JWST has revealed the apparent evolution of the black hole (BH)-stellar mass (M_BH-M_*) relation in the early Universe, while remaining consistent with the BH-dynamical mass (M_BH-M_dyn) relation. We predict BH masses for z > 3 galaxies in the high-resolution THESAN-ZOOM simulations by assuming the M_BH-M_dyn relation is fundamental. Even without live BH modelling, our approach reproduces the JWST-observed M_BH distribution, including overmassive BHs relative to the local M_BH-M_* relation.

We find that M_BH/M_* declines with M_*, evolving from ~0.1 at M_* = 10⁶ M_☉ to ~0.01 at M_* = 10^10.5 M_☉. This trend reflects the dark matter (f_DM) and gas fractions (f_gas), which decrease with M_* but show little redshift evolution down to z = 3, resulting in small M_*/M_dyn ratios and thus overmassive BHs in low-mass galaxies.

We use Prospector-derived stellar masses and star-formation rates to infer f_gas across 48,022 galaxies in JADES at 3 < z < 9, finding excellent agreement with our simulation. Our results demonstrate that overmassive BHs would naturally result from a fundamental M_BH-M_dyn relation and be typical of the gas-rich, dark matter-dominated nature of low-mass, high-redshift galaxies. Such overmassive, rapidly growing BHs may strongly influence the earliest stages of galaxy formation.