SignificanceData and theory reveal how organisms allocate metabolic energy to components of the life history that determine fitness. In each generation, animals take up biomass energy from the environment and expend it on survival, growth, and reproduction. Life histories of animals exhibit enormous diversity—from large fish and invertebrates that produce literally millions of tiny eggs and suffer enormous mortality, to mammals and birds that produce a few large offspring with much lower mortality. Yet, underlying this enormous diversity are general life history rules and trade-offs due to universal biophysical constraints on the channels of selection. These rules are characterized by general equations that underscore the unity of life.