The neo-Darwinian theory of evolution and adaptation by natural selection has got us used to considering organisms’ features as solutions to problems set by the environment. However, no living system faces an outside reality independent of itself: organisms and groups of organisms, by their constitution and activity, specify the content, events and properties of the world in which they live. In this perspective—the theory of enaction—, the organism does not need to be optimal but simply viable, and it appears as a fundamental actor of the becoming of its lineage at the scale of phylogeny.