A 2017 paper in Ecology and Evolution shows how the primary selection of metabolism and mass selects for life history transitions between virus, prokaryotes, larger unicells and multicellular animals.
Evolution on Earth have created distinct lifeforms with discrete size classes and unique life histories. These include i) virus with no metabolism, no cell, and almost no mass; ii) prokaryotes with a small cell, asexual reproduction, and a mass specific metabolism that increases with mass; iii) asexual unicellular eukaryotes with a larger mass than prokaryotes, and a mass specific metabolism that is first increasing and then declining with an increase in mass; and iv) large multicellular animals with sexual reproduction and a mass specific metabolism that declines with mass.
The original version of Malthusian Relativity was well-suited for the prediction of life histories and allometries in multicellular animals, but there was a major theoretical hole in relation to the natural selection of life history differences between virus, bacteria and larger unicells. All low energy organisms were simply selected to a lower size limit with no mass, and the paper in Ecology and Evolution shows that this is prevented by primary selection of metabolism.
All organisms have a quality-quantity trade-off where energy can be used on many small, or on a few large offspring. Because of this background selection against mass, it follows that net energy must increase superlinearly with mass before an increase in mass can be selected.
With mass-specific metabolism being selected as the pace of the resource handling that generates net energy for self-replication, it follows that an initial mass can be selected by a mass specific metabolism that depends on the mass of the molecular replicator at the origin of life. An initial sublinear dependence will select for virus-like replicating molecules with no intrinsic metabolism and practically no mass, but a small prokaryotic-like cell with an internal metabolism is selected by a superlinear dependence.
These cells can be selected into the larger life history class of unicellular eukaryotes by the gradual unfolding of feed-back selection from density dependent interactive competition. And yet another size class is selected when the feed-back is fully developed, and the positive dependence of mass specific metabolism on mass is vanishing with the evolution of complete metabolic pathways. This absence of a metabolic mass dependence selects for a multicellular animal, with the interactive competition of the fully developed feed-back selecting for sexual reproduction.