The big picture

Scientists generally agree that 4.6 billion years ago planet Earth formed from the dust surrounding our sun (that dust may have contained the complex organic molecules necessary for life). Then 3.6 million years ago simple single celled organisms appeared (probably bacteria) and 3.5 million years ago the simple single celled life form split into two different types of cell – and so the origins of biodiversity began. In those 3.5 million years life on earth went from the simplest life forms (Figure 1) to the complex diversity of life we see today (Figure 2).

Figure 1. Simple single celled organisms.

Figure 2. Complexity of life on earth.

The process that drives this long slow increase in diversity is evolution, through natural selection. As the planet evolved a plethora of habitats became available for the new life forms, organisms spread far and wide and slowly adapted to make the best use of all possible niches.

Planet Earth was not then, and is not now static. Even when all the habitats had been fully exploited new ones were developing as the restless earth destroyed others.

1. The earth tectonic plates move apart (divergent plate margins) and as they do they create new land. E.g. Iceland straddles the Mid Atlantic Ridge which is moving apart at about 2.5 cm/year and as it does magma wells up to create new land and so Iceland grows.
2. As the plates come together (convergent margins) oceanic crust is destroyed. E.g. the Nazca plates disappears under the South American plate.
3. Mountain chains are pushed up by the collision of continental plates E.g. the Himalayas.
4. New islands are formed due to volcanic activity at hotspots (Hawaii) and at island arcs – where the Pacific plate dives under the Eurasian plate the archipelago of Japan was formed.
5. New landforms where rivers enter water bodies and form deltas – deposition at the delta creates salt marshes and extends land out into the water body. E.g. Nile Delta.
6. Lakes are very transient features in the landscape and as time passes they fill up with sediment and create new habitats.

Planet Earth is dynamic and it changes constantly. There are two possible responses that life can make to these changes:

• Adapt and diversify. If the change is slow and progressive, life can adapt to the new conditions and biodiversity increases.
• Go extinct. If the change is too rapid, evolution is often too slow to keep pace and some organisms will be pushed to extinction. Five such events have occurred in the history of Earth they are call the Mass Extinctions. Many scientists name a sixth mass extinction – one that is driven by man.

The Story Of Earth And Life - Full Documentary- Blue Planet (https://www.youtube.com/watch?v=5e0hrstSn_I)

Evolution

Figure 1. The evolution of man.

Charles Darwin and Alfred Wallace introduced the idea that if you take any two creatures on earth and trace their ancestry back far enough you will find a common ancestor. If you could time travel back along the human line you would find a branch where we separated from chimpanzees between four and six million years ago. Travel forwards along the chimpanzees line and there will be a fork where they split from bonobos about three million years ago. Going back again to twelve to thirteen million years and you will find the point at which the simian group started to form (Figure 2).

Figure 2. Simian family tree.

If you continue your journey back 3.9 to 3.7 million years ago you will find the common ancestor of all life on earth - simple-single celled organisms. Evolution is the process that drove these and all other changes in the life on Earth.

Darwin published his theory of evolution in 1859 “On the Origin of Species” in which he laid out the evidence for his theory but it was not until the 1950s that the scientific community came to a consensus as to the mechanism for evolution – natural selection.

So what exactly is evolution?

Evolution is the change in the genetic composition and therefore the heritable traits of a population over successive generations and it can impact individuals and whole species. In a nutshell: evolution is descent with modification. That raises another question – what causes these modifications?

Theory of Knowledge

Discuss the ways of knowing Charles Darwin may have used to develop the theory of evolution.

A documentary about Charles Darwin's evolutionary theory (https://www.youtube.com/watch?v=SWh_Hj82qWM)

Alfred Wallace also proposed natural selection after his experience over many years of researching in Indonesia. The following video is a dramatisation of his work and life.

