When time passed, and the environmental conditions changed around the earth, the bacteria developed advanced organelles in their cells. then, the need for more body was felt to carry life functions like digestion, so they formed multicellular organisms. and it is to be noted that at time of bacteria i.e. the begining of earth, there was only water on earth. so simple multicellular organisms evolved in water. when land came,,,,, some of the aquatic organisms had developed more abilities....... like breathing with gills and lungs aside from skin. they stepped onto land (you can take the modern frod as an example. it used to live only in water initially) and developed organs for moving, breathing, and eating on land and these were called as amphibians.