[Evolution of cells]

Life has existed on earth for some 4 x 10(9) years. During most of this time, evolution took place at the level of cell evolution. The cells of presently existing organisms belong to two fundamentally different cell types, protocytes (of bacteria and archaea) and eucytes (of eukarya). Thanks to molecular phylogenetics, the path of evolution can now be traced back to its very beginnings, although the picture may be blurred by repeated horizontal gene transfer. A symbiogenetic origin of plastids and mitochondria is now very well documented, and it is being discussed also for some other constituents of eucytes, including even the cells nucleus. It could be demonstrated that not only did bacterial cells become incorporated into protoeucytes and transformed into organelles of their respective hosts, but also that endocytic eucytes have apparently been transformed to complex organelles by coevolution with host cells.