Lesson video

Hello, and welcome to this lesson from the Oak National Academy.

Today's lesson is taken from the unit Adaptations, competition, natural selection and evolution.

And the lesson title is "Fossils show that species change over time." Hi there, I'm Mrs. Wheat, and I'll be your teacher for today's lesson.

By the end of today's lesson, you'll be able to explain what fossils are and how they provide evidence that species change over time.

Let's have a look at our keywords.

We have five keywords for today's lesson, fossil, organism, minerals, extinct, and fossil record.

I'm gonna show you the definitions now, but don't feel like you need to memorise all of them instantly.

Take your time.

If you wanna pause the video and copy them down or just read them through, that's totally fine.

So here they are.

Today's lesson is in two parts.

First of all, we'll talk about the types of fossils that there are, and we'll talk about how they're formed.

And in the second half of today's lesson, we'll talk about how they provide evidence that species change over time.

But first of all, types of fossil.

There are two main types of fossils that we're gonna talk about today.

First of all, we're gonna talk about body fossils.

Body fossils are mineralized remains of once-living organisms. We'll talk about that word mineralized in the next few slides.

But mineralized is to do with minerals, which are naturally occurring chemical compounds of elements such as carbon, calcium, silicone, and iron.

And you find them in rocks.

So, again, body fossils are the mineralized remains of once-living organisms. And now fossils take thousands, sometimes millions of years to form.

So I'm not talking about organisms that have lived, you know, in the last few decades or so.

I'm talking about organisms that existed thousands of years ago or even millions of years ago.

So let's have a look at some examples.

So we've got, here we've got a fossilised skeleton of a fish formed thousands of years ago.

You might have seen this shape before.

This is an ammonite.

So this is a fossilised shell of an ammonite, which are organisms that lived in the sea and lived millions of years ago.

They're related to the modern-day species the nautilus, if you've seen that before.

We have our first dinosaur.

How could we have a lesson without fossils without talking about dinosaurs? This is an archaeopteryx's fossilised skeleton.

And here we've got a trilobite.

You might recognise this fossil as well.

Again, this is a species of organism that lived millions of years ago, and this is its fossilised exoskeleton.

So they didn't have internal skeletons like we do, like mammals do.

They had exoskeletons like insects, so hard outer shells.

And those fossilised really, really well, so there's lots of trilobite examples in the fossil record.

We've got an ichthyosaur.

What's really interesting about ichthyosaurs is that tonnes and tonnes of ichthyosaur fossilised skeletons have been found in Britain.

I think we often think about Britain as being quite a boring place with not many interesting animals.

But if we look way, way, way, way back, there were some really, really cool animals here once, and a palaeontologist, that's someone that studies fossils, called Mary Anning, who lived about 200 years ago, discovered a lot of these ichthyosaur skeletons in Britain.

Isn't that really interesting? And finally, we've got some petrified wood.

So it's not just animals that can be fossilised, plants can be fossilised too.

And so here we have a tree that's been petrified, so a form of fossilisation, and it's been preserved as a fossil, luckily so that we can study it.

Okay, so those are some examples of types of fossils.

Let's have a look at how these types of fossils are formed.

So these types of fossils, body fossils, are formed in the following way.

So first of all, you need an organism, something that was alive, and then that dies.

And the soft parts of it, like the skin, they decay.

But the hard parts, like the bones, the skeleton, they remain for a while longer.

So the remains, so the bones, whatever's left after decay has happened, get buried, covered over with a layer of sediment, something like mud or like sand.

So the remains now are buried under this sediment.

Over time, over a huge period of time, so we're talking thousands of years, so those layers of mud and sand build up on top of each other.

And there's so much mud and sand now, there's a huge amount of pressure from the top building down to the bottom, and that pressure turns the layers into sedimentary rock, a type of rock.

Okay, so now our remains are buried under this rock.

Eventually, water will seep into the remains and dissolve what's left.

And here we get the word mineralized again, minerals.

So minerals from the water.

Water that occurs in nature isn't just H2O.

It's got other dissolved things in it as well, yeah? So it's got these minerals, naturally occurring compounds of elements like calcium, like carbon.

So those dissolved solids that are inside the water, the minerals, they replace some of the remains, some of their elements that are naturally occurring in the remains.

And so that's the process of mineralization very briefly.

So when that happens, now we have a fossil that's created.

So what we have with the fossils, we don't have, unfortunately, it'd be amazing if we did, we don't have the exact dinosaur bone, and we don't have the exact piece of wood that existed millions of years ago.

