? Only maybe two dozen species of mammals survived. We think they were small, and they were generalists. They were able to eat all sorts of different food sources, so they weren't reliant on one thing that died off.

You know, they could eat seeds, scavenge other dead things, and survive that really bad day, those really bad sort of couple years after this. Now we just do want to note that other factors, specifically volcanic activity, may have contributed to this extinction event. Some people think that this extinction event was a bit of a longer extinction event with all this sort of global volcanic activity going on, and this impact sort of punctuated it and knocked a lot of things out sort of right in the middle. Alright. Well, with that, we're going to look at this more in examples and practice problems.

There will be more death and destruction to come. See you there.

The extinction at the end of the Cretaceous period is most closely associated with what?

• A. The extinction of many organisms that evolved in the Cambrian, such as trilobites.
• B. The extinction of the dinosaurs.
• C. Massive worldwide volcanic activity.
• D. Opening of terrestrial environments for colonization by vertebrates.

Which piece of evidence is consistent with the leading hypothesis for the cause of end-Cretaceous extinction?

• A. Massive lava flows have been found in Siberia, large enough to disrupt global atmospheric chemistry.
• B. Plant colonization of land led to a major increase in the nutrients available in the oceans and a worldwide decrease in oxygen levels.
• C. The super-continent Pangea broke up, forming many of the current continents during the Cretaceous.
• D. Worldwide, a layer of sediment dated to 66 mya contains iridium, an element common in asteroids.

Sixth Extinction

We now want to talk about what we're going to call here the 6th extinction. Now we talk about the big five, those big five extinction events that have happened in the Phanerozoic, but some scientists believe that a 6th extinction may be underway right now, and that's going to be due to human activity. Right? So we talked about those previous extinctions. It was due to things like, you know, massive volcanic activity and an asteroid slamming into earth, these sort of catastrophic events.

The catastrophic event this time is us. Alright? And this is well, people think this because the current extinction rate has been measured to be something like 10 to 100 times the background extinction rate as has been measured in the fossil record. And this rate, some scientists think, that 10 to 100 times may actually underrepresent the actual number of extinctions going on. Now why are they happening?

Well, again, it's us. Right? It's likely due to human impact or mostly due to human impact. Things like pollution, things like climate change, things like habitat loss. Right?

We just live in the world in a way that makes it harder for other organisms to live. Now we can look at the data here. Right? We have here a graph, from 0 to up to just 2% there, the percent of species that have gone extinct over time. And it's been measured here in different 100 year periods starting from 1500 up until well, it's measured in this graph up to 2010.

And what we see here is that, you know, the extinctions are pretty flat over time until just recently in those last 100 years, you get this huge uptick. Right? This big those slopes just get very steep. Now we'll just note right at mammals, the one that has the highest extinction rate, right, we're just at 2% of mammals that have been gone extinct. That's not a huge number, but it's the slope of the line that's pretty scary.

Right? If that slope continues or potentially gets steeper, we're going to be losing a lot of species. And remember, the fossil record tells us that it takes 5−10000000 years for similar levels of diversity to return. Now, of course, the species that go extinct, once they are gone, they are gone. But 5−10000000 years of living just with fewer species on the planet, I don't know.

It makes me kind of sad. Alright. Now with that, see you in the next video.

During the late Pleistocene, between 50,000 and 10,000 years ago, about 65% of megafauna species (animals with mass over 44 kg, or 100 lb) went extinct around the world. Australia and the Americas had the highest megafauna extinction rates with 100% of mammals over 1000 kg (~2,200 lbs) going extinct. Many scientists consider these extinctions as the leading edge of the world’s 6th major extinction event. If the causes of the late Pleistocene extinctions and the causes of the 6th major extinction event are linked, which of the following statements about the Pleistocene extinctions is most likely true?

• A. Volcanic activity in South America led to widespread ocean acidification.
• B. Modern humans moved into new areas at the end of the Pleistocene, notably into Australia and the Americas.
• C. A large asteroid impact in what is today Northern Canada occurred during the late Pleistocene.
• D. An adaptive radiation of new species forced these resident species into extinction.