The Forgotten Voyage: Alfred Russel Wallace and his discovery of evolution by natural selection (https://www.youtube.com/watch?v=Z1eQ6DadodA&t=20s)

Causes of evolution

The initial cause of evolution is the process of genetic variation, which is then passed on to the next generation because it gives an individual some kind of survival advantage. It is caused by a number of interlinking factors the most important being mutations and natural selection.

Mutations

• A mutation is a change in the DNA (determines everything about an individual) and it can be a single change or multiple changes. Evolution is usually the result of an accumulation of several mutations.
• Mutations are totally random and they may or may not manifest themselves in the individual.
• They may be beneficial, harmful or neutral but they are unconscious. Genes do not mutate to consciously change the individual – they just are.
• Mutations can have small and large-scale impacts. They can cause albinism (Figure 3) or they can make bacteria resistant to antibiotics (Figure 4).

Figure 3. Albino reticulated python.

Figure 4. Methicillin-resistant Staphylococcus aureus (MRSA).

Natural selection

• The gene variation has survival advantages.

The most important mechanism of genetic variation in this subject is natural selection but if you are interested to learn more or you also study biology, click here.

International-mindedness

All humans are the same species - Homo sapiens and yet there are marked variations. Consider what may have caused these changes.

Evidence for evolution

Below follows a few of the pieces of evidence that support the theory of evolution.

Artificial selection

One of the clearest pieces of evidence for evolution is our own success in artificial selection. Humans have taken a wide range of species and “evolved” them for particular characteristics that suit our purposes. Wild grasses have given us all our major cereal crops (wheat, rye, rice, barley etc.). And human domestication of the wolf has created a plethora of pet dogs.

Convergent evolution

Convergent evolution is where similar structures have evolved independently in different organisms without the presence of a common ancestor. E.g. the Sperm whale and copepods have both evolved the same buoyancy control but they are unrelated.

Embryology

Figure 5. Embryo stage of four different species.

Figure 5 Shows the similarities in the embryo’s of four different species, such evidence is taken as evidence for a common ancestor. As embryos develop the similarities are lost as structures differentiate.

Vestigial structures

These are parts of an animal that no longer serve any purpose but they serve as reminders of what the organism evolved from. E.g the Astyanax fasciatus mexicanus (Figure 6) is a blind cave dwelling fish that lost the need for sight many generations ago and yet it still has eyes.

Figure 6. Astyanax fasciatus mexicanus.

Biogeographical distribution

It was the biogeographical distribution of organisms that first prompted Darwin and Wallace come up with the theory of evolution. In particular they looked at the adaptive radiation of the finches on the Galapagos Islands (Figure 7). They noted that a single species would spread into various niches, adapt to different conditions and evolve into a new species. This is also an example of divergent evolution.

Figure 7. Four of Darwin's Finches.

What other local species demonstrate this? Hint look at my Twitter for #Evolution.

Fossil record

The fossil record shows the changes in organisms over time as the organism's structure can be compared to past and present species thus allowing the construction of a tree of life. Nautilus evolved from ammonites.

Be Aware

You should understand how evolution has caused biodiversity, but you must also appreciate that it is only part of a wider picture.

Speciation and natural selection

Speciation

"It is essential for genetic material to be able to make exact copies of itself; otherwise growth would produce disorder, life could not originate, and favourable forms would not be perpetuated by natural selection." - Maurice Wilkins, Nobel Prize for Medicine

This is a fun video showing the concept of speciation very clearly. Who is the narrator supposed to be? It is well worth watching any videos that he has narrated as they are great for this topic!

Definition

Speciation is the formation of a new species through biological processes.

Evolution drives speciation, but something has to happen to encourage changes in part of the population of organisms. That something is usually isolation, isolation can be:

• Geographic isolation: Populations are physically separated and can no longer interbreed (as shown in the video).
• Temporal isolation: Populations live their lives at different times of the day and so do not met to breed.
• Behavioural isolation: They have different mating rituals.

In these ways speciation creates more species which therefore increases biodiversity.

