Lesson video

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

Today's lesson is on common ancestors and transitional species, which means we'll be talking a lot about fossils and a lot about evolution.

This lesson is taken from the unit on Fossil Evidence, Selected Breeding and Explaining Evolution.

Hi there, I'm Mrs. Wheate and I'm gonna be your teacher for today's lesson.

By the end of today's lesson, you'll be able to explain the importance of transitional species and common ancestors in the fossil record.

Let's have a look at our keywords.

Our keywords for today's lesson are fossil record, all the fossils ever found, and their ages, provide a body of evidence called the fossil record, common ancestor, a species that several other species evolve from, for example, tigers and lions share a common ancestor, evolutionary tree, a diagram that shows the evolutionary relationships between different groups of organisms, and transitional species, a species that shows intermediate characteristics from evolutionary ancestors and modern species.

So to break that down for a second, intermediate, kind of meaning in between, a species or a group of organisms that shows characteristics that link an evolutionary ancestor to a modern species.

So don't worry about trying to understand all of those right now.

We're gonna go through them in lots of details as they come up in the lesson.

If you wanna pause the video or I'll just be quiet for five seconds, you can read them through again, but if you want a bit longer, maybe pause so that you can write them down and think through them by yourself.

Today's lesson is in two parts.

For the first part of the lesson, we're gonna talk about common ancestors and evolutionary trees.

And in the second half those lesson we're gonna talk about a really famous example of a transitional species called Archaeopteryx.

But first of all, common ancestors.

All of the fossils ever found and their ages provide a body of evidence called the fossil record.

So here below I've got a selection of fossils, got an ammonite fossil, a trilobite, some leaf imprints, so imprints or footprints, they count as fossils as well, and we've got an ichthyosaur skeleton as well.

So the fossil record could provide evidence of how the characteristics of species have changed over time, when organisms first appeared or went extinct, and the evolutionary relationships between species.

And that's what we're gonna focus on in this lesson, how the fossil record provides evidence of the evolutionary relationships between different species.

So often it is really helpful to use diagrams and pictures to help us understand something abstract as opposed to just using words.

So in biology we can use evolutionary trees, and these are diagrams that show the evolutionary relationships between different species.

So I've got an evolutionary tree here of the lion and the leopard, and this shows us how related in evolutionary terms these two species are.

And presenting it this way can be a lot more helpful than me just describing to you with dozens of words how these two organisms are related in evolutionary history.

Evolutionary trees are also called phylogenetic trees, but in today's lesson I'm just gonna be referring to them as evolutionary trees.

So these diagrams show how later species evolved from earlier species, or you could call those earlier species, their ancestors.

These diagrams also show which species share a common ancestor, and that's the focus of the first half of this lesson.

A common ancestor is a species that several other species evolve from.

So if we look at this evolutionary tree, we've got a lion and a leopard here, these two species share a common ancestor.

So now I'm gonna show you how to look at one of these diagrams and figure out where the common ancestors are.

So on the previous side, we were just looking at a very small part of this evolutionary tree.

And now this evolutionary tree shows the evolutionary relationships between species of big cat, so not just the lion and the leopard.

It's showing the relationships between the tiger, the snow leopard, the jaguar, the lion, and the leopard.

And remember, this is an evolutionary tree, and so we have some treelike terminology for it.

So the branching points, so if each of my lines are branches, where those branches split or divide or they branch off, that's where the common ancestors are.

And you can see I've marked those here with my purple arrows.

The evolutionary lines branch, because the common ancestor evolved into several different species.

Let's take a look at this example to help us understand that.

So in this evolutionary tree, we see that species A evolved into two different species, B and C.

So species A is the common ancestor of species B and C, but why might species A have evolved into two different species? This example is gonna be set at three different locations over the continent of Africa.

Imagine a species A had split into two populations that moved to different habitats.

So here we have the common ancestral species A living in Eastern Africa, and let's imagine that half of the population went north into the Sahara Desert, and some of the population went south into Sub-Sahara Africa.

And so we've got two very different climates here.

Each population could have evolved differently due to the different condition in each habitat.

Eventually the two populations would be so different to each other and to A, that we could classify them as separate species, B and C.

