Why does it make sense that life started so small? Why does it make sense that it started in water?

What does it mean to be an “multicellular organism”? The cellular slime mold Dictyostelium discoideum exists initially as a collection of separate cells moving over the forest floor. In times of starvation, these cells come together to form a “slug” that moves to more fertile territory. Eventually, it will change into a new structure that will throw off separate cells again. Is this an organism or a colony of “cooperating cells”? JUSTIFY YOURSELF

One of the distinguishing features of animal cells is that they have NO cells walls (as found in plants and fungi). Why would such a feature be advantageous for plants/fungi but disadvantageous for animals?