0:00 [Music] hello and welcome to our life sciences lessons today we're going to be looking at the first lesson in a series on the history of life let's have a look and see what
0:45 it entails we know about the history of life from fossils so the first thing we're going to be looking at is a little bit about fossils what are fossils how do fossils come about are there different types of fossils and then later on we will look at what the fossils are telling us for this lesson these are the topics we're going to be covering we're first going to look at how fossilization occurs what types of fossil there are how we
1:31 know how old they are because we're going to be talking in millions of years put all the information together in a timeline of what's happened on the earth over many years and then we will look at what causes the changes in what we find first of all what is a fossil and a fossil is basically the remains of some hard parts of animals that have actually turned to stone a fossil looks exactly like a bone but it is actually stone and if we look at this picture
2:17 that is a fossil mammoth and all of these bones that you see are actually stone so we're going to have a look at a little bit more about how fossils work oh we also find fossils of not parts of the animals but signs that they've actually been there like footprints in ancient mud and all of these fossils tell scientists a story about what life was like many millions of years ago first thing we need to look at is how do
3:04 living things when they die how do they actually become fossils and that's the process of fossilization and this picture gives us an overview of exactly what has to happen to form a fossil because not all animals that lived on earth actually became fossils a very small percentage of them only let's have a look first of all the animal has to die preferably near a river so the bones can be buried then soft parts decay leaving only the bones because it's near river sediment gets laid down over the bones and over many years many layers of sediment are brought down by
3:51 rivers and cover the skeleton as millions of years go by water percolates through the rock goes into the skeleton and the skeleton actually changes to rock because of movement of the earth and river action the layers are removed by erosion and eventually the fossil is exposed so scientists can actually have a look at it and try and work out what went on step number one okay first of all the animal has to die
4:37 the soft parts decay and only the hard parts get left behind then the animal has to be buried sediments have to be laid down on top of the skeleton this occurs over many many years and eventually mineralization occurs which is when minerals go into the teeth and and convert them to a fossil so the fossil is basically a combination of the original bone and the new minerals now this
5:24 takes millions of years erosion occurs rocks get pushed upwards water carves in into the rock and we end up with the fossils being exposed and only then do scientists actually know there's a fossil there and they can then start examining it and seeing what we can learn from that for example the earth under our feet this very minute might look like that it might have hundreds of fossils in it but we will never know until some sort
6:11 of excavation occurs or the landscape is eroded away by water now there are different types of fossil they all give us information but they're formed in a slightly different way so let's have a look at these different types of fossils okay the most useful ones are body fossils which are basically parts of a skeleton for example this picture here is a fossil of a sea mollusk so body fossils can only form from the hard parts
6:56 and sometimes as in this example the fossilization actually shows you that the animal's body when it was alive was made of lots of little compartments so sometimes you can see internal detail the most common fossils are the ones shown here what we call mineralized or petrified fossils which bones become stone then other things can happen for example these two pictures are carbonized fossil when the carbon in the living organism undergoes a chemical change
7:41 and that is how coal is produced so coal is actually a fossil then these are very very interesting because sometimes living things can be preserved in different ways for example this here that is a stone but it's a fossil of tree resin and you can see an insect got trapped in that tree reason and the insect is now part of the fossil this picture here is very interesting because this is a baby mammoth that was actually discovered as the ice
8:28 sheet retreated in siberia that little mammoth had been frozen in ice for hundreds of thousands of years and in this exact same situation this is a picture of a man and that is a mammoth tusk and as the ice is melting in siberia many of these mammoth tusks are actually being exposed and people are going out collecting them and then giving them to museums or selling them but boy isn't that rather large
9:17 another type of fossil is what we call a mold and what happens there the organism gets trapped the sediment hardens and the organism decays afterwards and if you have a look at this picture you can see that is a mold because you can see the animal was there it's not there anymore but you can see its shape quite clearly then what sometimes happens is other minerals fill the shape and then the fossil is called a cast but if you look inside a cast you can't see any internal detail
10:02 as we saw in the body fossil then the last ones are trace fossils and there were many trace fossils sometimes it's footprints they even have fossil done which actually tells them quite a lot about what the animals ate okay this is a series of dinosaur footprints hundreds of million years ago a dinosaur walked on soft mat and just to give you an idea of how large the dinosaur feet were those are the three toes compared to a human hand
10:50 that brings us to the end of the first section which is looking at fossilization before your brains start bursting let's take a break [Music] welcome back from your break we're going to carry on looking at fossils but what we're going to be looking at now is how do scientists know how old fossils are
