Early computational studies of the rTCA cycle focused on whether it was unique or representative of a larger family of possible cycles. Morowitz et al. found that while the 11 molecules comprising the rTCA cycle are among a subset of organic chemical space that satisfies a set of feasibility criteria for primitive carbon fixation, they are not unique in doing so. Over a hundred other known molecules also satisfy these constraints, comprising about 0.004% of relevant organic chemical space. Other researchers later extended this approach by enumerating additional chemical subspaces in search of rTCA alternatives. 8, These findings suggest that multiple alternative carbon fixation cycles might have been chemically possible in a prebiotic context. However, these simple filters do not capture the full requirements of a plausible prebiotic carbon fixation cycle. Alternative pathways may have faced insurmountable kinetic or thermodynamic barriers under early Earth conditions or may have been less accessible to subsequent evolutionary optimization. Understanding these constraints is crucial for determining why the rTCA cycle, among many theoretically possible options, became a central feature of modern metabolism.