The themes introduced in the last two sections stress life’s unity: Cells form the basic unit of all life; DNA, RNA, and proteins carry out the molecular functions of all cells; and metabolic reactions build and break down macromolecules. We need only look around us, however, to recognize that for all its unity, life displays a remarkable degree of diversity. We don’t really know how many species share our planet, but reasonable estimates run to 10 million or more. Both the unity and the diversity of life are explained by the process of evolution, or change over time.