According to scientists, life on earth began between 4 and 5 billion years ago. You and I are products of that long chain of life. All living things change over time. These changes occur through a process called evolution . Evolution, in fact, means slow change .
Natural selection is the process by which evolution works.
Changes in living things actually happen by accident. These changes are called random mutations . Most often, random mutations prove to be harmful to an organism and cause it to be less likely to reproduce. A common example is the modern giraffe. Mutations in early giraffes gave it a longer neck.