Examiner Tip

Make sure you understand how the concepts of evolution, speciation and biodiversity are linked.

Natural selection

Environmental change gives new challenges to species: those that are suited will survive, and those that are not suited will not survive.

Natural selection is the gradual evolutionary change that results from genetic variation in each generation. The various heritable traits will endow individuals with different rates of reproductive success, which then become more or less common in a population. Traits that increase the survivability of individuals will become more common as their bearers will pass them on to their offspring, and vice versa.

Darwin developed the theory of evolution through natural selection based on empirical evidence, some of which was gathered during his voyages on The Beagle. This was long before our understanding of genetics and so was based on the observable characteristics of organisms. We now know that the observable characteristics are determines by genes.

Theory of Knowledge

To what extent can a theory be based solely on empirical evidence?

The theory of evolution is underpinned by four basic ideas, firstly that species change over time and space, secondly that all organisms share a common ancestor, thirdly that change is gradual and finally that it is driven by natural selection.

The first three points had been known for some time and discussion revolved around the "how?" How do species change gradually over time? How can all the species have originated from one ancestor? Darwin presented an explanation for the mechanism of natural selection as the means by which all these changes can take place:

1. Within a population of one species, there is genetic diversity, which is called variation.
2. The offspring of fitter individuals may inherit the genes that give them that advantage.
3. Due to natural variation, some individuals will be fitter than others and therefore have a comparative advantage.
4. Fitter individuals are more likely to survive long enough to reproduce more successfully than individuals who are less fit.

Variation

Individuals of a species show a wide range of genetic diversity or variation. If they do not, survivability of the species is in question. This variation may be in the physical or behavioural attributes e.g. size, fur colour, eye colour and mating calls (Figure 1).

Some of these variations will benefit individuals in the struggle for resources and certain individuals will survive longer and leave more offspring, thus the occurrence of those beneficial traits will become more frequent.

Figure 1. Variation in zebra.

Inheritance

For natural selection to operate on a trait and have any long-term impact to drive evolution it must:

• Be heritable – this is the case because the variation in individuals is caused by differences in genes (inheritance units).
• Give an advantage in the competition for resources.

Comparative advantage

Many organisms are r-strategists, that is they produce more offspring than can be supported in the local habitat. This causes competition for resources and the individuals with the “best” characteristics will out-compete their contemporaries and are more likely to survive to reproductive age. This is a comparative, not an absolute advantage.

“…as natural selection acts by competition for resources, it adapts the inhabitants of each country only in relation to the degree of perfection of their associates” - Charles Darwin, On the Origin of Species, 1859.

Differential survival and reproduction

Natural selection works on the variation already within a population – it does not create that variation. Variation is caused by random genetic mutations that may or may not be beneficial to an individual. For the individuals with a beneficial trait survival and reproductive success rates are higher and the traits are passed on to the next generation, for those without it chances are lower and the “defective’ trait slowly erodes from the population. Over time the species will evolve.

Survival of the fittest

Survival of the fittest is a very useful concept as it encompasses the most important aspects of natural selection: survival and reproductive success (secure a mate to reproduce and pass your genes on to the next generation). However, what constitutes fitness is variable through time and space; biggest, strongest and fastest may not always be the best for survival. Successful traits in the savanna would not be successful in the tundra, and as the climate warms what constitutes “fitness” will change. Fitness is thus comparative not absolute. Below follows some examples of this.

Longevity is not necessarily useful

Lonesome George was a Pinta Island tortoise that lived to be 102 years old. Although he lived a long time he was the last of the subspecies so in evolutionary terms he was not successful because he failed to pass his genes on to the next generation.

Figure 2. Lonesome George.

