In this topic you will learn how to classify organisms of the past.

How much do we know about the great meat-eating dinosaur Tyrannosaurus rex? It was 46 feet long (14 meters) and 20 feet tall (6 meters), and it weighed 17,600 pounds. It walked upright on two powerful legs. Its tiny arms were too short to even reach its mouth. How do we know so much about Tyrannosaurus rex? We have the evidence it left behind.

A fossil is any evidence of an organism that lived in the past. Fossils often are skeletons preserved in rocks. People are interested in the history of organisms that lived in the past. As scientists learn more about an organism's past, they must consider that organisms change over time. Change in living things over time is called evolution.

To find the age of a fossil, scientists can compare it with other fossils. They can do this by studying the rock layer in which a fossil is found. The oldest fossils are in the oldest layers, which are at the bottom. Younger fossils are found in later, upper rock layers.

Can organisms that seem different be related? Scientists compare limbs to understand what is similar and different about organisms. They see which have similar features and might be related.

Related organisms have a common ancestor. Scientists look for evidence of common ancestors when they classify organisms. For example, whales are more closely related to humans than to sharks. You can see this by looking at whale flipper bones. They are much more like human arm bones than are shark fins.

Another clue to finding similarities among organisms comes from before they were even born. An undeveloped animal or plant is called an embryo. An embryo changes before it is ready for the world. Some features are lost in certain animals as the embryo grows.

Many organisms are extinct, which means they no longer live on Earth. Dinosaurs may have become extinct when huge asteroids or comets hit the Earth. To help classify extinct organisms, scientists can compare fossil embryos with each other and with modern embryos.

Learning about extinct organisms can help scientists classify living organisms. It can also help us understand the history of life on Earth.