What tectonic regimes operated on the early Earth and how these differed from modern plate tectonics remain unresolved questions. We use numerical modelling of mantle convection, melting and melt-depletion to address how the regimes emerge under conditions spanning back from a modern to an early Earth, when internal radiogenic heat was higher. For Phanerozoic values of internal heat, the tectonic regime depends on the ability of the lithosphere to yield and form plate margins. For early Earth internal heat values, the mantle reaches higher temperatures, high-degree depletion and differentiated into a thicker and stiffer lithospheric mantle. This thermochemical differentiation stabilises the lithosphere over a large range of modelled strengths, narrowing the viable tectonic regimes of the early Earth. All the models develop in two stages: an early stage, when decreasing yield strength favours mobility and depletion, and a later stabilisation, when inherited features remain preserved in the rigid lid. The thick lithosphere reduces surface heat loss and its dependence on mantle temperature, reconciling with the thermal history of the early Earth. When compared to the models, the Archean record of large melting, episodic mobility and plate margin activity, subsequently fossilised in rigid cratons, is best explained by the two-stage evolution of a lithosphere prone to yielding, progressively differentiating and stabilising. Thermochemical differentiation holds the key for the evolution of Earth’s tectonics: dehydration stiffening resisted the operation of plate margins preserving lithospheric cores, until its waning, as radioactive heat decays, marks the emergence of stable features of modern plate tectonics.