In this topic you will learn about evidence that shows that organisms change over time.

There is evidence that ancient animals existed on Earth. The most abundant evidence comes from fossils. Fossils are any remains or traces of an organism that have been preserved in Earth's crust. Fossils include preserved forms of bones and teeth, and imprints of shells, leaves, and insects. Fossils can tell us when and where an organism lived, what it looked like, and perhaps even how it lived. When most organisms die, they decay and do not form fossils. If an organism is buried quickly, before it decays, it may become a fossil.

Most of the fossils we find are left behind by organisms that no longer live on Earth. As changes in the environment took place, some species were not able to survive. When a species dies out because of changes in the environment, it becomes extinct. Dinosaurs are extinct organisms.

As some species became extinct, other species developed. Fossils suggest that the newer species developed from changes in earlier species. The idea that species change over time is called evolution. This idea is a theory, supported by fossil evidence. Most scientists believe that evolution explains how so many different forms of life have developed on Earth.

To understand the evolution of life on Earth, scientists need to be able to tell how old a fossil is. To date a fossil—that is, to tell how old it is—scientists often compare its age with the rock in which it is found. Relative dating is based on the location of the sediment layer in which the fossil is found. Older rock layers lie beneath younger rock layers. The arrangement of sediment layers tells the order in which organisms appeared and disappeared, but it cannot reveal exactly when each organism was alive.

Radioactive dating is used to determine the actual age of a rock layer or fossil. Radioactive elements constantly give off energy. As the energy is released, the radioactive element slowly decays into another element. The rate of decay is different for each radioactive element. By comparing the amounts of each element in a fossil, scientists can tell how many years ago the fossil was formed.

Scientists use fossils and other clues to organize the events in Earth's history into a time line called the geologic time scale. The scale is divided into four major sections, called eras. Except for Precambrian time, each era is divided into smaller time segments, called periods. Each division covers a certain amount of time in Earth's history and is based on the types of organisms that lived on Earth during that time.