This study draws evidence from the fossil record and from molecular biology to develop and support the theory that complex cells are symbiotic unions of bacterial cells. Bringing together research from disparate fields such as micropalaeontology, molecular biology and symbiosis research, the authors discuss: the evolution of metabolism; the eukaryotic host cell; the evolution of the meiotic cell; the acquisition of mitochondria and plastids; and the controversy over the origin of motility organelles.