section_path: Discovery and classification/Classification
section_name: Classification
paragraph_text: Classification
  
  The classification of archaea, and of prokaryotes in general, is a rapidly moving and contentious field. Current classification systems aim to organize archaea into groups of organisms that share structural features and common ancestors. These classifications rely heavily on the use of the sequence of ribosomal RNA genes to reveal relationships among organisms (molecular phylogenetics). Most of the culturable and well-investigated species of archaea are members of two main phyla, the " Euryarchaeota " and the Thermoproteota (formerly Crenarchaeota). Other groups have been tentatively created, such as the peculiar species Nanoarchaeum equitans — discovered in 2003 and assigned its own phylum, the " Nanoarchaeota ". A new phylum " Korarchaeota " has also been proposed, containing a small group of unusual thermophilic species sharing features of both the main phyla, but most closely related to the Thermoproteota. Other detected species of archaea are only distantly related to any of these groups, such as the Archaeal Richmond Mine acidophilic nanoorganisms (ARMAN, comprising Micrarchaeota and Parvarchaeota), which were discovered in 2006 and are some of the smallest organisms known.
  
  A superphylum – TACK – which includes the Thaumarchaeota (now Nitrososphaerota), " Aigarchaeota ", Crenarchaeota (now Thermoproteota), and " Korarchaeota " was proposed in 2011 to be related to the origin of eukaryotes. In 2017, the newly discovered and newly named Asgard superphylum was proposed to be more closely related to the original eukaryote and a sister group to TACK.
  
  In 2013, the superphylum DPANN was proposed to group " Nanoarchaeota ", " Nanohaloarchaeota ", Archaeal Richmond Mine acidophilic nanoorganisms (ARMAN, comprising " Micrarchaeota " and " Parvarchaeota "), and other similar archaea. This archaeal superphylum encompasses at least 10 different lineages and includes organisms with extremely small cell and genome sizes and limited metabolic capabilities. Therefore, DPANN may include members obligately dependent on symbiotic interactions, and may even include novel parasites. However, other phylogenetic analyses found that DPANN does not form a monophyletic group, and that the apparent grouping is caused by long branch attraction (LBA), suggesting that all these lineages belong to "Euryarchaeota".