[Symmetry]

I'm not talking about the difference between unicellular and multicellular life. I'm talking about the difference between small cells that resperate over their cell walls (prokaryotes and archaea) and cells that resperatory organelles (eukaryotes). Both algae and hydra are already over this barrier and I agree that once you're a eukaryote or the alien equivalent there's an easy path from there to complex multi-cellular interactions.

So certainly there's no missing link there. Heck, you've even got prokaryotes forming bacterial mats. The clear boundary is, again, between small cells that resperate over their surfaces and more complex cells that can resperate using structures throughout their interiors.