We have this mineralized version of it.

Okay, and then finally, over time, the rock the fossil is embedded in rises to the surface, and that's 'cause of geological activity.

You might already know that the Earth is a bit like a jigsaw puzzle.

There's these huge tectonic plates that the land kind of sits on on top.

And so those shift around, and that moves the rock.

And then the fossil and the rock that it's embedded in rise to the surface, and then the rock, the sedimentary rock that the fossils are embedded in slowly gets eroded away by wind, by rain, you know, by water, wherever it is.

And then part of the fossil could be revealed, or at least that makes it a lot easier when people are trying to dig these things up and not digging down way, way, way, way to the bottom of the sedimentary layer where the fossil was originally embedded in.

It's risen to the top slightly.

Okay, let's see if you understood that process and check your understanding.

Which of these statements is not part of the correct explanation for how this fossil of a fish was formed? Is it A, some parts of the animal decomposed, B, the animal's remains were dissolved by water seeping through the rock, C, the animal's remains turned to rock, D, the animal's remains were replaced by minerals, or E, the animal's remains were buried in sediment? You've got five seconds of thinking time now, or you can pause the video and take a bit more time.

Then click play when you're ready to hear the answer.

Okay, how did you do? Let's find out.

Let's see.

So which of these statements is not part of the correct explanation for how this fossil of a fish was formed? So it is C, the animal's remains turned to rock.

So fossils aren't rocks.

Fossils are the mineralized remains, or body fossils are the mineralized remains of organisms that lived thousands, perhaps millions of years ago.

They're not the same thing as rocks.

They're embedded and buried inside rock, but they're not the same as rocks.

Okay, let's try another one.

Put the steps in the correct order to describe how this fossil was formed.

So A, the animal's remains were dissolved by water seeping through the rock.

B, the animal died.

C, the material around the animal was compacted to form rock.

D, the animal's remains were buried in sediment.

E, the animal's remains were replaced by minerals.

So take five seconds thinking time.

Pause the video to give yourself enough time to think.

Click play when you're ready to see the correct order.

Okay, let's check your answer.

So the first statement was B, the animal dies first.

Then we had the animal's remains were buried in sentiment.

Material around the animal was compacted to form rock.

Next, the animal's remains were dissolved by water seeping through the rock.

And finally, the animal's remains were replaced by minerals.

Well done if you got that right.

The other type of fossils we're gonna talk about are trace fossils.

So trace fossils are imprints that an organism has left behind.

So here we have some imprints of a leaf.

So maybe this leaf fell to the ground onto some wet mud and then was buried in sediment.

There's nothing hard in a leaf, really, to be preserved, and so the soft part of the leaf would have decayed eventually.

But then the imprint in this example was preserved underneath those layers of sediment, which eventually, over years and years and years, turned to sedimentary rock, and then someone found this fossil.

You can have dinosaur footprints an example of a trace fossil.

So similar thing.

A dinosaur's walking through something wet, like wet mud, and then the layers of sediment buries it over millions of years, harden, turn to rock, and then inside that rock was this fossil.

And also, we've got some fossilised eggs.

So fossilised eggs and nests are also example of trace fossils.

They're not the organism theirself, but they tell us a lot about the organism's behaviour and habits.

So those are our two examples of types of fossils and how they're formed, and I wanna talk about some things that aren't fossils.

So skeletons of recently deceased organisms are not fossils.

So here we've got a model of a human skeleton.

First of all, it's a model, so definitely not a fossil.

And second of all, even if it was a human skeleton for someone that had lived 100 years ago, that still wouldn't be a fossil.

The process of fossilisation takes thousands of years.

So in a body fossil, the hard parts of the organism, such as the bone, have been replaced by minerals.

That's a key feature of a fossil.

This process, as I've said, takes thousands to millions of years to happen.

The youngest fossils of organisms that have died that we have found, that we have, are approximately 10,000 years old, okay? From organisms that died 10,000 years ago.

So any remains of organisms that have lived in the last hundreds of years, that won't have had enough time to fossilise.

So that's not an example.

That's why those aren't fossils.

So there are other types of remains of organisms that are very useful to study, but they're not fossils.

And again, they could be from organisms from thousands of years ago, but they don't quite qualify as fossils.

So these include remains of organisms that have been preserved in amber, ice.

Oh, sorry, amber.

Amber is like tree sap or tree resin.

So yeah, also organisms that have been preserved in ice, organisms that have fallen into tar pits.

