Archaea
In the past they had been classed with bacteria as prokaryotes (or Kingdom Monera) and named archaebacteria, but this is a mistake. The Archaea have an independent evolutionary history and show many differences in their biochemistry from other forms of life. They are now classified as a separate domain in the three-domain system. In this system, the three distinct branches of evolutionary descent are the Archaea, Bacteria and Eukaryota.