All of us have heard about extinctions of dinosaurs, mammoths and dodos  

The fossil record tells us that life extends about 3.8 billion years ago. Through this immense period of time we see that there've been many radiations and many extinctions but what stands out is that there were five big mass extinction events that affected biodiversity on our planet, and we know today that these extinction events shaped the biodiversity that we see today. In the next five weeks on our course

Extinctions: Past and Present we will talk first about the emergence of life and we'll see what the fossil record tells us about the origin of life and how life diversified. In the second week we'll begin to talk about the big five extinction events, how each of these extinction events affected life at the time and forced extinction events that led to other diversifications. In the last week of the course we'll talk about the current extinction event that is facing biodiversity today. In this course we will have the opportunity to talk to many different scientists, paleontologists, ecologists, botanists, biologists, ornithologists and each of them will share with us their research expertise about the threats that biodiversity is facing today.

In the end I hope you will be able to appreciate how these current extinction events are being driven by humankind, how we can curb the onslaught of extinction of modern organisms, and in the end, hopefully we'll be better stewards of our planet's biodiversity.

Welcome and introductions

I am Anusuya Chinsamy-Turan, a paleobiologist based at the University of Cape Town in South Africa. I am fascinated by the amazing biodiversity of the present and of the past. I especially love exploring how life on earth has been shaped by the mass extinction events that occurred in the distant past. Paleontologists, using the fossil record, study this history of the development of life on earth.

In the course, we will closely examine at the five major mass extinction events and how they shaped the biodiversity of the time. At present we are facing the prospect of a sixth extinction event. This represents a crisis for current biodiversity, and we will have conversations with various scientists about how they understand what is happening to modern organisms and ecosystems.

Although we are talking about events which happened in the far distant past, the evidence from fossil record can reveal an extraordinary amount of information; and there new discoveries are frequently made.

Each week I outline the significant developments involving the extinction and radiation of life, and interview scientists who research how we know this. Here radiation refers to the proliferation of species over a relatively short period of geological time. The five weeks of the course are:

Week One - The overview of a history of life on earth

Week Two - The first and second mass extinction events

Week Three - Movement onto land, and the largest mass extinction event

Week Four - The fourth and fifth mass extinction events

Week Five - Threats facing organisms today

My own research is in the field of palaeo-biology, which involves reconstructing extinct species as once living animals. I do this by studying the microscopic structure of the bones of extinct and extant vertebrates. In parallel with my career, I have always been passionate about communicating science. I have added some links below to some of my recent research including determining the sexual differentiation of giant extinct birds, deducing dinosaur growth patterns from fossilized bones and some even more recent work on the Australian Thunder birds that I have been involved in.

https://ugc.futurelearn.com/uploads/files/0f/8c/0f8c2120-7fa6-4c21-972c-a69a8e67c3f3/Extinctions_charts_annotated.pdf

Besides telling us about the origin and diversification of life on Earth, the discontinuities in the fossil record also tell us about the catastrophes that life encountered, the so-called extinction events. So it is very important that we all realize that extinction is a normal part of life on Earth. No species lives forever so there's always this background extinction rate that we see in the fossil record. The background extinction rate may be only ten to twenty percent of species per million years. If you break this down even further, we see this means that 10 to 20 species out of every 100, every million years will go extinct.

This is actually a very low scale and, against this background of normal extinction rates, a few more severe extinction rates stand out quite starkly. These are the five big extinction events, they are called the Big Five because these extinction events annihilate more than fifty to seventy-five percent of all living species at the time. The first of these big five extinction events occurred at the end of the Ordovician about 443 million years ago. The second biggest extinction event occurred at the end of the Devonian, about 373 million years ago. The third one, the end of the Permian, occurred about 252 million years ago. This one is considered the biggest extinction event that ever occurred.

It was soon followed by the end of the Triassic extinction event that occurred about 208 million years ago and then the last one, the last of these big five, is known as the end of the Cretaceous extinction event. This one I'm sure most of you have heard about, it occurred 65 million years ago and this is the one that ended the reign of the dinosaurs on Earth. The most common features of the big five extinction events are that many species, more than fifty to seventy-five percent, go extinct.

