Overmassive black holes and little red dots naturally form in simulations.

The origin of supermassive black holes remains a long-standing problem in astrophysics. Recent James Webb Space Telescope (JWST) observations reveal an unexpectedly abundant population of overmassive black holes at z > 4-6, at which the black hole masses lie far above local scaling relations and are not reproduced by present cosmological models1-5. How such overmassive black holes form and rapidly grow within young galaxies has remained unclear. Here we present fully cosmological radiation-hydrodynamic simulations that self-consistently follow the birth, early growth and emergent observable signatures of supermassive black holes in protocluster environments. We find that heavy seeds on the order 106 M⊙ naturally form, exceeding typical theoretical expectations by an order of magnitude. These seeds rapidly develop dense, optically thick disks whose strong electron scattering produces broad Hα emission comparable to that seen in little red dots6-10. Sustained super-Eddington accretion then drives fast growth to about 3 × 107 M⊙ by z ≃ 8. To our knowledge, this is the first demonstration that unifies little red dots to a short-lived, enshrouded phase of heavy-seed formation, which naturally evolve into the overmassive quasars detected by the JWST and ultimately the progenitors of today's supermassive black holes.