Evolution, in biology, complex process by which the characteristics of living organisms change over many generations as traits are passed from one generation to the next. The science of evolution seeks to understand the biological forces that caused ancient organisms to develop into the tremendous and ever-changing variety of life seen on Earth today. It addresses how, over the course of time, various plant and animal species branch off to become entirely new species, and how different species are related through complicated family trees that span millions of years.
Evolution provides an essential framework for studying the ongoing history of life on Earth. A central, and historically controversial, component of evolutionary theory is that all living organisms, from microscopic bacteria to plants, insects, birds, and mammals, share a common ancestor. Species that are closely related share a recent common ancestor, while distantly related species have a common ancestor further in the past. The animal most closely related to humans, for example, is the chimpanzee. The common ancestor of humans and chimpanzees is believed to have lived approximately 6 million to 7 million years ago. On the other hand, an ancestor common to humans and reptiles lived some 300 million years ago. And the common ancestor to even more distantly related forms lived even further in the past. Evolutionary biologists attempt to determine the history of lineages as they diverge and how differences in characteristics developed over time.