11:36 you've probably heard dinosaurs existed so many million years ago but how do scientists know that they actually did exist that time and scientists have worked it out by working out how old the fossils are or dating the fossils and there are two methods that scientists use for dating fossils the most accurate method is the second method and that tells you how many million years each particular fossil is first type of trying to work out which
12:21 fossils are older is something called relative dating and because fossils are found in sediments which are laid down in water the oldest fossils are always at the bottom and as more sediments get laid down more fossils get laid down and the younger fossils are actually at the top and this is called the principle of superposition in other words the lower ones are old the lower layers are older than the upper layers and this is a picture of the grand canyon in america
13:06 and the grand canyon from top to bottom lays open 300 million years of fossils and the oldest fossils would be right at the bottom in these white layers here and the younger fossils would be closer to the top the most scientific the most accurate way of dating is called absolute dating in other words it gives you a figure and it's also called radiometric dating because of the technique used to actually work out the age this is more accurate and it is a little bit
13:54 complicated so i'll try and explain it as simply as i can first of all in any minerals of any type there are some atoms that are radioactive and those are the ones that are shown here in red and scientists know what the proportion of radioactive atoms actually are when an organism dies those radioactive atoms gradually decay or break down and here they're shown in green so scientists can look and according to how many of the atoms
14:40 that were radioactive have broken down they can work out how old the rock is the time taken for half of the radioactive atoms to break down is called a half life and the figure for a half-life is different in different elements for example this picture shows uranium and uranium is one of the common elements that is used to do this radiometric dating uranium breaks down to lead and for half of the uranium atoms to break down it takes a billion years so the
15:25 half-life of uranium is a billion years so when scientists look at uranium they just work out how many of the uranium atoms are still radioactive and that gives them an idea of how old the rock is now not all elements are good for this there are three elements that are very suitable the one is uranium which we looked at the other one is potassium and there's another one that you might have heard of and that is carbon dating and each of these three different elements gives us an idea of how old the rocks are
16:13 uranium and potassium are used to date very old rocks and carbon dating is used for organic matter from 50 000 years ago to now now this might sound complicated but once the scientists have worked it all out it's actually quite easy to do because they take a geiger counter which can measure radioactivity they measure how much radioactivity is in the rock and then they look at a graph like this and they can then work out if half of the atoms have decayed
16:59 they will look and say aha this rock is so many years old and it's different for the different elements to make it a bit easier for you we're going to have a look at how radio carbon dating is actually used radiocarbon dating the half-life is 5 000 years so if they found like for example that baby mammoth we talked about they would simply measure the radioactivity in that baby mammoth and they would say okay 50 of that baby mammoth's radioactivity has been gone or has decayed and that tells us
17:45 that the baby mammoth was 5 000 years old now to compare the two different types of dating we have relative dating here so let's have a look relative dating that can only tell you which is the oldest and which is the youngest rock absolute dating which is the radiometric dating that can give you much more accurate figures and if you have a look if they work out it's 120 million years the margin for error is about 10 years so it might be 125
18:31 million years old as the rock gets older the margin of error gets a little bit bigger but it still gives us a good idea of how long ago those organisms existed now what time frame are we talking about scientists have estimated that the earth was formed 4.6 billion years ago that's 4.6 000 million years the figures are quite mind-boggling and in those 4.6 billion years things have changed quite a lot
19:19 the earth changed from a hot inhospitable sphere to a place where life could exist and of course humans are the end product of life on earth if we have a look at this diagram that's the start of the earth 4.6 billion years ago and that's now where we are this shows us all sorts of things that happened the important ones is that way back here that was when the first evidence of life existed they found fossils of tiny tiny
20:07 microscopic organisms but they were very simple organisms it's only much later that proper cells developed cells with the nucleus and for most of the time in history any things that were alive were all unicellular and it's only about 800 million years ago that multicellular organisms were first found in the fossil record and if we have a look some of them are very strange and do not exist today then of course we also had our favorites the dinosaurs which
20:53 existed well 100 million years ago but in terms of the earth's age that was actually quite recent because a timeline like this is extremely confusing we are going to be looking at a simplified version of the timeline and we are only going to be looking at living things and things that affected living things so let's have a look at the timeline we're going to use okay this blue square is our timeline starting at the beginning the origin of the earth 4.6
21:38 million years ago and right at the top is the present time and two-thirds of the way along three or three thousand million years ago and one thousand five hundred million years ago and what we're going to do is we're going to be having a look at what happened as time moved from ancient earth a long time ago and how it changed to conditions that we have on earth now and that is how we look at a timeline showing us what happened on the
