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Fossils, preserved evidence of ancient organisms, reveal Earth's history. Formed in sedimentary rock, fossils' ages are estimated through layer analysis and radiometric dating. These methods help place fossils in Earth's fossil record, providing a timeline of the evolution of life on Earth. Created by Khan Academy.

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Video transcript

- [Narrator] When I was 12 years old, I went on vacation to Alaska with my family. While walking down one of the cold stone covered beaches, I spotted a large pile of rocks that seemed to have slid off the side of one of the hills that connected to the shore. These rocks caught my eye due to their peculiar outer rust color that contrasted with the rest of the rocks on the beach. As I began curiously looking through them, I accidentally knocked a few over. Splitting some in the process. But to my surprise, one of the rocks that split in half had a secret hidden within it. Inside with the imprint of a tiny pine cone and nearby was rock covered in the imprints of leaves. I was so excited because that day I'd found real plant fossils, but what exactly are fossils and what can they tell us? Fossils are preserved evidence of organisms that lived in the distant past. They can be made up of many different things like bones, shells, plants, or even markings like footprints that tell us about an organism's behaviors. These preserved remains are mostly made possible by specific kind of rock known as sedimentary rock. Sedimentary rock is formed from compressed layers of sediment, which is a mixture of rocks, minerals, and organic matter. Sediment is the result of erosion, which is when bits of rock or soil break down and get deposited somewhere else. Sediment is deposited in layers, one on top of the other. over millions of years, these layers become compressed, eventually solidifying and forming sedimentary rock. But how do the fossils even get inside of these rocks? Well, let's look back at the fossil I found. If I had to guess, a long time ago, some leaves and a pine cone must have fallen to the ground and gotten covered in sediment. As more sediment layered on top of them, the plants pressed an imprint of their shape and patterns into the sediment. Over time, the sediment compressed into rock and the original plant decayed, but its imprint stayed inside the rock. Then finally, my fossil sat until I literally stumbled upon it. So now that we know what fossils are, what can they tell us? As I mentioned before, fossils are like little time machines that can show us what types of organisms where alive millions or even billions of years ago. And they can show us how life on Earth has changed over time. But in order to unlock these secrets, scientists have to estimate the ages of the fossils they find. This can be done in two main ways. Either by looking at which layers of the sedimentary rock the fossils are found in or by using a process known as radiometric dating. Let's talk about layer analysis first. As you know, sedimentary rocks are formed in layers with each new layer forming on top of an old layer. This means that the layers that are closer to the surface tend to be newer than the layers that are deeper in the ground. This means that a fossil found in a deeper layer of undisturbed sedimentary rock will be older than the ones found closer to the top. By looking through these layers, we're able to not only tell how old a particular fossil may be, but we can also look into the patterns of fossils over multiple layers to see how life on Earth has changed over time. The second way scientists estimate a fossil's age is through a process known as radiometric dating. Radiometric dating tells us how old a fossil is by studying its chemical properties. Specifically, radiometric dating looks at the decay, or the loss of particles, from certain radioactive atoms in the fossils. Scientists know how fast this decay happens. So by measuring the amount of decay that has occurred, scientists can estimate the age of the fossil. Once scientists know how old the fossil is, it can then be added to Earth's Fossil Record. Earth's Fossil Record consists of all the fossils found on Earth along with their relative ages. And it provides us with the big picture view of the history of life on Earth. And with that, you now know how fossils are formed and dated. Let's go back through the layers of this lesson and go over what we've learned today. Fossils are preserved evidence of organisms that lived long ago. This evidence of life is most commonly found in layers of sedimentary rock. Now, depending on how deep they are in the layers of sedimentary rock or their results from radiometric dating, fossils can be given an estimated age and be placed on Earth's fossil record. Now, when it comes to the fossil I found, it wasn't a major discovery like the discoveries of the Ichthyosaur, Plesiosaur fossils found by paleontologist Mary Anning, but it made me realize that discovering fossils is not carried out strictly by professional scientists. You and I can both find fossils too. All we need is a keen eye, some patience, and maybe a little bit of luck.