In nature there is no notion of joining forces to make something "better" or more complex. There's no plan at the outset to build an animal, only endless incremental tinkering and change.
There's plenty of well-studied cases of specialization in behavior of bacteria when, for example, they hit quorum and you start to see some emergent macro properties. I don't see why there couldn't be more complex extensions of that behavior. No multicellularity required there.
