Evolution is a theory in biology that postulates that the various types of animals and plants have their origin in other preexisting types and that the distinguishable differences are due to modifications in successive generations. The theory of evolution is one of the fundamental keystones of modern biological theory.

In 1721 the French philosopher Montesquieu affirmed his belief that a few early species had multiplied into present ones and that differences between animal species might increase and decrease. This belief that species could change into other species formed the basis of the idea of transmutation, or evolution.

Georges Buffon, an 18th-century naturalist, raised the possibility of the horse being related to the ass, and by 1760 the Swedish taxonomist Carolus Linnaeus had concluded that species could vary. In 1809 Jean-Baptiste Lamarck studied differences between species and varieties and drafted an evolutionary tree from tiny animals up to human beings, with branches showing ancestry shared by different groups.

It was Charles Darwin who presented the treatise on evolution that would revolutionize all later biological study. His observations of related but different species living in adjacent geographic areas, of the structural similarity between living forms and fossil remains in the same area, and of the differences between species living on neighbouring islands in the Galapagos Islands formed the basis of his Origin of Species (1859).

The heart of Darwinian evolution is the mechanism of natural selection. It proceeds from the fact that there are a greater number of offspring produced by most organisms than can survive to maturity. A high rate of mortality--through starvation, predation, disease, and accidents--reduces the population of those individuals less adapted to survive. The surviving individuals, which may have the smallest variation that enables them to live longer and reproduce, pass on their advantage to succeeding generations, which may evolve superior means of adaptation.

Darwin lacked an inheritance theory to explain the passing on of advantageous variations from one generation to the next. This gap was filled by the 20th-century science of genetics. In Genetics and the Origin of Species (1937), Theodosius Dobzhansky applied Mendelian genetics to Darwinian theory, contributing to the new understanding of evolution as the cumulative action of natural selection on small genetic variations in whole populations of an organism.

Part of the proof of evolution is provided by the fossil record, which shows a succession of gradually changing forms leading up to the present. Structural similarities among living forms also point to common ancestry, as do similarities in the embryonic development of different species. Molecular biology--particularly the study of genes and proteins--provides the most detailed evidence of evolutionary change.

The beginnings of organic ("Darwinian") evolution occurred in primordial waters, when cells were formed by living organisms surrounding organic compounds. As the food supply available in the liquid medium was exhausted, the development of the food-metabolizing process began, converting the primitive units into autotrophs and heterotrophs. Later advances were the organization of chromosomes, DNA, and nuclei for passing on characteristics, greatly expanding the powers of natural selection through adaptation. When multicellular life brought the differentiation of body parts, with differing structures and functions, survival value was enhanced.

When organisms began colonizing land, about 500,000,000 years ago--after billions of years in the water environment--methods of feeding became consequential. Species better able to adapt and colonize new environments became widespread (adaptive radiation).

The evolutionary ancestors of humans were able to adapt to different environments through time. The vestige of a tail with which humans are born is evolutionary evidence of an ancestor that was arboreal, a quadruped. The next development in primate evolution, using the hands to swing from branches, was the habit of brachiation. This was followed by the adoption of a terrestrial life by humanlike primates, the australopithecines, some 3,000,000 years ago. These beings had some apelike and some human characteristics.

Increased mental powers, brain size, and neurological complexity are characteristics of the genus Homo, which includes such extinct humanlike forms as Java Man, Peking Man, Heidelberg Man, and other pithecanthropines.

Direct fossil evidence of the earliest members of the human species, Homo sapiens, is widely distributed, placing the geographic origin of the species in doubt. Homo sapiens can be defined by the number of anatomical characteristics its members share, such as a particular cranial capacity, a vertical forehead, a rounded back of the skull, and limb bones adapted to an erect posture and gait.

Although the theory of evolution is accepted by the overwhelming majority of the scientific community, presentation of this theory has aroused considerable controversy from Darwin's time to the present. Most objections have come from theologians, primarily fundamentalists, who feel that Darwin's assertion that species are continually changing conflicts with literal interpretations of the Bible, in particular that all species of living things were created by divine design.

Excerpt from the Encyclopedia Britannica without permission.