Generation of reactive oxygen species (ROS) such as superoxide ions is an important signalling mechanism in mammals and plants. Yeast and bacteria possess systems that allow them to respond to ROS produced during oxidative stress but do not actively produce them as messengers. So when did ROS generation become a signalling mechanism and why? Gareth Bloomfield and Catherine Pears suggest the answer lies at the advent of multicellularity (see p. 3387). They show that the social amoeba Dictyostelium, which straddles the boundary between unicellular and multicellular life (since it can exist in either form), can generate superoxide. The ROS is produced in response to a secreted factor early in development during the transition to multicellularity, which occurs when the organism runs out of food. The authors show that low-molecular-weight superoxide scavengers or overexpression of the superoxide-scavenging enzyme superoxide dismutase can block Dictyostelium aggregation during the transition to multicellularity;in addition, they find that it significantly reduces expression of genes important for early development. Bloomfield and Pears therefore conclude that a superoxide-dependent signal is critical for initiation of development in Dictyostelium. They also note that ROS play important developmental roles in animals and plants, suggesting that this mechanism might have arisen to provide the signalling diversity that multicellularity demands.