Conversation so far:
about how long ago did multicellular life forms appear on earth
Multicellular life forms appeared on Earth 600 million years ago.
give some examples of muticellular organisms.
All species of animals, land plants, and most fungi are multicellular.
what is the origin hypotheses for muticellularity?
A group of function-specific cells aggregated into a slug-like mass called a grex, which moved as a multicellular unit. This is essentially what slime molds do.

Question: what is a popular theory?
Self-contained rewrite: what is a popular theory for the origin of muticellularity?