Mapping the history of atmospheric O ₂ during the late Precambrian is vital for evaluating potential links to animal evolution. Ancient O ₂ levels are often inferred from geochemical analyses of marine sediments, leading to the assumption that the Earth experienced a stepwise increase in atmospheric O ₂ during the Neoproterozoic. However, the nature of this hypothesized oxygenation event remains unknown, with suggestions of a more dynamic O ₂ history in the oceans and major uncertainty over any direct connection between the marine realm and atmospheric O ₂ . Here, we present a continuous quantitative reconstruction of atmospheric O ₂ over the past 1.5 billion years using an isotope mass balance approach that combines bulk geochemistry and tectonic recycling rate calculations. We predict that atmospheric O ₂ levels during the Neoproterozoic oscillated between ~1 and ~50% of the present atmospheric level. We conclude that there was no simple unidirectional rise in atmospheric O ₂ during the Neoproterozoic, and the first animals evolved against a backdrop of extreme O ₂ variability.