Strong paleontological and molecular evidence indicates that life first appeared on Earth about four billions years ago, in the form of prokaryotes, or non-nucleated unicellular organisms, such as archaea and bacteria. Comparative genetic studies of bacteria and archaea with the first unicellular eukaryotes have shown that different species were originated from ancestral prokaryotes through progressive mutations in DNA replication. Although the stability in genome duplication is crucial to the perpetuation of species, genetic changes or mutations are also important to both the evolution of a new species and the survival of a given species. Mutations occurring in germ cell lines are transmitted to the offspring, thus causing genetic variation that may or may not offer survival advantages, or disease. Mutations, therefore, are the primary source of genetic variation. The secondary process of producing genetic variation is DNA recombination that often occurs during meiosis. Recombination results from...