Hypothetical evolutionary pathways to pyrenoids from condensation. Model simulations propose that free Rubisco (A) and carbonic anhydrase (CA) could proceed to a phase separated condensate of Rubisco in the presence of a condensing protein factor/linker (e.g. EPYC1 in the Chlamydomonas pyrenoid, or CsoS2/CcmM in α/β-carboxysomes) with CA in close external proximity (B) or co-condensed inside the condensate (C) (Long et al., 2021). Due to the net proton release during Rubisco carboxylation and subsequent decrease in internal pH, co-condensation with CA would favor conversion of CO₂ to $HCO3−$⁠, and thus elevate CO₂. Although the condensate could partially restrict outward diffusion, other models suggest that the condensate would have to be large (i.e. >3 µm in radius) or be surrounded by a diffusion barrier (Fei et al., 2022). Both models indicate that evolution of Rubisco condensation is feasible in the absence of additional Ci uptake components (e.g. LCIA and HLA3). Following condensation, pyrenoid evolution could proceed in several ways, including development of a starch sheath (D) to restrict diffusion of CO₂ out of condensate, and/or a traversing thylakoid membrane that could allow regulatory re-localization of CA to within the condensate when required (E) (i.e. when the CCM is induced), and, in some cases, both combined (F). A comprehensive array of the diversity of pyrenoid architectures is illustrated in Barrett et al. (2021).