The thermodynamic concept of time is directly applicable to the discussion of life processes. Charles Darwin and Herbert Spencer's theory of evolution for the first time related growth and life to developing complexity. The evolution of life is revealed as an irreversible temporal development of complex systems. In the framework of nonequilibrium thermodynamics, evolution can be understood in terms of symmetry breaking. This lies at the root of life's time arrow. But many different biological temporal rhythms must be distinguished. In the course of evolution, these rhythms were superimposed on each other into complex time hierarchies, encompassing everything from elaborate economic systems to single living organisms.