Welcome back today we're going to talk about mass extinctions. These episodic events that wipe out much of life on the planet and play an important role, in life's evolution. We're going to begin by talking about the notion of extinction itself, and of course the most famous extinction, the loss of the dinosaurs. Then talk about the Big Five Extinctions. The great modern extinctions. Modern, in the last 500 million years. That shaped the evolution of life on the planet. Then talk about the ongoing process of extinctions that are happening right now. And finally, turn to the question of the fate of humanity. And speculate about when we, will become extinct. The notion of extinction itself is in some ways a radical notion. Today, the typical two or three year-old seems to know about dinosaurs, be fascinated by them. A fascination that for many continue, throughout the rest of their lives. And the idea that there were, was ancient life and this ancient life is now here no more. It seems to be just common knowledge. Yet, when the idea was proposed, 200 years ago, it was a radical notion. Early biologists like Linnaeus, classifying life on the planet, didn't think about extinct lifeforms. Scientists and everyday people, assumed that the life that was on the planet today. Represented the diversity of life through all time. It was work by Georges Cuvier, a French scientist working at the time of the French Revolution, that presented and developed this radical idea. This idea of catastrophism, the idea that the Earth episodically underwent radical change. What Cuvier was doing, he began by studying elephant skulls. He looked at skulls of the Asian elephant, and the skull of an African elephant. And he noticed, that the different elephant types, had distinct teeth. And that he could classify the elephants, by their teeth. And he was able to show that you had, two distinct species. He then went on, to study other elephant skulls. Skulls that were in the collection in Paris, and skulls that were described to him, by his correspondents. And Cuvier recognized, that some of the skulls were from animals that were no longer alive. He identified mastodons, giant elephant-like creatures, that roamed both Europe and North American but are now extinct. And it was Cuvier's work, that opened people's mind to the idea that there are life forms that existed in the past that are no longer here today. So let's go back and talk about some of these life forms and begin by thinking about what life was like, during the age of the dinosaurs, the Cretaceous Age. Life back then was warmer, fern like plants covered much of the Earth's surface, it was time not just of ferns but also of the first flowering plants. There was a rapid co-evolution taking place of both flowering plants and bees. As bees were able to fertilize the plants, the plants were able to spread by fertilization. The plants fed the bees, and these two grew together. And are actually a wonderful example of co-evolution. We like to think of ourselves as central to life on the planet. Yet not, there weren't humans then. Not only that, while there were mammals, they were relatively unimportant. This world of the Cretaceous however, changed very dramatically one day 65 million years ago. When all, it seemed to be all life was wiped out. Now the Cretaceous is just one of many different geological ages. Geologists, like to classify time, in terms of different periods, or ages. Epochs. The Cretaceous Period, was the last period of what's called the Mesozoic Era. It was preceded by the Jurassic and the Triassic Periods. Each one of these, periods had distinct lifeforms that dominated the Triassic world, the Jurassic world, the Cretaceous world. Today we live, in the Cenozoic Era. And our modern time, is called the Holocene. I like this illustration to show us the notion of geological time. First to remind us, how old the Earth is. The Earth is about 4.6 billion years old, it's first half billion years was a period of heavy bombardment. Life emerges here, the atmosphere becomes dominated by oceans, by oxygen. About at halfway through the Earth's life to date. Multicellular life only starts to emerge here, in relatively recent times. And complex life form, have only existed, for about the final 10% of the history of the earth. With the first land animals appearing here and our dinosaurs, dominating for about 230 million years ago to about 65 million years ago. One of the nice things about this figure, is it reminds us, that the zetazoic. Represents only about 1% of the history of the Earth, and the age of the dinosaurs, was not so long ago. Why did the dinosaurs go extinct? Well, we know that there's, we look at the geological record. There was this very distinct transition, about 65 million years ago, at a boundary in the Earth's geological record, called often the Cretaceous/Tertiary or the K-T boundary. K being the letter used to symbolize Cretaceous because sea was already taken by an earlier age. This is also often called in the literature, The Cretaceous Paleogene boundary or the K-T boundary. The Paleogene is really a more modern name to refer to the this transition, or the time just after this transition. When we look at the geological record we see a very dramatic change. Before the K-T boundary, we see, not just dinosaurs, but life forms like, these Terrasaurs, these pretty frightening looking flying reptiles. All these things go extinct, about half the species that are found in the geological record, below the boundary disappear at the boundary. And while the most dramatic changes of course and the loss of the dinosaurs, we also see the loss of lots of marine life forms. There are many forms of, say, plankton, that you find below the boundary, and they disappear completely, when you cross the boundary. Something dramatic happened 65 million years ago. A key piece in understanding that transition, was work done by Walter and Luis Alvarez, a father-son team. Walter Alvarez