22:24 earth in the past [Music] welcome back from your break we are going to have a look at a timeline which shows us the history of the earth so let's get started the blue bar is the timeline with the dates filled in and we are
23:09 going to be looking at certain points on this timeline and saying well what was the earth like at that particular time in history what types of animals existed at that particular time if any 4600 million years ago earth was formed it formed from dust and gases from the solar system and if we look at the picture not a very hospitable place to live
23:56 it was a molten sphere hot temperatures the atmosphere had carbon dioxide methane nitrogen water vapor no oxygen it wasn't a very nice place to live because because the earth was so hot volcanoes were erupting there wasn't an atmosphere to stop meteorites from out of space hitting the earth so this is what it looked like volcanic eruptions meteorites hitting the earth not a very hospitable place for life to
24:44 exist about 4 000 million years ago the first life on earth came into being these were simple cell-like structures not even proper cells and they didn't contain dna they contained rna but they were alive then 3 900 million years ago proper cells life as we know it with dna came into being
25:32 these were simple prokaryotic cells and you should know from what you've done on cells that the word prokaryotic means that they didn't have a nucleus they were very simple organisms similar to the bacteria which exist today but these were very different because there wasn't any oxygen so they used carbon dioxide from the atmosphere and inorganic minerals and this is an example of a hot spring that exists today and sulfur is forms the yellow in the hot springs and even today there are some of these very very simple
26:20 bacteria which don't need oxygen they use sulfur carbon dioxide and energy from the hot springs to be able to survive then 3500 million years ago bacteria as we know them existed and these were weird because they used light to photosynthesize so they could make their own food they didn't have chloroform and they didn't give off oxygen
27:06 but these at least were closer to the bacteria that we know today then three thousand million years ago the first proper photosynthetic bacteria which are called cyanobacteria they had chlorophyll and they used light to produce food by photosynthesis and they released oxygen as a waste product they had a primitive type of chlorophyll now these cyanobacteria exist today
27:52 and the picture shows some of the cyanobacteria that are actually found in water today fossils of these bacteria exist and this is a place in australia and all of these funny-looking mounds are actually stromatolites and they are fossils of these very very simple photosynthetic bacterial cells that lived all those millions of years ago now tom's going by
28:38 the cyanobacteria are busy undergoing photosynthesis and this causes the oxygen levels to rise and that poisoned many of the bacteria that relied on carbon dioxide because the photosynthetic bacteria were using carbon dioxide the carbon dioxide levels dropped so the composition of the atmosphere is changing then 2 000 million years ago the first eukaryotic
29:26 cells develop what does eukaryotic mean we know that eukaryotic means that they have got a proper nucleus and if we look at this picture there's the nucleus that's why it's eukaryotic but the important thing is they also had organelles okay and here are some of the organelles in this particular unicellular organism scientists think that organelles probably arose when cells engulfed other bacteria and did not digest them and the other bacteria kept on living inside them and helping them
30:12 to survive 800 million years ago the first multicellular organisms occur and they were probably a ro produced by unicellular organisms that clumped together for example in this picture here there are a whole lot of unicellular organisms clumped together and they found that when they stay together they were safer and they then started carrying out different life functions and helping each other out and that is how multicellular organisms
31:01 came into existence then 550 million years ago there was such a big change because from the simple microscopic bacteria and some multicellular cells which had existed for 150 million years within a relatively short space of time and i'm talking about geological time so it's about 10 million years is a relatively short space of geological time all these weird and wonderful different living things came into existence
31:48 and that is called the cambrian explosion and that is when life exploded onto the scene in all its amazing diversity this is an artist's impression of some of the weird organisms which existed we still find some today that are quite similar but they're quite a lot that do not exist in that form today oh there's also a little snail a little little mollusk but 430 million years ago living things started being able to grow
32:35 onto land animals left the sea and developed the ability to walk on land as the sea levels dropped some plants managed to survive on land so after that time land ecosystems also came into being and that in a nutshell is the history of the earth i think your brains are a bit bursting by now so let's take a break
33:27 [Music] welcome back to our life sciences lesson on fossils and fossilization and the history of the earth what we've looked at so far in this section is we've looked at fossilization how it occurs how fossils are made we then looked at the different types of fossils we looked at fossil dating which means how do we know how all the different types of fossils are and then we have just finished looking at a timeline of the history of the earth and what
34:14 we're going to be looking at now is we're going to be looking at some of the changes that happened in the earth to cause all these weird and wonderful living things to be able to exist so what we're now looking at we're now looking at what are the changes that have actually occurred over time and why did they cause living things to change okay first thing is in the last 400 million years the continents have moved for example 400 million years ago this is what the earth looked like there