The extinct species spanned a wide range, there's no selectivity in terms of whether it's marine or terrestrial habitats that are being affected, and the other common feature is that they have been within a relatively short time frame which may relate to a single or cluster of causes. The other thing about each of these extinction events, and we will see this very clearly in the lecture series, is that after each of these extinction events there is a dramatic turn over in the organisms that are present and the organisms that become dominant thereafter.

Besides telling us about the origin and diversification of life on Earth, the discontinuities in the fossil record also tell us about the catastrophes that life encountered, the so-called extinction events. So it is very important that we all realize that extinction is a normal part of life on Earth. No species lives forever so there's always this background extinction rate that we see in the fossil record. The background extinction rate may be only ten to twenty percent of species per million years. If you break this down even further, we see this means that 10 to 20 species out of every 100, every million years will go extinct.

This is actually a very low scale and, against this background of normal extinction rates, a few more severe extinction rates stand out quite starkly. These are the five big extinction events, they are called the Big Five because these extinction events annihilate more than fifty to seventy-five percent of all living species at the time. The first of these big five extinction events occurred at the end of the Ordovician about 443 million years ago. The second biggest extinction event occurred at the end of the Devonian, about 373 million years ago. The third one, the end of the Permian, occurred about 252 million years ago. This one is considered the biggest extinction event that ever occurred.

It was soon followed by the end of the Triassic extinction event that occurred about 208 million years ago and then the last one, the last of these big five, is known as the end of the Cretaceous extinction event. This one I'm sure most of you have heard about, it occurred 65 million years ago and this is the one that ended the reign of the dinosaurs on Earth. The most common features of the big five extinction events are that many species, more than fifty to seventy-five percent, go extinct.

The extinct species spanned a wide range, there's no selectivity in terms of whether it's marine or terrestrial habitats that are being affected, and the other common feature is that they have been within a relatively short time frame which may relate to a single or cluster of causes. The other thing about each of these extinction events, and we will see this very clearly in the lecture series, is that after each of these extinction events there is a dramatic turn over in the organisms that are present and the organisms that become dominant thereafter.

The five big extinction events

We startk by getting a sense of the immense age of life on earth as deduced from the fossil record. We observe that throughout the earth's life-history, life forms have become increasingly diverse and complex, and later in the week, I will interview a microbiologist about microbes and their successful reign on earth.

Ongoing low-level extinctions form a basic part of the development of life. We know from the fossil record there is a 'background extinction' rate in which 10-20% of species per million years disappear. Against this background extinction rates, there are a few catastrophic extinction events, which punctuated the history of life on earth. These events were devastating for many species, but are also associated with creating conditions for major environmental changes and the emergences of many new forms.

Five mass extinction events really stand out as having had a profound effect on the development of biodiversity. During each of these events more than 50- 75% of all species living at the time were wiped out.

The five mass extinction events that interrupted life on Earth, starting from earliest to most recent:

End Ordovician 443 million years ago (mya). Also known as the 'Ordovician-Silurian' mass extinction

End Devonian 373 mya. Also called 'late Devonian'

End Permian 252 mya. Biggest extinction event - also known as the 'great dying' almost destroyed all life on earth

End Triassic 208 mya. Also know as 'Triassic-Jurassic' mass extinction

End Cretaceous 65 mya. Also know as 'Cretaceous-Tertiary' or 'Cretaceous-Paleogene' extinction event

Extinction events chart

Two defining features of the mass extinctions are that many species go extinct within a relatively short time frame (caused by one or a cluster of causes) and that the extinction affects a wide range of habitats and species. The impact of the extinction event is always that there is a dramatic change in the organisms which come to dominate in the phase immediately after the extinction event.

A glossary of terms

There are a lot of scientific names used in the course, and we have compiled a short that may assist you. It does not include all the terms used, only a selection of the ones that are most important for this course. There is no need to memorise the technical or scientific terms used in the course in order to understand the core course content.