So they started off as being all the same thing species A, but because something happened, for example, some of them migrated to a drier place, some of them migrated to a wetter place, and over thousands or millions of years, enough changes accumulated between these two different groups of populations that they became different species.

Let's check to see if you've understood so far.

So which branching point represents a common ancestor of the jaguar, the lion, and the leopard? You can take five seconds or you can pause the video if you want some more time to think.

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

The answer is A and C, well done if you got that right.

B is incorrect, that's a common ancestor for the tiger and the snow leopard.

D is also incorrect.

That's a common ancestor for the lion and the leopard, but not also for the jaguar.

Okay, so we could identify the common ancestors on an evolutionary tree, let's add a bit more.

So this evolutionary tree shows that the lion and the leopard have three common ancestors.

So A, C and D, those are all their common ancestors.

However, D is the most recent common ancestor.

Sharing this most recent common ancestor means they are more closely related to each other than any other species on the evolutionary tree.

So the lion is more related to the leopard than it is to the jaguar, the snow leopard or the tiger, and vice versa, the leopard is more related to the lion than it is to the jaguar, the snow leopard or the tiger, because they share this most recent common ancestor.

Right, let's see if that made sense.

What is the most recent common ancestor of the tiger and the snow leopard? Take five seconds or click pause if you want a bit more time, click play when you're ready to see the answer.

Let's look at the answer, it is B, well done.

There's only one correct answer for this one.

So whilst A is also a common ancestor of the tiger and the snow leopard, it's not the most recent common ancestor.

So far, we look to evolutionary trees that I've read from left to right and they're on their side, but that isn't always the way they have to look.

So they can be shown in different shapes and different directions.

So if we look at another example, this evolutionary tree here is still showing all the same relationships, evolutionary relationships between those species.

Okay, if you stop, maybe pause the video, take five seconds, have a look at it, you'll see that all the common ancestors are still in the same position.

It's just that instead of being read from left to right, it's now being read from bottom to top.

So take a few seconds to have a look and get yourself familiar with how these two are, in fact, the same thing.

Okay, hopefully that convinced you or maybe you had a discussion about it.

But so despite the fact that the newest evolutionary tree has been rotated, it still shows the same evolutionary relationships between these species.

Let's look at some more examples.

So here they're kind of more triangular shape as opposed to rectangular shape, but this is still showing the same relationships.

And so is this one.

So again, the first one is read from bottom to top, still showing the information, the other one is read from top to bottom, still showing the same information.

And again, if you want some more time to process that, I'll be quiet for five seconds, or you can pause the video to have a look at those really carefully to convince yourself, oh wait, no, those are showing the same thing.

Okay, let's check if that made sense.

These evolutionary trees convey the same information.

Is that true or false? Take five seconds or pause the video if you want more time.

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

It is true, even though in the first example we have a more square diagram being read from left to right, and in the other example of the evolutionary tree, we have a more triangular diagram that's being read from bottom to top, these evolutionary trees are still showing the same information as each other.

The position of the sun bear and the sloth bear, and also the polar bear and the brown bear, they've been rotated.

But because these species were at the end of the same branch, you can rotate them without changing any of the evolutionary information that's conveyed.

If that didn't make sense, please take some more time to have a look at them really carefully and maybe have a discussion if anyone's nearby you as to why these are the same.

So these are more examples of things that you can change in evolutionary trees that don't change the information that's being conveyed.

So on these evolutionary tree diagrams, humans and chimpanzees are in different positions.

So we can see it in the first one, chimpanzee is on top, human is on the bottom, on the next one, human is on top, chimpanzee is on the bottom.

So this change in position still, it doesn't affect the information that's conveyed about their evolutionary relationships.

All that happens is things at the end of a branch, so the chimpanzee and the humans, they're branched together, they're paired together, all that's happened is it's been rotated.

That doesn't change the relationship.

They've still got the same number of common ancestors and the common ancestors are still in the same position.

So this isn't changing anything that's being conveyed about their evolutionary relationships.

However, so on these evolutionary diagrams, orangutans and gorillas are in different positions.

So this change in position does affect the information that's conveyed about the evolutionary relationships.

So chimpanzees and humans, they were at the end of the same branch.

