How Life Made the Leap From Single Cells to Multicellular Animals -- For billions of years, single-celled creatures had the planet to themselves, floating through the oceans in solitary bliss. Some microorganisms attempted multicellular arrangements, forming small sheets or filaments of cells. But these ventures hit dead ends. Then, more than 3 billion years after the appearance of microbes, life got more complicated. Cells organized themselves into new three-dimensional structures.