G protein-coupled receptors (GPCRs) allosterically activate heterotrimeric G proteins and trigger GDP release. Given that there are 800 human GPCRs and 16 different Ga genes, this raises the question of whether a universal allosteric mechanism governs Ga activation. Here we show that different GPCRs interact with and activate Ga proteins through a highly conserved mechanism. Comparison of Ga with the small G protein Ras reveals how the evolution of short segments that undergo disorder-to-order transitions can decouple regions important for allosteric activation from receptor binding specificity. This might explain how the GPCR–Ga system diversified rapidly, while conserving the allosteric activation mechanism. The paper by Tilman Flock et al can be viewed here.