Orangutan and gorillas were not at the end of the same branch, they're on different branches.

So changing their position does affect the information that's conveyed about them.

The common ancestors now are in a slightly different order, and so it is telling you a different thing about how they're related.

Okay, let's see if we understood that.

These evolutionary trees convey the same information.

Is this true or is this false? Take five seconds or pause the video if you want more time, click play when you're ready to see the answer.

Let's check our answers.

So this is false.

So the direction and the style of the tree has changed.

That's fine, that's not what the problem is.

The problem is if you looked really carefully, you can see that the position of Asiatic black bears and American black bears has been switched.

And this does change the information that's being relayed here.

So if we look at the first diagram, the Asiatic black bear and the sloth bear have two common ancestors.

But if we look at the second example, now the Asiatic black bear and the sloth bear only have one common ancestor.

So switching these bears, because they weren't at the end of the same branch, does change the information that's being relayed here.

Well done if you got that right, let's try another.

Okay, so same species, but slightly different evolutionary trees again.

So do these evolutionary trees convey the same information? Is that true or is that false? Take five seconds or pause the video if you want more time, click play when you're ready to see the answer.

That is true.

So all that's changed here is the style and the direction of the evolutionary tree.

So the first evolutionary tree, it's read from left to right, and the second evolutionary tree, it's read from bottom to top.

And again, the first evolutionary tree is more right angled, the second evolutionary tree is a bit more triangular, but that doesn't change the position of the common ancestors.

So they're telling you the same thing.

Well done if you got that right.

Okay, let's do one more, so what about these? These evolutionary trees convey the same information.

Is that true or false? Take five seconds or pause the video if you want some more time.

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

It is false.

So if you look at the second evolutionary tree, the brown bear and the sloth bear have changed positions, and they weren't originally at the end of the same branch, so you can't switch them.

This is now showing on the second diagram that the sun bear and the brown bear have a most recent common ancestor.

That's not true.

And again, it's showing that in the second diagram that the polar bear and the sloth bear have a most recent common ancestor as opposed to the first diagram where it's the sun bear, the sloth bear, and then the polar bear and the brown bear that share the most recent common ancestors.

Well done if you got that right.

This is our first practise to ask for today's lesson.

This evolutionary tree shows the evolutionary relationships between horses, donkeys and zebras.

Take a few seconds now just to look through it and familiarise yourself with it.

Okay, so use the worksheet to answer the following questions about this evolutionary tree.

Number one, on the diagram, circle all the common ancestors of the Tibetan wild donkey and the mountain zebra.

Number two, who is more closely related? Is it domesticated horses and the wild horses, or is it the African wild donkey and the Asiatic wild donkey? And then justify your answer.

So explain why it is that you think that.

And finally, redraw the evolutionary tree onto the template on the worksheet.

So it still has to convey all the same information.

It just is gonna be in a different orientation.

Okay, so you'll need to pause video now to give yourself enough time to do the questions, good luck.

Let's have a look at the answers.

So on the diagram, circle all the common ancestors of both the Tibetan wild donkey and the mountain zebra.

So you should have circled these ones.

Okay, who is more closely related, the domesticated horses and the wild horses, or the African wild donkey and the Asiatic wild donkey? So it is the domesticated horses and the wild horses who are more closely related.

So you need to justify the answer.

And the reason is domesticated horses and wild horses share the same most recent common ancestor.

The most recent ancestor for the Asiatic wild donkeys is not the same as the most recent ancestor for the African wild donkeys.

Okay, and then you need to redraw the evolutionary tree onto the template on the worksheet.

And this is what that should have looked like.

Well done on completing that practise task.

Okay, we're done with the first part of the lesson.

Let's move on.

So we've looked at common ancestors and how to identify them on evolutionary trees.

Now we're gonna talk about a really important example of a transitional species, the Archaeopteryx.

Did you know that birds evolved from dinosaurs? So here I've got a photo of a Velociraptor model.

Not a real thing obviously, they live millions of years ago, no cameras around, no humans around to take the photo.

And here we've got a photo of a heron, which is a modern day bird.

