This rock contains a fossilized marine animal known as a crinoid. Scientists study fossils of plants, animals, and other organisms in order to better understand what life was like on Earth many years ago and how it has changed over time. Fossils are important evidence for the theory of evolution.
Fossils are the preserved remains of animals, plants, and other organisms from the distant past. Examples of fossils include bones, teeth, and impressions. By studying fossils, evidence for evolution is revealed. Paleontologists are scientists who study fossils to learn about life in the past. Fossils allow these scientists to determine the features of extinct species. Paleontologists compare the features of species from different periods in history. With this information, they try to understand how species have evolved over millions of years (Figure below). 
There are many layers of rock in the Earth's surface. Newer layers form on top of the older layers; the deepest rock layers are the oldest. Therefore, you can tell how old a fossil is by observing in which layer of rock it was found. The fossils and the order in which fossils appear is called the fossil record. The fossil record provides evidence for when organisms lived on Earth, how species evolved, and how some species have gone extinct. Geologists use a method called radiometric dating to determine the exact age of rocks and fossils in each layer of rock. This technique, which is possible because radioactive materials decay at a known rate, measures how much of the radioactive materials in each rock layer have broken down (Figure below). Radiometric dating has been used to determine that the oldest known rocks on Earth are between 4 and 5 billion years old. The oldest fossils are between 3 and 4 billion years old. Remember that during Darwin's time, people believed the Earth was just about 6,000 years old. The fossil record proves that Earth is much older than people once thought.
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