This explanation never fully satisfied me. I get that we're more complicated than swamp water bacteria. But if we're made up of cells, doesn't it stand to reason that at some point clusters of single-celled organisms got together to synergistically carry out collective goals to create this organization? I don't mean that a group of cells collected in pond water and discovered that they could interact in such a way as to suddenly become human. I mean that over time, as groups of cells came together, more and more complex "societies" developed to produce this strangely complex creature we are. Or my cat is. Or the tree in the back yard is.
It's the selfish gene, taken up a level to the collective germ.
This TEDTalk by Bonnie Bassler on how bacteria communicate is a much more satisfying answer--not simply because it agrees with my juvenile hypothesis, but because it actually explores the case for multicellular organization down to the molecular level. Take a look: