Archaea

Single-celled non-nuclear organisms, distinguished from eubacteria (“true bacteria”), usually living in extreme environments, both groups belonging prokaryotes.
They were originally thought to be evolutionarily older than the true bacteria;. However, it is now known that these groups have evolved parallelly and are equally old. Genetic studies have shown that archea are much different from bacteria and are more closely related to eukaryotes than bacteria, and may even be the starting group for eukaryotes.