Some are so radically different that paleontologists have created new phyla in order to classify them.
( Cambrian Period.) The first vertebrates, animals with backbones, appeared about 400 million years ago; the first mammals, less than 200 million years ago.
Beginning in the 1960s, a related scientific discipline, molecular biology, enormously advanced knowledge of biological evolution and made it possible to investigate detailed problems that had seemed completely out of reach only a short time previously—for example, how similar the genes of humans and chimpanzees might be (they differ in about 1–2 percent of the units that make up the genes).
This article discusses evolution as it applies generally to living things.
Applications of evolutionary theory to plant and animal breeding are discussed in the articles plant breeding and animal breeding.
An overview of the evolution of life as a major characteristic of Earth’s history is given in community ecology: Evolution of the biosphere.
All living creatures are related by descent from common ancestors.
Humans and other mammals descend from shrewlike creatures that lived more than 150 million years ago; mammals, birds, reptiles, amphibians, and fishes share as ancestors aquatic worms that lived 600 million years ago; and all plants and animals derive from bacteria-like microorganisms that originated more than 3 billion years ago.
Natural selection occurs because individuals having more-useful traits, such as more-acute vision or swifter legs, survive better and produce more progeny than individuals with less-favourable traits.
Genetics, a science born in the 20th century, reveals in detail how natural selection works and led to the development of the modern theory of evolution.
Hereditary variants were thought to arise naturally in populations, and then these were either selected for or against by the contemporary environmental conditions.