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The debate about early Earth differentiation focuses on the processes responsible for the formation of protocrust(s) and continental crust of felsic (SiO₂ ≥ 55 weight %) composition. One aspect of this debate is how Hadean zircons fit into an ultramafic environment. On the basis of experiments, thermodynamic modeling, and elemental partitioning, we show that felsic melts could have been generated by shallow interaction between primordial serpentinized peridotite and basaltic magmas on Earth and Mars. On the basis of the hafnium isotopic evolution of Hadean detrital zircons worldwide, we infer that these interactions allowed for the formation of extensive Hadean felsic crust (4.4 to 4.5 billion years ago), which, in turn, would account for up to 50% of the present continental crustal mass. A similar process may have occurred on Mars. The serpentinized protocrust had a dual role in the primitive planetary environment: to provide ingredients for the continental crust and to enable life to emerge on water-bearing terrestrial planets.