The natural selection of metabolism and mass selects lifeforms from viruses to multicellular animals.

I show that the natural selection of metabolism and mass can select for the major life-history and allometric transitions that define lifeforms from viruses, over prokaryotes and larger unicells, to multicellular animals. The proposed selection is driven by a mass-specific metabolism that is selected as the pace of the resource handling that generates net energy for self-replication. An initial selection of mass is given by a dependence of mass-specific metabolism on mass in replicators that are close to a lower size limit. A sublinear maximum dependence selects for virus-like replicators, with no intrinsic metabolism, no cell, and practically no mass. A superlinear dependence selects for prokaryote-like self-replicating cells, with asexual reproduction and incomplete metabolic pathways. These self-replicators have selection for increased net energy, and this generates a gradual unfolding of population-dynamic feed-back selection from interactive competition. The incomplete feed-back selects for larger unicells with more developed metabolic pathways, and the completely developed feed-back for multicellular animals with sexual reproduction. This model unifies the natural selection of lifeforms from viruses to multicellular animals, and it provides a parsimonious explanation where allometries and major life histories evolve from the natural selection of metabolism and mass.