The diversification of metazoans during the latest Neoproterozoic and early Cambrian has been attributed to, among other factors, a progressive rise in surface oxygen levels. However, recent results have also questioned the idea of a prominent rise in atmospheric oxygen levels or a major or unidirectional shift in the marine redox landscape across this interval. Here, we present new carbonate-associated uranium isotope data from upper Ediacaran to lower Cambrian marine carbonate successions. These data provide evidence for short-lived episodes of widespread marine anoxia near the Ediacaran-Cambrian transition and during Cambrian Age 2 (ca. 525 Ma). We suggest that biotic turnover and resulting ecological restructuring, triggered by marine redox fluctuations rather than progressive oxygenation, were the dominant drivers of the Cambrian explosion. Episodes of harsh environmental conditions against a backdrop of Proterozoic–Phanerozoic oceanic oxygenation on the eve of the Cambrian explosion could have, by promoting ecosystem restructuring, spurred the diversification of the Cambrian Evolutionary Fauna.