So tar is part of crude oil, part of oil like how petrol is a part of crude oil, which is what, you know, obviously people put in cars to make their cars run.

So tar is like one of the thickest layers of crude oil.

And in some places on the planet, in Los Angeles, actually, in California in America, you have these huge, big, kind of like swamps where all this kind of tar is.

It's really, really sticky, a bit like quicksand, and once you get into it, it's really difficult to get out.

So sometimes, unfortunately, animals have kind of wandered into these tar pits, not realising, and they've been preserved in those.

So, very sad for the animal, but very, very useful for the scientists studying these animals.

And also, bogs.

Again, similar principle, like boggy, swampy areas.

So just, like, really, really wet, marshy land.

Animal kind of wanders into it, can't get out of it, dies, and then their remains are preserved.

So these are really, really useful to study.

They're not fossils because no mineralization has taken place.

They're not preserved inside sedimentary rock.

But actually, they can be in some ways more useful to study because a fossil is the mineralized remains of something.

It's not exactly the same.

It's not the exact dinosaur bone.

It's not the exact piece of wood.

But something that's been preserved by these methods is a bit like they've just been put in the freezer for thousands of years.

And so it's still the same woolly mammoth body with largely the same chemical composition that it was when it died.

It's just decayed really, really, really, really slowly 'cause they've been preserved in these conditions without very much oxygen.

So, really, really interesting, really useful to study, but not fossils because no mineralization is taking place.

They're not inside sedimentary rock.

Okay, let's see if you understood that.

Which of these could be a fossil? Is it A, this insect that's been preserved in amber, B, this petrified wood, or C, this dinosaur footprint? Take five seconds or pause the video.

Click play when you're ready to see the answer.

Let's look at the answers.

So which of these could be a fossil? B could be a fossil, petrified wood.

Wood can fossilise as well.

And C, footprints.

This is an example of a trace fossil.

So the insect in amber is not a fossil.

There's no mineralization here, and nothing's been preserved in sedimentary rock.

Let's have a go at another one.

Which of the following is definitely not a fossil? So is it A, we've got the skeleton of a creature that looks like a lizard, B, we have a nest that's got some eggs in it, and C, we have a bone which still looks like it's got some flesh on it.

So again, take five seconds or pause the video now to give yourself some extra thinking time.

Which of the following is definitely not a fossil? Okay, it is C.

C is definitely not a fossil.

So in A, the skeleton is fossilised.

We can see that it's in some sedimentary rock.

B, nests and eggs, those can also be fossilised.

C, this looks like a bone that someone's been cooking with recently, so this is not thousands of years old.

That is not a fossil.

Even though it belonged to something that used to be alive, doesn't make it a fossil.

It's not been mineralized.

It's not undergone fossilisation.

Well done if you got those right.

Let's have a go at our first practise task for today's lesson.

So the photo shows a pebble.

It's a type of rock.

Izzy says, "The pebble can't become a fossil because it is already a fossil." Is Izzy correct? Explain why or why not.

Number two, the photo shows a tree.

Sofia says, "Trees can't become fossils, only animals can." Is Sofia correct? Explain why or why not.

And number three, the photo shows a type of lizard called a bearded dragon.

Explain how it could become a fossil.

So you'll need to pause the video now to give yourself enough time to think about your answers and to write them down.

Click play when you're ready to see the answers.

Good luck, everyone.

Okay, let's see how you did.

Is Izzy correct? Explain why or why not.

So Izzy is incorrect.

Pebbles cannot be fossils as they were never alive.

Fossils are mineralized remains of organisms, so a living thing from thousands, sometimes millions of years ago.

Is Sofia correct? Explain why or why not.

Sofia is also incorrect.

Fossils are remains of once-living organisms. Anything that's been alive could be a fossil, including a plant, like a tree.

Okay, number three.

The photo shows a type of fossil called a bearded dragon.

Explain how it could become a fossil.

When the organism dies, it needs to be buried under sediment like mud or sand.

Its soft parts, like the flesh, will decay, leaving hard parts, like the skeleton, behind.

More sediment layers would then need to build up on top of the remains, which increases the pressure.

The layers of sediment would then become sedimentary rock.

Water would then need to seep into the remains and dissolve them.

Minerals from the water would need to be left behind, forming a fossil.

Well done if you got those right, everyone.

We've completed the first part of the lesson.

We've talked about types of fossil and how they're formed.

Now we're gonna talk about how fossils provide us evidence for how species change over time.

Fossils provide evidence about the characteristics of organisms that lived a long time ago.