Weaknesses are not always weaknesses

In the human population sickle cell anemia is regarded as a weakness because it reduces the oxygen carrying capacity of the blood and can have some complications. Sickle cell is a recessive gene which means it can only cause full blown sickle cell anemia if the gene is inherited from both parents. If the gene is inherited form only one parent the trait does not develop fully and thus does not usually cause any major problems so it would not be selected in or out of the global population through natural selection. However, it does impart some level of immunity to the malaria parasite. So, in areas where malaria is prevalent the sickle-cell trait gives a distinct survival advantage and so is more common than in the general human population. Figure 3a and b show how closely the distribution of sickle cell anemia and malaria coincide.

Figure 3a. Sickle cell anemia.

Figure 3b. Malaria distribution.

Changing advantage

This famous example involves the British peppered moth Biston betularia. There are two variations of this moth – light (Figure 4a) and dark (Figure 4b). Their habitat was the light coloured trees and lichens on which the fed.

In the 1800s the light variation dominated because they were well camouflaged and predation levels were lower, birds could see the dark variety more easily so more of them were eaten and fewer survived to reproduce.

During the industrial revolution pollution killed the lichens and blackened the tree bark thus camouflaging the darker variety. The lighter variety then suffered higher predation levels as the birds picked them off and left the dark ones to reproduce. The balance changed and the darker variety dominated.

That was not the end of it though. With the introduction of the clean air act in 1956 the air became less polluted the lichens re-established themselves and the trees returned to their natural lighter colour. This once again favoured the lighter coloured moth and the balance returned to pre-industrial revolution levels.

International-mindedness

The theory of evolution is an internationally accepted theory within the scientific community. Is it internationally accepted by the religious community?

Plate tectonics

Plate tectonics as a scientific theory is based on the concept of continental drift, which was first put forward in the early part of the twentieth century. The theory explains the motion of the earth’s lithosphere (outer layer) and the impact that has had on the distribution of the continents (Figure 1).

Theory of Knowledge

Consider the role of three ways of knowing in the development of the theory of plate tectonics.

Figure 1. Earth then and now.

According to the theory, the lithosphere is broken up into seven or eight major plates and several minor ones. The plates may be continental plates (lighter in colour and density) or oceanic plates (darker in colour and more dense). These plates move relative to each other along with the continents that ride on them. This movement has significant impacts on evolution and creates many opportunities for an increase in biodiversity:

1. The separation and movement of the continents creates new islands and moves the continents into different climatic zones forcing evolutionary change.
2. The movement creates four types of plate boundaries each associated with different types of activity with varying impacts on biodiversity.
3. Hotspots and other volcanic activity create new land.

Movement of the landmasses​

Watch the video closely – can you recognise any of today’s continents?

​Early land distribution

Between 300 and 100 million years ago Earth had a single massive land mass called Pangaea (Figure 2). A single landmass such as this would have distinct climatic cycles, very different form the present day. The continental interior is an exceptionally long way from the rain bearing winds that blew off the oceans and this would have generated an extremely dry climate. Life would have evolved to deal with the extreme conditions just as life has adapted to the extremes of the Atacama Desert (driest place on earth today).

Figure 2. From Pangaea to today.

The equatorial area of Pangaea would have life forms and diversity similar to that of the Amazonian rainforest. Evidence in the form of coal seams suggests that present day United States and Europe occupied this equatorial position. Coal is laid down in tropical swamps.

Life would have evolved and diversified to fill all the ecological niches available on Pangaea. However the tectonic forces that created Pangaea were still active, so the plates continued to move (Figure 2). The result being that between 175 and 55 million years ago Pangaea broke up into the continents we know today and they started the long haul to their current positions.

During this time climatic conditions would have changed on all continents as they moved through latitudinal ranges. Fortunately for life on earth, plate movement is slow (usually quoted as – the rate at which your fingernails grow) so the organisms had time to evolve and adapt to the new conditions. In some areas this would have reduced the biodiversity as conditions became harsher but in other cases biodiversity would have increased as conditions favoured life.

