Welcome to this lecture of the Origins course, where we will be looking at mass extinction events. Mass extinctions are catastrophic events in the history of our planet, where a large percentage of species disappeared within a relatively short time span. This meant that dominating groups of organisms disappeared, but also allowed new ones to radiate, drastically restructuring the fauna and floras of the Earth, while redirecting the course of the history of life on the planet. My name is Bent Lindow, and I am a vertebrate palaeontologist and educator at the Natural History Museum of Denmark. This first video will review what defines a mass extinction, and we will briefly look at the history of how scientists came to recognize their importance in the history of life on Earth. Finally, we will look at how geologists and palaeontologists study mass extinctions. The second video will take us on a journey back to the greatest mass extinction of all time some 251 million years ago, at the end of the Permian Age. Here around 90% of Earth's species disappeared in just a few tens of thousands of years. Finally, the third video will take us back some 65 million years and look at the Cretaceous-Tertiary mass extinction, which claimed two thirds of Earth's species. We will travel to a locality in eastern Denmark, which preserves physical evidence of this disaster. Extinction is pretty much the norm for all species on Earth, and eventually a species will either evolve into a new one, or go extinct. 98% of all known species are extinct. They are only known from fossils. Through the lens of evolution, extinction is a breakdown of evolution, so to speak. Evolution by natural selection is a process, whereby a species is continually adapted to its environment and changes in it. However, if the environment changes too rapidly for the species to adapt successfully, it disappears. Evolution breaks down, so to speak. And as Earth's environments continually change due to changes in climate and suchlike, extinction is perfectly normal. It has been estimated that within a period of 1 million years, between 5 and 10% of the all living species disappear. This is called the background extinction level. At the same time, new species evolve, and the average duration of a species is set at 5 million years. However, this can vary between groups of organisms, ranging from durations of 15 million years to as little as 100,000 years. Mass extinctions are events or "biotic crises", where extinction rates suddenly peak above the normal background extinctions. They are sudden short periods, where the amount and diversity among living organisms plummet. The rate of extinction vastly outmatches the origin rate of new species. In other words, many more species disappear than newly evolved within a short time span. To qualify as a mass extinction event, geologists and palaeontologists generally agree that it must display these common features: More than 30% of Earth's species must disappear. The event must encompass a broad span of ecologies and groups of organisms, both in the sea and on land. It must be worldwide. And it must happen within a short time span. Extinction level is significantly higher than background extinction rate Before we delve into how modern scientists study mass extinction, it is worthwhile to look at how the concept of mass extinction developed in the geological and palaeontological sciences. Beginning in the early 1800s, geologists had started to subdivide the past into a succession of different time periods. At the time, the absolute age or duration of these periods was not known, as radiometric dating would not be developed until the 1950s. Instead, the subdivisions were based on the kind of fossils found in various rock layers. It was quickly noted that different species, kinds of organisms and also faunal combinations characterized different time periods. As one moved from one set of deposits or period to another, new taxa and organisms appeared, and the previous ones disappeared. Indeed, a given taxon or species was only found in one specific time span and would never reappear again later in the geological column. This became known as the principle of faunal succession - specific sets of various plants and animals living in one time period being replaced by new sets. The question of why species disappeared rose quickly. This gave rise to two revolutionary insights: one was the notion of extinction - that a species or organism could disappear completely from the surface of the Earth - and also the notion of catastrophism. One of the main proponents of the idea catastrophes or properly termed "revolutions" in the prehistory of the Earth was the French zoologist and palaeontologist Georges Cuvier. Amongst others, in his time Cuvier founded the science of comparative anatomy, and showed conclusively that species could even become extinct. Now, Cuvier also worked on deposits in the area around Paris. He pioneered the use of fossils as tools to correlate between geographically disparate geological deposits. Here, he noted that layers containing the fossil remains of land-living animals were topped by layers containing the fossil shells and skeletons of sea-living animals. He also noted that there were large differences between the land-living animals from one layer to another, while the marine animals changed little from layer to layer. Cuvier used this to explain the extinction and faunal successions of groups of organisms with catastrophes. From time to time a worldwide revolution - probably a flooding event, where the sea rose, as indicated by the fact that these events impacted much harder on terrestrial than marine animals, would sweep over the planet, clean out and eliminate many species. Following the catastrophic revolution, a new set of organisms would appear or be introduced. These would then exist for a period of time, until they too were eradicated by a new catastrophe. And so forth. Cuvier died in 1832. And in his final years, another opposing viewpoint about the succession of life on Earth appeared which quickly came to dominate at least the English-speaking part of geology: gradualism. Between 1830 and 1833, English geologist and lawyer Charles Lyell published the three volumes of his hugely influential book Principles of Geology. In these books Lyell argued that only modern, observable phenomena were needed to explain processes in the Earth's past - anything else was dangerous, un-scientific speculation. Lyell's argument can be simply summed up as "the present is the key to the past". In order to explain observations from the geological past, we should invoke natural processes in the present by finding similar observations. For example, on the wall behind me is a slab of rock with a number of parallel ridges on it. The ridges are asymmetric: one slope is longer than the other. All