Here we use the theory of ecological stoichiometry to propose and provide a preliminary test of a novel hypothesis that the Cambrian ‘explosion’ may have been triggered by changes in circulating P availability in the biosphere. We exposed living stromatolites from a spring-fed stream in Mexico to a gradient of P enrichment to examine subsequent effects on stromatolite C : P ratio and on the primary grazer, an endemic snail. Consistent with a previously hypothesized stoichiometric ‘knife-edge’, snail performance was maximal at intermediate P-enrichment, indicating in situ stoichiometric constraints because of high stromatolite C : P ratio along with high sensitivity to excessive P intake. These results are consistent with the idea that stoichiometric constraints may have delayed the evolutionary proliferation of animals in ancient stromatolite-dominated ecosystems and also suggest that high food P content can significantly impair consumers. We propose that ecosystem P availability may have impacted both the expansion and decline of animal taxa in the history of life.