had won the Nobel Prize for earlier work in physics, working together with two Chemists, Frank Asaro and Helen Michel. And they were looking at the geological record looking at this boundary layer, between the part of the geological record that had dinosaurs, the Cretaceous period. And the more modern period, that didn't have dinosaurs, or any of the other life forms we talked about. They found a thin transition boundary layer, made of clay. When they took this clay back to their lab, they found a very surprising result. This clay layer, was rich in Iridium. Why Iridium? Well, Iridium is a rare earth, it's not very common on the Earth's surface. Iridium is a very heavy element and as the Earth formed and cooled. Most of the Iridium on the planet, sank to the Earth's interior, and the Earth's core is rich in Iridium, but we very rarely find Iridium at the Earth's surface. However when we look at meteors, of course we think they're, we were sampling lots of asteroids we see the same thing. They are rich in Iridium. They would when the meteors formed think the Iridium had not fractionated out. And when a meteor comes in, it deposits a lot of Iridium. What Alvarez and his collaborators concluded, was a giant meteor struck the Earth, about 65 million years ago. And when it struck the Earth, it scattered Iridium and other materials, all over the Earth. And that this destructive explosion, is what killed the dinosaurs. This idea received greater confirmation, when geologists discovered the Chicxulub asteroid. And where this Chicxulub asteroid hit as a meteor, here, in Yucatan. The Yucatan peninsula, is this thumb that sticks up from Mexico. Into the Gulf of Mexico, located right here, and what the geologist found, was an impact ring. And they found all around here sinotes and sinkholes that marked out a trough. They then found, when they looked in the ocean basin here, they also found signs of this crater hit. And I think there's now a consensus that a meteor struck the Earth here 65 million years ago. And when this happened, it not only kicked up lots of dust and water, but it created devastating effects through out the Earth. It's immediate effect, was to generate an enormous title wave a mega tsunami over 100 meters high. That swept through the Gulf of Mexico into the southern portions of the United States and into Mexico. This explosion generated tremendous amount of heat, probably stimulated volcanic activity throughout the planet. Certainly lit lots of wildfires as the explosion propagated through the Earth and the Earth's atmosphere. These wildfires, generated lots and lots of dust, kicked up this dust into the atmosphere. These enormous explosions throughout the planet. Filled the upper atmosphere with dust. This dust layer blocks sunlight and after this explosion, the earth was probably in darkness for several years. This killed much of the plant life and when the plant life died, the animals that relied on that plant life that ate those plants. Were wiped out this phenomenon was similar to what people talked about when they thought about the possibility of nuclear winter. During the Cold War, one of the many fears of what would happen associated with a nuclear war. Is that that nuclear war would not only kill much billions of people, when the nuclear weapons hit major cities in North America and Europe. But that the dust kicked up by those nuclear weapons would block the sun. And the survivors of the initial nuclear attack. Would eventually all die, because there'd be no sunlight, no plants, no food. We think that's what life was like for the dinosaurs, probably any life form, or land life form that lived right around here, was destroyed by the tsunami or killed by the initial explosion. But the rest of the planet was probably devastated, not by the direct effects of the explosion, but by the side effects. The wildfires, the volcanic, potentially the volcanic activity. And the additional the dust that was kicked up, and its effects, on the plant cycle. The evidence for this, these effects have grown, over the past decades there's very clear geological evidence for the tsunami. Here's a figure showing where we think the tsunami, the extent of the tsunami, and studies of the geological record at various sites along this boundary, find clear evidence for the tsunami bed. We find microtektites and shocked quartz evidence for a powerful explosion that kicked material up when the meteor struck near Yucatan. We see through out the planet a coal layer, showing signatures of a mass fire. We find this thick clay through out the planet, but thickest in the south-east portions of the United States, where the material from this explosion landed. Because it was so close to the Yucatan. In retrospect the idea that we had a violent event, that devastated the Earth shouldn't be surprising. When we look around the solar system, we see violent events happening all the time. Where they were looking at ice at the poles at Mercury past violent events like that lead to Venus' slow retrograde motion, the Earth's massive moon, or Uranus' tilted spin. The craters on moons like Phobos or some of the moons we explored on the outer solar systems, all show evidence that meteor impacts are common. And I think for many of us the most dramatic case that convinced us that this is happening all the time is looking at what happened to Jupiter. When the Shoemaker-Levy comet, plowed into its atmosphere, creating enormous explosions. Now Jupiter, we think, is not a planet that holds life. But the fact that we see these dramatic collisions, are a reminder that these are ongoing events. And we should not be surprised that 65 million years ago. A meteor hit the Earth and was able to do such devastation. So, I want you to think about the rates of these events, and think about how often they might occur, on other systems. And think about what that might mean for evolution. And then we'll come back and we'll talk more about these extinction events.
