This document focuses on the changes in living things during the long history of life on earth-on what is called biological evolution. Many kinds of cumulative change through time have been described by the term "evolution," and the term is used in astronomy, geology, biology, anthropology, and other sciences. More than 99 percent of the species that have ever lived on the earth are now extinct, either because all of the members of the species died, the species evolved into a new species, or it split into two or more new species. Mammals now live in a world that was once dominated by reptilian giants such as Tyrannosaurus rex. Trilobites that populated the seas hundreds of millions of years ago no longer crawl about. Similarly, the fossil record reveals profound changes in the kinds of living things that have inhabited our planet over its long history.
Fossils of primitive microorganisms show that life had emerged on earth by about 3.8 billion years ago. The earth itself formed shortly thereafter, when rock, dust, and gas circling the sun condensed into the planets of our solar system. Powerful telescopes reveal new stars coalescing from galactic dust, just as our sun did more than 4.5 billion years ago. Other changes are more gradual but much more dramatic when viewed over long time scales. Streams wash dirt and stones from higher places to lower places. This simple fact is obvious everywhere we look.