Living organisms are unique in their ability to generate and replicate ordered systems from disordered components. Generation of order, replication of the individual, and evolution of the species all depend on the successful utilization of external energy derived from chemicals and light. The information for reproduction is encoded in nucleic acids, but evolution depends on a limited variability in replication, and proceeds through the selection of individuals with altered biochemistry. Essentially all biochemistry is catalyzed; therefore, altered biochemistry implies altered or new catalysts. In that sense catalysis is the medium of evolution. We propose that a basic property of enzymes, at least as fundamental as reaction rate enhancement, is to adjust the reaction path by altering and eventually optimizing the reversible interchange of chemical, electrical and mechanical energy among themselves and their reactants.