Imagine pond scum: there is an interface between two fluids, and living cells are confined to this 2 dimensional structure. complexity has evolved hierarchical up to the point of having internal organelles (essentially eucaryotes) giving the cell some thickness, but that's been maxed out. A cell must have one side in the top fluid and one side in the bottom fluid to survive without rupturing. Furthermore, each side has a distinct material that is meant for the respective fluid (that is, they can’t pitch or flip over).
Now they’ve evolved a single cell thickness biofilm, daughters sticking together and performing different roles within the community.
But, given that it must remain only a single cell thick, how might it continue to evolve more complexity? How might it even develop intelligent behavior?