 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 turn to stone over time. They're called fossils. So in this video, let's 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 term, fossils are basically remains. Remains of once living animals or in general we'll say organisms. Because they need not be just animals, they can be plants as well or maybe even microorganisms like bacteria. And what kind of remains do these living things leave behind after their death? Well, there could be many. It can be actual bones or it could be bones which are turned into stones, 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'll 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 few months maybe, the whole flesh gets decomposed by microorganisms and then it just leaves behind the 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 like some volcanic eruption or maybe due to earthquake, the whole thing gets buried. Or the most common way in which this can happen is if these guys die very close to a water source, like say a lake. So let me do that. Let me show you that. So imagine this one died somewhere close to a lake. And so after dying, somehow let's say it gets pushed into the lake, its flesh is completely decomposed, and then it sinks all the way down to the bottom, thud. 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 these 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. 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 present in these rocks, water will start coming into the earth. And some of that 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, let's 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 let's look at this claw over here. As water reaches that, water contains a lot of microorganisms. So those microorganisms might start decaying that claw. Maybe some parts of that claw starts getting dissolved in 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. The water contains a lot of minerals inside of it. It can be minerals of calcium, silicon, phosphorus. All those minerals now come in place, come in this place over here. And over time, the water starts evaporating. Well, 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, that 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 intact. 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 intact. This is some prehistoric reptile. But in some other cases, you might just find one of the parts. So for example, look at this one. It's an actual picture of a skull, which is fossilized. It's 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. These are also fossils. And these contain entire insects, not just parts of it, but the 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 gets hardened. So if there are some insects very close to the tree and they get stuck in that resin, they will 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 that the sticky material does not allow bacteria to move and carry out decomposition. So even the bacteria get 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 what 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. OK, 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.