How do organism become multicellular?

Answer 1

Basically the organisms produce more cells. Just because you are bigger than a baby, does not mean that your cells are bigger than the baby's cells. If this were the case, the cell would have difficulty retrieving oxygen and nutrients, as well as getting the waste out of the cell. In order to be efficient yet to grow at the DNA designated size, the cells make identical copies of themselves (as in, they produce more cells) using the cell cycle (made up of interphase, mitosis, cytokinesis).