In the long history of Earth (about 4.5 billion years) most species have become extinct. The fossil record (see How We Know) shows what many extinct animals and plants were like. These life forms have never come back. Why did they disappear? Are we next?
Extinction occurs when drastic changes occur in climate or living conditions. A given species may or may not be adapted, for life under the changed conditions. About the same time that extinction occurs, new species may appear if they have traits that are useful in the new environment. New species typically have a genetic make-up that allows them to take advantage of the changed life conditions. New species appear because they happen to be adapted to the changed conditions.
Changes in the Universe
Scientists also have evidence that the non-living universe has been changing for millions of years.
Such living and non-living changes are call "evolution." Many people (but only a few scientists) do not believe in evolution, arguing that the idea is "just a theory." Scientific theories are not simple opinions. They are supported by observable evidence and data. Theories that stand the test of time should be shown much respect. Scientists use theories
Absolute proof for a theory is not necessary. A theory deserves support as long as it remains useful and cannot be disproved through experimental evidence. No theory in biology meets these requirements better than the theory of evolution.
In "modern" times, ideas about evolution were first introduced around 1865 by Charles Darwin and Alfred Wallace. When they found out about each other's work, Darwin and Wallace decided to publish at the same time. But it was not evolution that they discovered. What they discovered was how evolution occurred.
Talk about evolution has always created religious arguments. In the time of Darwin and Wallace, people who thought evolution made sense were subject to vicious ridicule and condemnation as evil people. To a lesser degree, the same views exist today.