Not biologist, but I wonder whether these model organisms are valid. One might expect that today's single cell organisms may have had multicellular ancestors, but have kept the--I struggle for the word--the required genetic machinery for multicellular form in dormant/unexpressed fashion, but otherwise relatively intact and ready for expression given certain evolutiinary presures. This would be in contrast to the original evolution of multicellular life. Edit: I see others here have expressed this idea already, and more elegantly.