So that heron evolved from a dinosaur, isn't that weird? So the common ancestor of birds belonged to a group of dinosaurs called theropods, and this group includes Tyrannosaurus Rex and Velociraptors.

So this wasn't known until the late 19th century when evidence was found in the fossil record.

So in the 1860s, a single fossilised feather was found by palaeontologists in Germany.

Palaeontologists are people that study ancient living things, often through fossils.

So these scientists were really confused because no one had ever seen any evidence of birds so far in the fossil record before.

More research was conducted, and eventually the owner of the feather was found, it was a species of bird-like dinosaur called Archaeopteryx Lithographica.

And here we have here a more complete fossilised skeleton of that organism.

So the thing that is so interesting about this species is that Archaeopteryx Lithographica shows characteristics of both dinosaurs and birds.

So it is an example of a transitional species.

Transitional species show intermediate, these kind of in-between characteristics, from evolutionary ancestors, in this example, that's the dinosaurs, and modern species, in this example, that's the modern birds.

So if we look at my photos before, well, my pictures below, we've got a photo of a model of the Velociraptor, and we've got an artist impression of what an Archaeopteryx would look like.

Again, this species lived millions of years ago, we don't really know what it looks like.

And this is one of our best guesses of what it looks like.

And then we have a photograph of a modern day bird, the heron.

So you can see from the photo without having to look at the skeletons that we've got some things in common here.

You can see maybe the jaw is quite similar from the Velociraptor to the Archaeopteryx, but obviously it's also got feathers, which makes it more similar to the heron, although a lot of scientists now are pretty sure that Velociraptors did have feathers.

And this kind of "Jurassic Park" model of what Velociraptors look like is a bit outdated.

But anyway, it's showing characteristics of both the ancestral species, the evolutionary ancestors, the dinosaurs, and the modern day species of the birds.

And so that's what makes it a transitional species.

True or false, Archaeopteryx Lithographica is an example of a transitional species, is that true or false? Take five seconds or pause the video if you need some more thinking time.

It is true, now justify your answer, explain why it's true.

Is it true because it shows characteristics of birds but no characteristics of dinosaurs, or is it true because it shows characteristics of both dinosaurs and birds? Take another five seconds or pause the video if you want some more time.

It is true because it shows characteristics of both dinosaurs, the evolutionary ancestor, and birds, the modern species.

Well done if you got that right.

Let's bring it back to thinking about common ancestors.

So this section of an evolutionary tree shows that the Archaeopteryx and modern birds share a common ancestor.

However, this common ancestor has not yet been discovered.

This could be described as a missing link between the Archaeopteryx and modern birds.

You could also describe it as a gap in the fossil record.

Now, there's lots of reasons for gaps in the fossil record.

One of these is because the conditions for creating fossils are very rare.

So fossilisation, you have often, if we're talking about body fossils, you have the remains of a species that needs to be very quickly covered in a layer of sediment, like mud.

So it can't be completely eaten by a predator, otherwise, there's nothing left to fossilise.

It can't just decay very quickly, otherwise, again, there's not much to fossilise.

So for this reason alone, most of the fossils that have been found have been of marine animals or marine creatures, organisms. And that's because when a marine animal dies, and if it's not eaten, it's more likely it's gonna fall to the bottom of the ocean eventually, the bottom of the ocean often is like muddy sand, and these layers build up on top of each other, and that makes the perfect conditions for fossilisation.

But again, most creatures don't die in this way that they can be preserved.

And so, most of the organisms that have ever existed have not been fossilised.

These conditions are really rare, particularly for land organisms, organisms that live on land.

So fossils, the ones that did actually become fossilised went through kind of this very specific procedure that meant that they weren't preserved as body fossils.

Those could also be destroyed by geological activities such as earthquakes, before we even discover them.

And also, many early forms of life didn't have bones and were soft bodied like a jellyfish, and which means they can't really form body fossils and they're much less slightly to form trace fossils.

So for these reasons and a few others, but these are the main reasons that we don't have a complete fossil record, there are gaps in the fossil records.

Okay, let's see if you understood that.

The fossil record is complete and includes fossilised remains of an organism from every species that ever existed.

Is this true or false? You can take five seconds or pause the video if you want some more time to think.

