Primitive autonomous organisms, the bacteria, are essentially composed of a tough membrane that separates the inner environment from the exterior. Hooked onto this membrane is a dense string of circular double-stranded nucleic acid of the DNA type. The remainder of the inner compartment is made of the protein-synthesizing mechanism. In these bacteria, the genetic material (DNA) is not kept in a separate compartment and these unsophisticated organisms are called prokaryotes. In more evolved living forms, the genetic material is kept apart within a small globule called the nucleus. Cells provided with a nucleus are eukaryotes.
The multiplication of primitive autonomous single-celled organisms such as bacteria proceeds through a duplication of the genetic material inside the cell. After duplication, the two genomes move to opposite ends and the cell splits in two, each daughter cell carrying one genome. This straightforward cellular multiplication allows the transfer of only a limited amount of information. In bacteria, only one million nucleic acid bases can be duplicated without the introduction of substantial errors. As a consequence, the number of proteins synthesized by these primitive autonomous organisms remains low and further evolution is jeopardized.
A more exacting duplicating mechanism on one side, sustained by a doubling within the cell of the amount of genetic information carried by each member of a species, is an achievement that allowed the synthesis of more proteins, the coping with unforeseen conditions and the attainment of a new evolutive level. This diploïdy of the genome was gained by the genetic exchange that occurs during sexual mating. This sexual mating is practiced by evolved eukaryotic unicellular organisms, which thereafter moved further up the evolutive ladder by assembling into colonies. The differentiation of the cells composing the colony allowed the appearance of multicellular organisms.