>>14
Simple, it takes small steps. A single celled organism mutates so that it sticks to other organisms but allows a little space for sea water to circulate in the cluster. This means that cells inside are protected from the elements and the orgnaism doesn't have to spend so much energy on it's cell membrane. Every time a clump of this cluster of cells comes off the exposed cells die and create a barrier to protect the cells inside and cells which evolve the abilities to improve this process survive better.
Eventually the organism intentionally causes pieces of it to fall off by erecting them on storks, these pieces have very similiar genes and cooperate instead of compete. Etc etc...