So often, this is the only evidence we have about these organisms. So we think about dinosaurs.

They existed millions of years ago.

The last dinosaurs died out 65 million years ago.

No one alive has ever seen a dinosaur, but we know so much about them.

If you ask any three-year-old, they'll be able to name you a dinosaur, right? And all that information comes from studying their fossils, not actually seeing them with our own eyes.

Comparing fossils to living organisms shows us how species have changed.

So some fossils are of organisms unlike anything still alive on Earth.

These species have gone extinct, so they're not around anymore.

There are no individuals of that species left alive.

Here in my example, I've got a fossil skeleton of a woolly mammoth.

Again, there's no human on Earth that's ever seen a living woolly mammoth.

Almost everything we know about woolly mammoths comes from studying their fossils.

Studying fossils provides evidence about the original organisms, including size, their shape, body structures, and the time period they lived in.

And sometimes even scientists can tell what colour these organisms have been because of tiny amounts of pigment that have been left behind in the fossil.

So we can draw conclusions about the habitats in which the original organisms lived based on where the fossils are found.

And food chains based on the fossilised remains of organisms trapped inside other organisms. So if we've got one type of dinosaur found inside another type of dinosaur, we can conclude that, okay, well, that small dinosaur, that was part of the bigger dinosaur's diet.

Let's check to see if all that made sense.

Dinosaurs lived on Earth a very long time ago and have been extinct for millions of years.

Where does our knowledge about dinosaurs come from? Is it from A, from photographs of living organisms, B, from observations made by people alive at the time, C, from the fossils of their remains, D, from books and museums? Take five seconds or pause the video to give yourself extra thinking time.

Click play when you're ready to see the answer.

Okay, so the answer is C, from fossils of their remains.

So there's no photographs of dinosaurs.

What looks like a photograph on this slide is an artistic impression of what we think a dinosaur looked like.

B, from observations made from people at the time.

There were no humans alive at the time there were dinosaurs, so it's not like we have writing about this is what a dinosaur looked like.

D, from books and the museums. Okay, but where does the information come from for the books and the museums? It comes from fossils of their remains.

Well done if you got that right.

Fossils also show us that organisms change over time.

Horses are a really well-researched example of this.

There are so many fossilised horse skeletons, including tonnes of examples of complete fossilised skeletons, not just parts of it.

So through fossil evidence, we can see the development of horses from a small mammal, small woodland mammal, about the size of a dog, about 45 centimetres tall, to much larger modern horse, well over a metre in height.

So here we have, again, some of that small, like, forest-dwelling creature slowly developing over millions and millions of years into the modern horse.

So studying the fossilised bones of these animals show us how these species change over time.

So the leg bones got a lot longer, the species got taller.

So here, again, we've got an example of, well, this is an artist's impression of the forelimb of an early horse.

And we can see the toe structure is slowly changing.

The number of toes is different.

The leg bones are getting longer, eventually until we have the leg bones of a modern horse.

So looking at fossils shows us how species change over time.

Okay, let's check our understanding.

What evidence can fossils provide about an organism? Is it A, the organism's shape and size, B, what sort of habitat the organism lived in, C, what the organism might have eaten, or D, what time period the organism lived in? Take five seconds or pause the video to give yourself extra thinking time.

Click play when you're ready to see the answer.

Okay, let's go through the answers.

Could it be A, the organism's shape and size? Yes, we can tell that from fossils.

B, what sort of habitat the organism lived in? Yeah, we can also tell that from fossils.

C, what the organism might have eaten? Yes.

D, what time period the organism lived in? Yes, we can tell that from fossils as well.

Well done if you got that right.

So there are limitations to what fossil evidence can tell us.

So there are gaps in the fossil record because conditions for creating fossils are super, super rare.

We talked about how to make a fossil in the first half, our first part of the lesson.

There's a lot of stages, and along the way, a lot of things can get disrupted, which means that fossil didn't form, you know? If the bones get crushed, there's not gonna be enough left for fossilise.

There's so many different things that could happen that could destroy fossils or just mean that the fossil isn't made in the process.

Remember, the remains need to get buried in a layer of mud or sand.

That doesn't always happen, and so those remains are just gonna end up decaying completely and not be fossilised.

So fossils can be destroyed by geological activities, such as earthquakes, before they're discovered.

So many forms of early life as well didn't have bones, so didn't have skeletons, and they were soft-bodied, like this picture of a jellyfish.

Jellyfish has no bones in it.

Very, very soft.