The change from a single massive ocean (Panthalassa) to several smaller seas and oceans would have had a similar impact on marine biodiversity. Tectonic movement created a mid oceanic ridge where shallow water offered new niches for organisms to occupy. Similarly continental shelves and coastal water would have created new environments and niches. Biodiversity in the oceans would start to increase.

Early departures

Antarctica, Australia and India with Madagascar attached split off from the main landmasses very early in the break up of Pangaea (130 – 110 million years ago). About 90 million years ago India separated from Madagascar and headed northwards where it later collided with Eurasia, whilst Australia and Antarctica continued to their present positions. As a result of this Madagascar and Australia have very unique flora and fauna with a high degree of endemism and are good examples of how isolation can cause speciation.

Definition

An endemic species is unique to a particular location and not found elsewhere.

Madagascar is the fourth largest island in the world with 5% of the world’s plant and animal species – 80% of which are endemic. Some of the species were there when the island split form the mainland and others immigrated from neighbouring landmasses. However they arrived, the species had the island to themselves and they rapidly diversified in to all available niches. To read more about Madagascar go to this site.

Figure 3. World famous lemurs of Madagascar.

Australia, the largest island in the world also has a very high degree of endemism:

• Over 90% in plants, insects and amphibians.
• Over 80% in mammals and reptiles.
• 45% in birds.

In addition to the isolation factor, Australia is a very large tectonically stable landmass with a huge range of habitats for species to exploit. Australia has approximately 140 species of marsupials (pouched mammals), most of which are now endemic. Marsupials first evolved in North America, spread into South America, Antarctica and finally Australia (when they were all connected). The unique thing about the Australian marsupials is that they have evolved instead of the placental mammals and now occupy the niches that the placental mammals occupy elsewhere. Early isolation allowed this diversification to happen and protected the marsupials from the spreading competition of placental mammals on other continents.

Examiner Tip

Questions about biodiversity may focus on the role of isolation as a cause for speciation. Make sure you understand this link and can explain it using good examples.

Figure 4. The iconic Australian kangaroo.

New positions

All continental landmasses are in different position now as compared to millions of years ago. We have already discussed the climatic impacts of the massive landmass of Pangaea but what is the significance of today’s positions?

We all know that different areas of the world have very different climates, caused largely by their latitudinal positions. The only organisms that can survive in an area are those that have evolved and adapted to the temperature, precipitation, and sunlight levels and the seasonal patterns they all show. So a wide range of climatic zones means evolutionary adaptation and a wide range of organisms. Figures 5a-c gives some idea of the range of vegetation in the different climatic zones, vegetation provides habitats and niches for organisms so more plants generally means more animals and greater biodiversity.

Figure 5a. Low latitude tropical rainforest.

Figure 5b. Mid latitude temperate woodland.

Figure 5c. High latitude tundra.

The range of climates seen on Earth provide diverse habitats, niches and food sources, all of which contribute to evolution through natural selection high biodiversity. If Earth had the same climate everywhere it is highly likely that biodiversity would be significantly reduced.

International-mindedness

Consider the differences in the attitude of the religious communities to plate tectonic theory and the theory of evolution.

Examiner Tip

It is important that you understand and are able to explain how climatic variation impacts evolution through natural selection and thus biodiversity.

Plate boundaries

Theory of Knowledge

Scientific knowledge is changing all the time so how can we be sure that this version of the theory of plate tectonics is correct?

Plate boundary action

As the video points out there are three types of plate boundary. The first two types - divergent and convergent plate boundaries have distinct landforms associated with them. They have both had significant impacts on biodiversity – positive and negative. The third plate boundary is where plates slide past each other at a transform boundary. These are rare and the only major one is the San Andreas Fault which has very limited impact on evolution or biodiversity.

