There are several kingdoms with multicellular life. Plants, animals, most fungi, some protists (many protists are more closely related to animals, fungi, or plants than they are to each other for that matter). See: https://en.wikipedia.org/wiki/Eukaryote#Five_supergroups
Note e.g. that animals and fungi are closely related on this diagram, but there are unicellular fungi (notably yeast). I would guess that this means the common ancestor between animals and fungi would have been unicellular. At any rate, both are far from plants, and other protist species unrelated to all of these are multicellular.
Endosymbiosis is generally accepted to also have occurred with chloroplasts. I believe there are also contemporary observed cases of protists consuming photosynthetic organisms and becoming immobile.
Note e.g. that animals and fungi are closely related on this diagram, but there are unicellular fungi (notably yeast). I would guess that this means the common ancestor between animals and fungi would have been unicellular. At any rate, both are far from plants, and other protist species unrelated to all of these are multicellular.
Endosymbiosis is generally accepted to also have occurred with chloroplasts. I believe there are also contemporary observed cases of protists consuming photosynthetic organisms and becoming immobile.