So that means they can't form body fossils, and it's much, much harder for them to form trace fossils 'cause if they push against wet mud, they're not leaving much of an imprint 'cause there's nothing hard in their body.

Some other limitations of fossils.

So fossils are often incomplete or broken when they're found.

To understand what the original organism looked like, scientists have to interpret the available fossil evidence.

They have to draw upon their own expertise and judgement , and they have to use a bit of imagination.

So this photo here, so these are all the pieces that belong to one fossilised dinosaur skull.

So a scientist had to use all their expertise to be able to recreate this into what they believe that the dinosaur skull originally looked like.

So even the fossils we have can't tell us everything about extinct organisms. So this is because not all members of the species are the same.

There's variation between them.

I'm sure you already know this.

If you're in a classroom, you can look around the classroom.

You guys all look different, yeah? So just because we have one fossil of a Tyrannosaurus rex, it doesn't tell us what every single Tyrannosaurus rex of the species looked like.

If we only had male fossils of a species and we hadn't found out any female fossils of that species, males and females of species can look really, really different, yeah? So that's another limitation for fossil evidence.

Let's see if you understood that.

An archaeopteryx is an extinct animal.

The image shows an artist's idea of what an archaeopteryx looks like based on fossil evidence.

Which of the following statements are true? A, we can be certain what an archaeopteryx looks like based on fossil evidence.

B, the artist might have had to use their imagination and fossil evidence to create the image.

C, there are no living archaeopteryx to look at, so we cannot be certain of what they look like.

Take five seconds or pause the video.

Come back when you're ready to see the answer.

Let's have a look at the answers.

It is B, the artist might have had to use their imagination and fossil evidence to create the image.

And C, there are no living archaeopteryx to look at, so we cannot be certain what they look like.

Well done if you got that right.

This is the final practise task for today's lesson.

Well done for making it here.

So, this fossil of a stegosaurus dinosaur is kept at the Natural History Museum in London, UK.

It has been given the nickname Sophie.

The stegosaurus is a very well-known type of dinosaur.

Again, ask any three-year-old.

They know what a stegosaurus is.

But we have only found fossils from fewer than 10 individuals of stegosaurus.

Sophie is the most complete stegosaurus fossil ever discovered.

So here we've got some pictures or some artists' impressions of what we think stegosaurus might have looked like.

So many different pictures of stegosaurus have been made.

Here are some questions about that.

So number one, the artists of these pictures may have used fossil evidence to help them.

Describe what fossil evidence can tell an artist about the stegosaurus.

Number two, is it possible to decide which picture looks the most like a real stegosaurus? Explain your answer.

Number three, explain why none of the pictures might look exactly like a real stegosaurus.

So, pause the video here.

Give yourself some thinking time, some time to write down your answers.

Click play when you're ready to see the answers.

Okay, let's see how you did.

So the artists of these pictures may have used fossil evidence to help them describe what fossil evidence can tell an artist about the stegosaurus.

So, fossil evidence could tell the artist about the organism's size, shape, structure, habitat, time period they lived in, and food chain.

Number two, is it possible to decide which picture looks the most like a real stegosaurus? Explain your answer.

There's no way to know for sure which picture looks the most like a real stegosaurus.

There are no living stegosauruses left to compare with the drawings.

The artists will have had to use their imagination, at least in part, to create these drawings.

So finally, number three, explain why none of the pictures might look exactly like a real stegosaurus.

Fewer than 10 stegosaurus fossils have been found, so all of our knowledge of what stegosauruses look like come from these few individuals.

These individuals might not be representative of what most of the species looked like.

They might have been all really old or all really young.

There would be variation between individual stegosauruses caused by differences in their genes and environmental factors.

The fossilised individuals that have been found may have been different ages, sex, or species.

Stegosaurus remains may have been damaged or changed during the fossilisation process or in the very long period before the fossil was formed.

Scientists have to use their judgement and imagination when piecing individuals back together from fossilised parts and fragments.

Great job if you got that.

Well done on all your hard work in today's lesson.

Let's summarise what we learned to help us remember it.

Body fossils are the mineralized remains of once-living organisms. Trace fossils are imprints that an organism has left behind.

The youngest fossils that have been discovered are roughly 10,000 years old.

Fossils show that some species have gone extinct and provide evidence that the features of some species have changed over time.

Fossils cannot tell us everything about organisms that lived in the past, but they can provide us evidence about what they looked like and the habitat they lived in.

We'll done on all your hard work today.

I'll see you back here really soon for our next lesson.