The soil amoeba Dictyostelium is an excellent model system in which to study how environmental signals regulate development. Starvation induces the amoebae to aggregate into a slug, which migrates to the soil surface. Here it `culminates', forming a fruiting body of spores and stalk cells. Culmination is O2-dependent and, on p. 3349, West and co-workers reveal that the enzyme prolyl 4-hydroxylase-1 (P4H1) acts as an O2 sensor during this stage of Dictyostelium development. Culmination normally requires O2 levels above 10%. But, the researchers show, disruption of the P4H1 gene increases this requirement so that culmination is blocked at ambient O2 levels. By contrast,overexpression of P4H1 reduces the O2 requirement of culmination to below 5%. Because P4H1 is an orthologue of the prolyl hydroxlases that sense O2 levels in animals, the researchers suggest that it functions as part of an ancient mechanism for O2 sensing that predates the evolution of animals and that, in Dictyostelium, regulates culmination.