of the long slopes on all the ridges all face in the same direction, while the short slopes all face in the other direction. To explain these, I should go out and find a similar form in recent nature: current ridges at the bottom of present-day streams. They are made of loose, unconsolidated sand, but have the same shape as the fossilized ones on the surface of this rock. By observing the present, I can explain the past: in this case what I have behind me is a piece of the fossilized bed of an ancient stream. Lyell elaborated much more on his concept, which he named the principle of uniformity or uniformitarianism. However, he actually used uniformitarianism in four different ways in his books: The first was Uniformity of law: The laws of nature - such as chemical processes, the force of gravity and so forth - have been constant through time. They have not changed. Second was Uniformity of process: i.e. only observable modern phenomena and processes should be used to interpret the past. This is also known as actualism - we use processes observable in the present to explain events and processes in the past. Thirdly: Uniformity of rate: Processes in the past must have occurred at the same rate and scale as today. It is not allowable to imagine a process being speedier or larger than anything observable in the present. This notion is also known as gradualism. And fourth and finally: Uniformity of state: Lyell considered that change on Earth was both in a dynamic balance and yet cyclical - there is no progress in our planet's evolution. At any given time the climate, volcanism, type of deposition and types of organisms are in some kind of balance. Although the overall balance would change through time, at some point it would gradually revert back to a previous equilibrium. For example, fossils indicated that the present Tertiary era was a relatively cool period with large mammals dominating the land and sea, with birds in the air. This had superseded the Mesozoic era, which was a warm period, with large reptiles, dinosaurs, dominating the earth, the sea and the skies. Lyell contented that further back in time - in the Silurian before the Mesozoic - the climate would have been cooler and mammals would have been the dominant life-form once more. Needless to say, Lyell's fourth principle of "uniformity of state" was quickly refuted in the nineteenth century: fossils showed that life on Earth had irreversibly progressed from simple to more complex forms and there were no mammals in the Silurian. But Lyell's first, second and third principles were to remain well-accepted amongst English-speaking geologists for over a hundred years. It was Charles Lyell's third principle - "uniformity of rate", which was to completely eradicate the notion of catastrophes and mass extinctions from most of the geological and palaeontological world in the nineteenth century and most of the twentieth. Uniformity of rate became instead the principle of gradualism. This meant that change in the past could only have happened at the same rate and scale, as they do in the present. And this completely precluded any large-scale catastrophes or revolutions resulting in the mass disappearance of species, simply because none had been observed in the present. Thus dominating animal groups or faunas did not abruptly disappear, but instead gradually gave way to others. The gradualist viewpoint was supported, amongst others, by the discoveries made by the Danish palaeontologist Peter Wilhelm Lund. Lund excavated a number of caves in Brazil in the 1830s and 1840s. Here he found a number of large extinct animals, like this giant ground sloth and armoured glyptodont behind me. Now Lund began his career as a catastrophist, thinking that the skeletons were the remains of animals alive during a previous age, which had abruptly ended in great revolution or turnover. However, as his investigations continued and Lund excavated more and more caves, he began to find the fossil bones of species that were still alive in South America along the extinct ones. And when Lund finally found the bones of ancient humans in the very same layers as those of the giant extinct mammals, he completely changed his mind: there had been no major revolution or catastrophe, which abruptly ended the age of the giant mammals. Instead the age of the giant beasts had gradually changed into the current age of humans. The findings of Lund and other researchers, along with not a little bit of polemics by Lyell and his advocates, meant that gradualism became single ruler of the scientific roost amongst English-speaking geologists and palaeontologists. So the idea of large-scale simultaneous extinctions of many species, possibly resulting from upheavals on a global scale - mass extinctions so to speak - was completely dismissed. Only a few French and German palaeontologists would occasionally propose the notion again during the nineteenth and most of the twentieth century. This state of affairs would continue until 1980, when a seminal paper was published in the journal Science: In boundary layers between the Cretaceous and Tertiary periods, a team led by physicist Louis Alvarez had discovered vastly increased level of the rare element iridium. They proposed that the iridium was dust from an extra-terrestrial impact - an asteroid strike - on Earth some 65 million years ago. They also proposed that this extra-terrestrial impact was responsible for the abrupt extinction of the dinosaurs and many other species at the boundary between the Cretaceous and Tertiary periods. Now this paper immediately spurred many more investigations by geologists and palaeontologists, eager to either refute or prove Alvarez' and colleagues' contentions. Over the following years this resulted in more and more evidence for an abrupt extinction event, during which between 50 and 65% of Earth's species perished within a geological instant. And thus, the notion of mass extinctions came back in vogue amongst geologists and palaeontologists studying the Earth's history. Today, the idea of mass extinctions throughout Earth's history is well-established and well-documented. This timeline goes back some 542 million years ago, to the beginning of the Phanerozoic Eon. The many currently recognized mass extinctions are each shown with a skull-and-cross-bones signature. Five of these mass extinction events stand out as being exceptionally severe, claiming between 50 and 90% of Earth's species. They are known as "The Big Five" and are times in the planet's history when the global fauna and flora was dramatically restructured. We shall visit two of them in the following videos; first, the greatest of them all some 251 million years ago, at the end of the Permian era; second, the last of the big five, which took place some 65 million years ago at the boundary between the Cretaceous and Tertiary periods.
