The sequence of bases along the DNA molecule may be compared between species. If the base sequences are similar then the species are closely related. For example, there is a 1.2 percent difference between the DNA of humans and that of chimpanzees, and a 1.4 percent differences between humans and gorillas, making the chimpanzee a closer relative to humans than the gorilla. Humans and orangutans share 97.6 percent of their DNA, but this does not match the 98.8 percent shared by chimpanzee and human.
Evolution is the process of gradual change by which the characteristics of living organization are altered from generation to generation the changes in given organism eventually become so great that it can no longer be regarded as the same species. Fossil records show that such replacement of one species by a more advanced one has been taking place for many millions of years, and that most species have survived for only about ten million years. The idea that living things have all evolved from simple ancestors was first put forward by the ancient Greeks, but until Charles Darwin and Alfred Russel Wallace published their work on natural selection no one could give satisfactory explanation.
Darwin and Wallace noticed that plants and animals produce large numbers of offspring and that there is a struggle for existence among the progeny- competition to find food and avoid predators as well as a struggle against adverse environmental factors. They also realized that the progeny vary, and reasoned that only those most suited to their surroundings would survive. The progressive improvement shown throughout evolution is thus brought about by natural selection of the best in each generation.