After investigating Earth’s ancient paradoxes—Faint Young Sun paradox, Hadean zircon mystery, and Mesozoic megafauna enigma—we find that they all converge toward a unified resolution at 0.6g. We report that eight entirely independent proxies—encompassing mineralogical, geological, climatological, fluid-dynamic, biomechanical, aerodynamic, and thermodynamic datasets—consistently indicate a systematic reduction of the effective gravitational acceleration g_eff in deep time. These proxies, rigorously measured and cross-validated in this work, exhibit a multimodal convergence that is incompatible with the null hypothesis (1g0 in deep time). Unsatisfied with incomplete biological or atmospheric explanations, we introduce our framework. Our quantitative physical model, which remains independent from the empirical and hard observations reported, explicitly holds the universal gravitational constant G, Earth’s mass, and radius invariant, fully compatible with all tight astrophysical constraints. This is achieved by modeling the local effective gravitational acceleration g_eff(t) through advanced physics models, which permit g_eff(t) to undergo rare, episodic fluctuations across geological timescales. Our model achieves a present-day screening accuracy within 1 × 10−7, satisfying Lunar Laser Ranging (LLR) constraints. The framework is uniquely parsimonious as it explains those enigmas using a single physical variable. Non-biological independent validations are found within the Hadean paradoxes. Using finite-element analysis and allometric stress laws, we derive a robust biomechanical constraint: the survival of these taxa required g_eff ≲ 0.85 g0 during the Mesozoic Era. This figure, validated by Sobol analysis, transforms the fossil record into a Fossil Gravimeter, opening new horizons of research.