The cell is the building unit in living organisms both simple and complex. Consequently living things can be broadly classified into prokaryotes or eukaryotes depending on their cell structure. These two groups differ by their subcellular structure where eukaryotes have membrane bound organelles, while prokaryotes lack these organelles. Eukaryotic cell, therefore, contain complex structures formed by internal membranes and cytoskeleton whereas, prokaryotic cells are simple with less complex structured organelles. Theorists postulate that prokaryotes were the only form of life for millions of years before eukaryotes come into being through evolution. Additionally, the majority of prokaryotes are unicellular with an exception of a few such as cyanobacteria while eukaryotes are multicellular.