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Video transcript

- [Instructor] Are there any evidences for this theory of evolution? For example, we say birds are evolved from dinosaurs over millions and millions of years. And these dinosaurs are pretty famous, there are so many movies on them. But how do we know that they really existed, nobody has seen them in real life. Well, one of the evidences can be found inside earth. Look at these pictures. They are the remains of these ancient reptiles. But wait, what exactly are these, are they bones? How can bones stay like that for millions of years, don't they get decomposed? Well, these are bones which turned to stone over time, they are called fossils. So in this video, lets look at what fossils are, and how they are formed. Let's start by writing down what fossils are. So what are fossils? Well, in simple terms, fossils are basically remains, remains of once living animals, or in general we say organisms, because they may not be just animals, they can be plants as well, or maybe even micro-organisms like bacteria. And what kind of remains do these living things leave behind after they are dead? Well, there can be many. It can be actual bones, or it could be bones which are turned into stone as we will see how, or it could be footprints left behind, so many things. Basically, something should be left behind, and if it can stay there for thousands and thousands of years, we will call that fossils. Okay, so next obvious question would be, how do these fossils get formed. For that, let's take an example. Let's say your favorite dinosaur is over here, doing its dinosaur thing, and then something happens to it, and then it suddenly dies, with its tongue sticking out. Poor guy. Now usually what happens after this is, within no time, within a few months maybe, the whole flesh get decomposed by micro-organisms, and then it just leaves behind a skeleton, and then the bones will also get decomposed, but that happens very slowly, but they do get decomposed, and after a few years, absolutely nothing is left behind, everything is decomposed. And so you see, in general, most animals don't leave anything behind, no traces behind at all. So they don't get fossilized at all, okay? Then, how are fossils formed? Well, in order for an animal to become fossilized, what should happen is, before the bones have a chance to get decomposed, they should get buried underground. We will see that if they can be buried, then they can be fossilized. So how do they get buried? Well that can happen multiple ways. Might be due to natural calamities such as volcanic eruption or maybe due to earth quake, the whole thing gets buried. Or, the most common way in which this can happen is if these guys die very close a water source, like say a lake. So let me do that, let me show you that. So imagine, imagine this one died somewhere close to a lake. And after dying, somehow let's say it gets pushed into the lake, it's flesh is completely decomposed, and then it sinks all the way down to the bottom hard. Now these bones will take a lot of time to get decomposed, right? Now let's see what happens before it gets decomposed. You see this water contains a lot of stuff. It can contain mud, or stones, continuously these things keep entering into the water, and over time slowly these things start depositing down. And so some of this will start depositing on top of these bones as well. So there's one layer of these depositions formed, these things are basically called sediments, these can be sand, or mud, or stones, anything. And so as time passes by, more and more layers of that starts getting deposited, deposited, deposited, and that's how our skeleton can now get buried. And then, over time, this whole thing hardens, and turns into a rock. Well once this happens, there's a good chance that this can become fossilized. What happens now is due to some pores, some gaps which are penetrating these rocks, water will start coming into the earth, and some of the water will also make its way towards these bones. And as time passes by, the bones might start getting dissolved in this water, and the bones will get replaced by the minerals present in the water, eventually the entire thing, the entire thing will now be replaced by minerals, in other words the bones have now turned into a stone. This is how fossilization happens. Okay, lets rewind a little bit, and show you this process in a little bit more detail. So, let's say we zoom into one of the bones, let's consider the leg bones. Let me zoom in a little bit more, so we can look at it nicely. Okay, now lets look at this claw over here. As water reaches that, water contains a lot of micro-organisms. So those micro-organisms start decaying that claw. Maybe some part of that claw starts getting dissolved in that water, water is an excellent solvent. Over time things will get dissolved, and so that entire claw is now gone, leaving behind a hole over there. And so, that place is now completely filled with water. What happens next? Well that water is not pure water, that water contains a lot of minerals inside of it. It can be minerals of calcium, silicone, phosphorus, all those minerals now come in place, come in this place over here, and over time, the water starts evaporating, water evaporation can happen all the time. Water evaporates leaving behind these minerals. The process keeps on continuing until the whole thing is now replaced by these minerals. Let me shade that a little bit. All right, so now this claw has turned into a bone. Sorry, into stone. And the same thing might happen over here, maybe this part of the bone starts dissolving, now that gets completely filled with water, again there are a lot of minerals present over there, the water evaporates, leaving behind these minerals, and over time this whole thing is now covered with minerals, and so that shape now is covered in minerals, and again, that part has been converted to the stone. And this is how, slowly and steadily, small small small parts of it convert into stone, and eventually the whole, whole bone gets converted into a stone. So it's a perfect replica of that bone, it's just that it's now made of minerals, it's a stone. And this is how this entire body, over time, can convert into a stone. Let me zoom back now. And after many, many, many years, when people start digging over here, they will discover this rock, and eventually when they dig more, they will discover this fossil. And, of course, the entire bone structure may not be in tact, it's totally possible that some of the bones will be scattered around, but it's not uncommon. Like, for example, if you take a look at this picture, you can see pretty much the entire skeletal structure is in tact. This is some pre-historic reptile. But in some other cases, you might just find one of the parts, so for example look at this one. This is an actual picture of a skull which is fossilized, its a skull of a dinosaur. Anyways, there are other ways in which fossils can also be formed, let me just show you one more quickly. Take a look at this picture. You can see some insects trapped inside some transparent shiny stones, right? These are also fossils, and these contain entire insects, not just part of it, but entire insects. So what are these stones, what are they? Well these stones are basically hardened resins. So trees have a habit of releasing this very sticky material called resin, they usually do that when they get injured. And over time that resin can get hardened. So, if there are some insects very close to the tree, and they get stuck in that resin, they'll just stay there. And eventually, the resin gets hardened over time, and the insects will die inside of it. These hardened resins are called amber, and the insects that get stuck over there do not decompose, even after millions of years. And the reason for that is that, it turns out the sticky material does not allow bacteria to move and carry out decomposition. So even the bacteria gets stuck in it, so even the bacteria can be found in these amber. Anyways, whether you consider the tiny insects which got fossilized in amber, or you consider the giant bone fossils of dinosaurs, these have helped our scientists to discover extinct species. And by studying them carefully we have predicted what kind of characteristics they might have had, what kind of behavior they might have had, and even how long ago these animals might have lived, yes you can also do that, we'll talk about that in other videos. And as a result, they provide great evidence to the theory of evolution. Okay so, let's quickly recall what we learned in this video. Can you recall, what are fossils? And secondly, can you recall how fossils can be formed? If you have a tough time answering these, no worries, you can always go back and revisit the video.