Recent research provides an unprecedented account of the diversity and biomass of life, but the data also suggest unexplained patterns such as the co-dominance of very different life forms. We compile the planetary body size biomass spectrum across all taxa and investigate possible underlying forces. We find that small (10-14 g) and large (106 g) organisms vastly outweigh other sizes. The global spectrum reveals an allometric power exponent close to zero, with the marine spectrum in particular showing multiple closely packed modes that are compatible with metabolic food webs. All habitat realms share two distinct size modes that correspond well to the evolutionary innovations of unicellular and complex multicellular life forms, plus a smaller third mode representing unicellular endosymbiotic life. Each mode contains both producers and consumers. These findings show both differences and similarities across habitat realms and point to a size-based synthesis of microevolution, macroevolution, and macroecology.