Evolution of complex life from single cells:
The immediate, single-celled precursors of Animalia are known as the Choanoflagellates. They are single-celled organisms with a single, rear flagella, placing them in the family of amoeba, choanoflagellata, and animalia (Opisthokontos).
Single-celled choanoflagellates will often lump together into circular colonies that are identical to the formations of choanocytes in the first and most primitive animals, sponges.
A corresponding bridge between single-celled and metazoan life in plants can be found in the genus Volvox, where a bikont (double-flagellate) photosynthetic protist forms into reproductive colonies resembling large spheres, which reproduce smaller forms of the colony within themselves, until the new colonies burst the old one from the inside out. They can also reproduce asexually, the old fashioned way.