35:01 was north america south america asia africa india which was actually part of africa and australia and antarctica they were all part of a huge continent they were all joined together and that continent was called pangaea and because all the continents were joined animals could easily move from one of the continents
35:47 to the other plants their seeds could be blown and we would find plants moving across this huge continent called pangea but it didn't stay like that because the continents gradually drifted apart and we call that continental drift so let's have a look at what's happening there's africa and you can see there europe and asia has now split from africa india has split from africa and antarctica and australia were still together
36:34 200 million years ago if we look at 130 million years ago you can see what's happening to india india was rashing along on its way to join the rest of asia where it actually is now okay if we look at 70 million years ago india was still an island north america was still joined there's north america it was still joined to asia and north and south america were totally separate from each other as was
37:20 africa then if we look today you can see the position of all the continents in the world today very different to what it was 200 million years ago now an important thing to notice is that as these continents moved they moved from near the poles to near the equator or from near the equator to near the poles so conditions changed quite a bit and we can see this quite clearly because in antarctica which is now covered by a huge ice sheet they found fossils
38:05 of tropical plants like ferns which tells us that at one time antarctica was actually a tropical place india which is now very very hot was at one time in the south pole so continental drift changed conditions quite a lot as the continents moved another interesting point is how do we actually know that the continents did move and one of the pieces of evidence that shows that continental drift did occur is something called
38:53 biogeography and that's looking at where the fossils of certain living organisms that existed in the past are actually found and this supports the theory of continental drift and let's have a look at some of those at the moment okay first of all showing that africa and south america were once joined are fossil remains of certain types of reptile also showing that america south america and africa were joined are fossil remains of something that almost looked like
39:38 a crocodile but then even more interesting some of the other distribution of certain living things shows okay and if we look here okay this is where we find fossils of a particular very very interesting fern and it's found in south america africa india antarctica and australia and that actually gives the scientists the evidence they need to know that their theory of continental drift is actually a true theory it also helps us understand
40:25 how evolution occurs and when you get to matric and you have a look at evolution you'll find an understanding of this biogeography very useful then other things that happened is the temperature of the world changed from the hot molten ball that it was 400 million years ago it became a frozen wasteland there were ice ages in fact there were several ice ages and between them there were periods of global warming so the world temperatures changed and there were
41:11 several periods of warming and cooling and that led to at least five ice ages and this is what the earth looked like covered in ice have you ever seen the movies ice age they're actually based on fact because for example mammoths did exist and mammoths did survive the ice ages because they had these lovely lovely thick coats saber-toothed tigers did exist giant sloths like sid the sloth did exist
41:56 now in an ice age conditions change life doesn't become so easy let's have a look the whole world becomes frozen plants can't survive because the water they need to survive is frozen herbivores need plants no plants the herbivores have to migrate to find plants that they can eat to find food if the herbivores migrate the carnivores like sabretooth tigers also have to migrate because they have to go where their food is and in the ice age movies they have the great migration and that
42:42 shows that as the world became colder and colder there was no food and to be able to survive living things had to migrate so those movies are actually based on what scientists have been able to work out about what really happened in those ice ages during these ice ages as the temperature changed because much of the water became part of the ice the sea levels dropped that meant that something which was on the sea floor became land so any animal which existed then as the sea dried up
43:29 it had to adapt to be able to survive then when it got warmer the sea levels rose so large areas which were land have now become c and we have fossils that actually tell us this for example this picture here is the fossil of a whale which is found in the sahara desert in the middle of the desert because the sahara was once part of the mediterranean sea it was once part of the sea floor even our own karoo dry
44:14 hot and dusty has many different fossils in it which once lived in water because the entire kuru was once an inland lake so temperature changes also affected the sea levels temperature changes also affected gases and that's what we're going to look at now as time went by and the temperature got warmer photosynthesis occurred the proportion of gases in the atmosphere changed and any organism which needed a particular gas to survive if that gas wasn't available guess what
45:02 it didn't survive so as the composition of gases like co2 and oxygen in the atmosphere changed organisms which needed those gases died out and became extinct while other organisms developed ways to use those gases and they then became more common because they knew how to live in the changed atmosphere of the earth hopefully this has given you an idea of how things have changed in the 4 600 million years since the earth came into being i hope that what we have done has given
45:47 you a bit of insight into the history of our earth see you next time [Music] [Music] you
