Scientists think that all modern plants, animals, and fungi can be trace to protist ancestors because the protists is the root of all eukaryotic diversity. ... The function that do cilia and flagella in protists carry out is movement. The first multicellular animals presumably evolved from colonial protists (most likely choanoflagellates), and within the colonies, cells progressively differentiated from one another, acquiring specific functional characteristics. It may sound like a bit of a stretch, but in fact such a phenomenon can be observed in colonies of extant unicellular organisms, both prokaryotes and eukaryotes, where some cells of the colony can become differentiated under particular environmental conditions. We can speculate that early in the history of animals some differentiation genetic programs evolved, got stabilized, and became inheritable from one generation to the next. Once different populations of cells were established in the colony, the door was opened to further specialization. Specialized cells were not initially grouped in organs but were more like regionalized populations or layers of cells, as we see in sponges, and morphogenesis progressively evolved