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

That is false, why is it false? Let's justify the answer, explain why it's wrong.

So is it false because many fossils have been destroyed by geological activity or there are fossils of at least one organism from every species that has ever existed.

We just haven't found them yet.

Again, take five seconds or click pause if you want some more thinking time.

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

Okay, the reason it's false is because many fossils have been destroyed by geological activity.

There are other reasons, for example, early life forms being soft bodied and it being really difficult to fossilise soft bodied organisms and the conditions of fossilisation just being very rare in general.

But unfortunately, there aren't fossils of at least one organism from every species that ever existed.

That would make palaeontologists' lives so much easier, and it would be super interesting to have that to study and have that as a record.

But unfortunately, that's not the case.

So this is an evolutionary tree that shows the relationship between a group of dinosaurs called theropods and modern birds.

So you can take five seconds now or pause the video to just have a look through it and see if you can already start to see some of the evolutionary relationships between these organisms. Let's have a look at the questions.

Number one, almost all the animals in this evolutionary tree are extinct.

What evidence could have been used to construct this evolutionary tree? Number two, Jacob is learning about transitional species at school.

He says, "Modern birds evolved from the Archaeopteryx." Use the evolutionary tree to explain whether he's correct or not.

Number three, explain the significance of the discovery of Archaeopteryx in understanding the evolutionary relationships between dinosaurs and modern birds.

Number four, suggest why the common ancestor of the Archaeopteryx and modern birds hasn't been discovered yet.

So you'll need to pause the video to give yourself enough time to think about those questions and then click play when you're ready to see the answers, good luck.

Let's look at the answers.

So number one, almost all the animals in the evolutionary tree are extinct.

What evidence could have been used to construct this evolutionary tree? So the answer is fossil evidence.

So dinosaurs lived millions of years ago, over 60 million years ago, and they were around for like 200 million years.

So there weren't people around at the time to be taking photographs or writing accounts.

Modern humans have been around for hundreds of thousands of years, but not millions of years ago.

So what we know about dinosaurs comes from fossil evidence.

Number two, use the evolutionary tree to explain whether Jacob is correct or not.

So Jacob is incorrect.

The evolutionary tree shows that Archaeopteryx and modern birds share a common ancestor, and there are other examples of dinosaur-like birds where modern birds do directly descend from, but they were discovered after the Archaeopteryx.

The Archaeopteryx was the first bird-like dinosaur that was discovered that helped scientists kind of bridge this link between, oh, maybe birds descended from dinosaurs.

Okay, let's look at the rest of the answers.

So number three, explain the significance of the discovery of Archaeopteryx Lithographica in understanding the evolutionary relationship between dinosaurs and modern birds.

Archaeopteryx Lithographica is an important example of a transitional species.

Before Archaeopteryx was discovered, there was no evidence linking dinosaurs to modern birds.

Because of this discovery, much research has been conducted between the evolutionary relationship between dinosaurs and modern birds.

So finally, number four, suggest why a common ancestor of Archaeopteryx and modern birds hasn't been discovered yet.

So the conditions for creating fossils are rare.

So it could be that no individuals of this species died in the circumstances which allowed fossilisation to occur.

Geological activities, such as earthquakes or volcanic eruptions, can destroy fossils.

It's possible that the fossilised remains of this species have been destroyed by geological activity.

You couldn't have written about the common sort of species having a soft body because it should have been really obvious in this diagram, these were bird-like dinosaurs that had bones, that had skeletons.

So unfortunately, that one wouldn't apply to this situation.

Really well done if you got those right.

Great job on today's lesson.

I hope you found it really interesting.

Let's summarise what we've learned.

A common ancestor is a species that several other species evolved from.

Evolutionary trees can be used to help visualise the evolutionary relationships between groups of organisms. The fossil record includes transitional species with features of both evolutionary ancestors and modern species.

Species of Archaeopteryx are an important example of transitional species.

They were the first species which helped scientists link dinosaurs to modern birds.

There are gaps in the fossil record because the conditions for creating fossils are rare, geological activity can destroy fossils and organisms that don't have bones are difficult to fossilise.

Really great job, guys.

I hope you get a bit of a break now, and hopefully I'll see you soon for our next lesson.