Divergent plate margins

As the name would suggest divergent margins are where plates move apart and this creates opportunities for diversification:

• Ocean floors spreading throws up huge underwater mountain chains (mid oceanic ridges). As with mountain chains on the land this creates new habitats and niches that marine life can exploit.
• Oceanic ridges are associated with hydrothermal activity, which change water temperatures over a wide area and form the basis for entire food chains based on chemosynthesis. Check out this for some of the weird and wonderful creatures that live around the Mid Atlantic Ridge.
• As the Mid Atlantic Ridge developed a shallow section of the ocean allowed volcanic activity to break the ocean surface to form Iceland. The split (Figure 1) continues on land, Iceland grows and rift valleys form new habitats.

Figure 1. Thingvellir national park in Iceland: Rift valley.

Convergent plate margins

As plates move together they create mountains, volcanoes, land bridges and massive ocean trenches.

Approximately 90 million years India split away from Pangaea and headed northwards then somewhere between 35 and 55 million years ago it crashed into the Eurasian landmass to form the Himalayas. This huge mountain range created new habitats, diverted rivers and separated populations of animals. The Himalayas have had an indirect impact on biodiversity too. They prevent cold, dry winds blowing from the north making southeast warmer than other areas of the same latitude but areas such as the Gobi much drier. The Himalayas also block the northward passage of the monsoons thus increasing rainfall in Terai region. We have already looked at how climate impacts biodiversity.

Figure 2. The Himalayas.

The Isthmus of Panama formed around 12 – 15 million years ago as the North and South American plates collided and volcanic activity filled the gap. The land bridge effectively cut off the Atlantic and Pacific oceans from each other. This created some of the world’s major ocean currents (North Atlantic Drift and the Humboldt current), which had significant impacts on climate. The formation of the Isthmus had a direct impact on the flora and fauna of the Americas – species started to intermingle and hybridise, some species disappeared due to competition but others evolved to adapted.

Figure 3. Red eyed tree frog. Found from Mexico to Colombia.

Converging plates also create new land in the form of island arcs (Japan) and give rise to ocean trenches - the deepest places on earth (Marianas Trench). All creating new habitats for new life.​

Volcanic activity

Volcanic activity creates new habitats and niches both on land and in the oceans. Any new land created by volcanic eruptions is subject to succession as species colonise the area. E.g. the southern end of the Hawaiian island chain consists of a large active volcano, Kilauea. It’s most recent eruption started in 1983 and has added 206 hectares of land to the island, species invade the area and biodiversity increases (Figure 4).

Figure 4. Succession on Kilauea Crater, Hawaii.

If volcanic eruptions are in the oceans they create new islands and seamounts. We have already looked at the benefits that new land bestows on biodiversity. Seamounts are simply volcanoes that have not broken the surface of the ocean to form islands. This may be because they never reached the surface or because eruption ceased and the volcano has been eroded by the waves. Seamounts provide excellent opportunities for coral reefs to grow because they provide the shallower water necessary for coral to grow (Figure 5).

Figure 5. Manta ray swimming over Raja Ampat seamount, Indonesia.

International-mindedness

Consider alternative explanations of why the continents are where they are today.

How can you research these issues in ecosystems? Use Topic 2.5 to provide ideas. Why is it important to be researching these issues in ecosystems today? Use Twitter #Ecosystem or #Conservation to help you.

The following video is about researching in Papua New Guinea, an area of rainforest which has not been researched very much and is severely threatened by illegal logging and mining.

'Lost Land of the Volcano'

Mass extinction

If you put “mass extinction” into any search engine a number of the results will include something about five mass extinctions in earth’s geological and biological history. The question is what makes them “mass” extinctions? The key factor about a mass extinction is that it is one in which abnormally high numbers of species die out at the same time. Abnormally high as compared to the background or normal extinction rate.

Definition

Background or normal extinction is the standard rate at which species go extinct.

Mass extinction is a sudden global decrease in the number of species over a relatively short period of time.

Background extinction rates vary (Figure 1) between taxonomic groups, dinoflagellates having a very low extinction rate whilst mammals have the highest rates with most of the species only surviving for around one million years.

