The paper uses chemical kinetic arguments and illustrations by computer modelling to discuss the origin and evolution of life. Complex self-reproducing chemical systems cannot arise spontaneously, whereas simple auto-catalytic systems can, especially in an irradiated aqueous medium. Self-reproducing chemical particles of any complexity, in an appropriate environment, have a self-regulating property which permits long-term survival. However, loss of materials from the environment can lead to continuing decay which is circumvented by physical union between different kinds of self-reproducing particles. The increasing complexity produced by such unions (symbioses) is irreversible so that the chemical system evolves. It is suggested that evolution by successive symbioses brought about the change from simple, spontaneously arising, auto-catalytic particles to complex prokaryotic cells.