Hello, Bio 199 students. This is Dr. Halfhill, and this is the third lecture for Chapter 25, where in our first lecture we discussed what are some of the steps that may help us understand the possible origin of life on Earth through things like the production of monomers and the production of protocells, etc. And then in our second lecture, we talked about the history of life on Earth that was mostly...
dominated by prokaryotes, and then we describe the endosymbiont theory as describing how eukaryotes may have formed. Now in this lecture, we are going to discuss the history of mass extinctions on Earth and some of the impacts that we should expect to see after these large events that happen. And then when we return to class, we will finish out chapter 25 talking about what are some of the processes associated with changes in body plant. So off we go involving mass extinctions.
So this is a complicated chart and we will break it down in pieces. But overall, the fossil record chronicles a large number of events and those events give rise to mass extinctions. So there are definite evidences about global environmental changes that happen that are very rapid and being so rapid they're very disruptive and end up leading to the extinction of many organisms at one time.
So the majority of species were wiped away in these events. So when we're looking at this very complicated figure that's down below, the first thing that we'll look at is so what evidence of mass extinctions do we have? So So on the left side axis, this ends up being the extinction rate. So this ends up being the blue line.
So when we look at the blue line, we see that it is very bumpy. And there are definite large bumps that happen. So each one of those peaks or bumps represents a mass extinction event.
And some mass extinction events are going to be larger and more important than others. others. The axis on the right side represents the number of taxonomic families that exist on Earth.
And so this shows the red line. So when we look at this entire figure together, we see that the extinction rate, there are definite peaks. Each one of those peaks is going to be associated with a mass extinction event. But overall, the red line, which is the number of taxonomic families on Earth, earth continues to grow over time. So even though mass extinction events occur, the production of new species through speciation and Darwinian evolution leads to more and more taxonomic families that exist on earth.
So mass extinctions play a very important role because they are going to reset the organisms that are living at a particular time and then allow those organisms to go through evolutionary forces through the processes that we've seen on our first three chapters. And so one of the big ones is going to be called the Permian mass extinction event that we will discuss and another one is going to be the Cretaceous mass extinction event. Now these two mass extinctions are very important.
The Permian which is also called the Great Dying as well as the Cretaceous mass extinction event. And the Cretaceous extinction event led to the loss of many dinosaurs. So in the Permian extinction event, it claimed 96% of all marine animal species and 8 out of 27 orders of insects. So it had incredible impacts in both the ocean as well as the land. It was an incredibly disruptive, challenging time to be an organism on Earth.
Earth. And this Permian extinction event is hypothesized to be caused by volcanic eruptions. Now these volcanic eruptions happened in Asia and very specifically Siberia.
So it was a long time period of volcanism that changed the atmospheric chemistry as well as the water chemistry and made conditions so challenging that it was impossible to even imagine that it would happen. that you had all this death of complicated animal life. So it reset life, the stage, for other things to happen after this.
The other important thing is that extinction event that we'll talk about is going to be the Cretaceous extinction event. Now this doomed many marine and terrestrial organisms and most importantly this is hypothesized to be the cause of the loss of large dinosaurs that existed only leaving the birds. And we believe that this was caused by an asteroid strike.
Um this asteroid strike happened about 65 million years ago and it happened in the Yucatan Peninsula which happens to be where Mexico is at a crater called Chicxulub. And so there is evidence on the northern part of of Mexico of a a very large impact crater. And that impact crater led to very challenging conditions on life that led to a very large number of deaths of organisms that are there.
And in fact, by the Cretacean extinction event, it removed the dinosaurs as the dominant form of life on Earth and made way for the mammals that existed. exist today in what we see under current earth conditions. So we know that mass extinctions are actually quite common in earth's history. So then what happens after that? So this concept is going to be called adaptive radiation.
And in adaptive radiation, there is going to be evolution of organisms that survived the mass extinction event. And those organisms... that survive are going to go through a series of speciations and they are going to fill in all the available niches in this new environment caused by the extinctions of the previous organisms that live there.
So here you have a picture of a bird. Now this bird is going to be carrying seeds on its neck. So birds are able to travel long distances.
So after a mass extinction event occurs, birds are going to move around the environment and they are going to bring seeds from one location to another. And then those seeds are going to go through adaptive radiation, the plants that live there, where they are going to go through a period of rapid evolution and they are going to fill in the niches that we see that are going to be left by the mass extinction event. Thank you.
Now, Hawaii happens to be an incredible place to see adaptive radiation in action. So here's a picture of me on the top of the Maui volcano. If you ever have the opportunity to go there, it's amazing.
Go visit it. So Hawaii is one of the great showcases of adaptive radiation that we have on the planet. And the reason why we know this is that the Hawaiian island chain is a relatively new feature.
in the environment. So if you look across the age of the islands there, Hawaii has only been around for 0.4 million years, Maui has 1.3 million, and as you go to the west, those islands have been there for longer periods of time. And so when a island chain emerges from underwater volcanoes, there would not be any land-based organisms that live there.
So you would have a founding population, like the seeds that are on the neck of that bird, the birds would come, drop off the seeds, and then those founding populations can go through adaptive radiation, evolve rapidly into many different forms. And these forms often exist only in this particular location. And if we go back to our previous chapters, islands are always going to be a unique place to see unique forms.
So all All these different plants that you see here, they look very different from one another. Some have large leaves. Some have these crazy silver leaves with these long stalks. Some look like shrubs.
Others look like these tiny little circular forms. They're all closely related to one another. But adaptive radiation has allowed them to evolve to the unique conditions on each island. and they were able to fill in the available ecological niches that were there.
So after a mass extinction event, the survivors will repopulate the area and then go through evolutionary forces to produce new species, and those new species are going to fill in the available niches. So mass extinction events are common, and adaptive radiation is one of the things that we see after the... extinction events. So this is going to be the last of the recorded lectures and we will return to normal class on Monday. I appreciate your time.
Thank you very much.