Figure 1. Maximum mean extinction rates for major taxonomic groups.

The big five

The big five normally refers to the top five African game animals (African lion, elephant, and leopard, Cape buffalo and the rhinoceros), but here it refers to the five mass extinctions. They are all named after geological eras. The most famous mass extinction that almost everyone knows something about is the K-T (Cretaceous – Tertiary) extinction. This was the event that wiped out the dinosaurs. There have however been other more destructive mass extinctions than the K-T one.

Figure 2. The big five mass extinctions.

Ordovician-Silurian mass extinction (440 million years ago)

At this time the majority of life on earth was actually in the oceans so marine organisms were hit hardest by this extinction with over 80% of them being wiped out. The event lasted some four million years and consisted of two peaks of extinction. The peaks seem to coincide with the beginning and end of the most severe glaciation, which in turn was caused by a marked dip in atmospheric carbon dioxide levels.

Late Devonian mass extinction (375-360 million years ago)

There is a great deal of contention over this mass extinction. Firstly there is little scientific consensus about this event. Estimates for its duration range from 500,000 to 25 million years and the causes are even less clear. It is agreed that the late Devonian era was a time of significant environmental change that would have had a big enough impact on life on earth to cause extinction. What triggered those environmental changes is up for debate. It is agreed that this extinction wiped out around three quarters of all species on earth and once again marine life took a hammering. The coral reef biodiversity took over 100 million years to recover.

Figure 3. Modern day coral reef destruction.

Permian mass extinction (250 million years ago)

Also called The Great Dying, this was the biggest extinction event in earth’s history destroying around 96% of all species on the planet. Put another way – that means life’s current biodiversity evolved from the 4% of species that survived! Once again there is a great deal of contention as to the causes of this extinction. Some scientists favour a catastrophic event such as bolide impacts (meteors that burn up in the sky), large-scale volcanism or a sudden release of large amounts of methane form the sea floor. Other scientists propose gradual processes such as sea level changes, falling oxygen levels and increased aridity.

Figure 4. Bolide burning up in the atmosphere changes atmospheric composition.

Triassic-Jurassic mass extinction (200 million years ago)

Not one of the major events as regards actual extinction but very important as regards impacts. In the two or three extinction phases that took place in the final 18 million years of the Triassic period approximately half of all known species went extinct. This vacated terrestrial niches and allowed the dinosaurs to take over. Causes for these events are very hard to determine but favoured explanations include gradual climate change (probably due to volcanic eruptions releasing flood basalts), sea level fluctuations and asteroid impacts.

Cretaceous-Tertiary mass extinction (65 million years ago)

The most recent and the most famous mass extinction occurred when an asteroid hit the Yucatan peninsula in Mexico. Not only did it wipe out the dinosaurs, but some estimates suggest 80% of all life on earth went extinct. The key fact here is that many mammals and birds survived and were able to take over the niches previously occupied by the dinosaurs.

Figure 5. Meteor and dinosaur.

"We'll lose more species of plants and animals between 2000 and 2065 than we've lost in the last 65 million years. If we don't find answers to these problems, we're gonna be victims of this extinction event that we're at fault for." - Paul Watson (Environmental activist)

This quote heralds the sixth mass extinction.

Sixth (Holocene) mass extinction (right now)

International-mindedness

Consider the difference in the rates of extinction between different countries. What causes these difference?

Some people still deny this extinction even exists. The extent of this extinction is extremely difficult to establish and estimates vary wildly. Humans are definitely killing off species and many scientists agree we are doing so thousands of times faster than nature is creating them. The cause of this extinction is not up for debate, it is human activity. Make sure you can express and justify your own opinion on this issue.

Figure 6. Gorilla - just one of thousands of critically endangered animals.

Theory of Knowledge

The Big Five mass extinctions are based on fossil records. How can we be sure a modern animal has gone extinct? Beware the coelacanth!