Because the cell organelles of eukaryotes have different (polyphyletic) origins, the question arises as to whether the group is a unified clade or not. It is certain that the protists are not. Cell organelles are specialised units which carry out well-defined functions, like mitochondria and plastids. It is fairly clear now that all or most of these organelles have their origin in once-independent prokaryotes (bacteria or archaea), and that the eukaryote cell is a 'community of micro-organisms' working together in 'a marriage of convenience'. The first such events took place between ancient bacteria to produce the double-membrane class known as gram-negative bacteria. Since the gram-negative bacteria include the cyanobacteria, this was the first of several such events in the history of the eukaryotes.