History of the Theory of Evolution
Many ancient myths were attempts to explain how the various plants and animals were created. Thales suggested that all life came from water. Aristotle believed that living things could arise out of nonliving matter, and that more complex forms of life probably arose from simpler forms.
With the spread of Christianity, people throughout the Western world accepted the story of Creation as told in Genesis, the first book of the Bible. For hundreds of years no further scientific thought was devoted to the origin of life.
By the 18th century scientists had again begun to investigate the development of living things. Georges Louis Buffon thought that environment acted directly to change the structure of organisms. Erasmus Darwin and Jean Baptiste Lamarck each advanced the now discredited theory that organisms inherit characteristics acquired during the lifetime of their parents.
The basis for the modern theory of evolution was formulated independently by Charles Darwin, grandson of Erasmus Darwin, and by Alfred Russel Wallace. They introduced their ideas about natural selection in joint papers read to a London learned society in 1858. Darwin's Origin of Species (1859) made the theory famous. The Darwin-Wallace theory was incomplete, however, because the science of genetics was unknown at the time. Gregor Mendel discovered the laws of heredity in the 1860's, but his findings were neglected until 1900.
In 1937 the theories of natural selection and genetic variability were fused by Theodosius Dobzhansky, a Russian-born American biologist, in Genetics and the Origin of Species. Dobzhansky's theory, called the Synthetic Theory (because it synthesizes, or combines, previously existing ideas), states that evolution is a two-step process. In the first step, genetic variations occur through mutations or by a chance combination of inherited genes. In the second step, desirable variations—those that produce traits that help the organism survive—are passed on to the next generation and become permanently established in the species.
When the theory of evolution first came to public attention in 1859 it was met with a storm of protest and ridicule. Some people, misinterpreting the theory, thought it claimed that humans were descended from apes. (Actually, the theory states that humans and apes had a common ancestor that was neither human nor ape.) One religious objection to evolution was that it conflicted with the Biblical story of Creation.
In the United States some states passed laws making it illegal to teach evolution in public schools. In 1925 John Scopes, a teacher in Dayton, Tennessee, was brought to trial for violating such a law. Scopes was defended by Clarence Darrow, and William Jennings Bryan was an attorney for the prosecution. Although Scopes was found guilty, the trial helped to end much opposition to the theory of evolution, and the various antievolution state laws were eventually repealed.
In most countries, the theory of evolution achieved acceptance by most religious groups. Opposition to the theory remained greatest in the United States, where many groups adhere to creationism.
Almost all scientists agree on the fact of evolution, but there have been several disputes over the theory. In the 1960's, the principal dispute was between scientists of the neutralist school and those of the selectionist school. The neutralists rejected traditional Darwinism and contended that the genetic characteristics of an organism had no effect on the chances of its survival. The neutralist position lost support when most additional research found little evidence in its favor.
The theory of punctuated equilibrium was proposed in 1972 by the American paleontologists Niles Eldredge and Steven Jay Gould. A dispute arose between scientists who held this theory and those who believed that the evolution of a species remains at a slow pace. An increasing amount of fossil evidence has supported the theory of punctuated equilibrium.
In the late 20th century, sophisticated tools have allowed people to study the process of evolution at the genetic level. Studies of genes of various species have shown that there is a great variation in the gene pools of animals. The genes shared by members of a species are not same. By comparing the DNA of various species, scientists can find out how closely two species are related, and whether they share a recent common ancestor.
Evolutionary scientists who work with DNA analysis not just compare the DNA of living organisms, but also conduct analysis on the DNA of fossil animals and plants to find out more about their evolutionary history. DNA analysis of fossil remains of our human ancestors has established the theory of a common ancestor of all humans.
