Author SummaryProteins often interact with other proteins and assemble into complexes. Here we show that the flexibility of individual proteins is important for their recruitment to complexes, as it facilitates the formation of asymmetric interfaces between different subunits. The role of flexibility becomes increasingly important as a greater number of distinct proteins are packed together within a single complex: the more distinct subunits, the more flexible those subunits need to be. A consequence of this is that, when a protein complex gains a new subunit during evolution, the newer subunit will tend to be more flexible than the older subunits. This suggests that we may be able to partially reconstruct the evolutionary history of a protein complex by considering the flexibility of its subunits. We also find that the types of protein complexes an organism forms are closely related to the flexibility of its proteins, with eukaryotic species, and particularly animals, using their increased flexibility to assemble complexes involving more distinct components.