The early Cambrian encompasses both the Cambrian Explosion and the subsequent Botoman–Toyonian extinction, yet the mechanisms driving these two events remain vigorously debated. A globally coherent seawater strontium isotope (87Sr/86Sr) framework has been lacking for this critical interval. Here we present a new high-resolution 87Sr/86Sr record derived from 48 carbonate samples of eastern Siberia, which we integrate with previously published datasets from South China, Mongolia, and Morocco.Our data reveal a rapid rise in seawater 87Sr/86Sr of ~0.0006, from ~0.7081 in Cambrian Stage 2 to ~0.7087 in Stage 3, synchronous with the first appearance of diverse skeletal taxa.Using a simple box model, we demonstrate that this isotopic shift is best explained by enhanced continental weathering, which delivered radiogenic Sr from sources such as the Pan-African orogenic belts and regolith developed on the Great Unconformity. Increased denudation likely elevated phosphorus and other nutrient fluxes, thereby stimulating carbonate biomineralisation and fuelling metazoan diversification.We further present the LOWESS-fitted ⁸⁷Sr/⁸⁶Sr curve for the first two epochs of the Cambrian. The secular trend is characterized by a decline from the Ediacaran–Cambrian boundary to ~523 Ma, followed by a sustained rise reaching ~0.7087 by ~517 Ma, with a transient plateau at ~517–515 Ma that coincides with the Sinsk transgression—suggesting that sea-level fluctuations modulated the supply of radiogenic Sr to the ocean. We propose that shallow-marine anoxia, widespread drowning of carbonate platforms, and environmental perturbations associated with the Kalkarindji Large Igneous Province collectively acted as synergistic drivers of the Botoman–Toyonian extinction. Our integrated isotope–box model approach offers a quantitative framework for linking tectonic weathering, ocean chemistry, and biological turnover, providing new insights into how Earth's surface processes regulated the tempo of early animal evolution.