When first discovered in 1977, these microscopic single-celled organisms were classified as Bacteria. However, later genetic and biochemical analyses showed that they were more closely related to the Eukaryotes (which include humans). The researchers who discovered this, Woese and his colleagues, proposed the addition of the “domain” as a taxonomic level above that of kingdom, with all life classified within one of three domains, now called Bacteria, Eukaryotes, and Archaea. As of 2010 more than 250 species of Archaea had been described, most fitting into one of two groups, Euryarchaeota and Crenarchaeota. Many of these live in extreme environments – places where we once believed no life could exist.

(Woese et al., 1990; Kimball, 2010; UCMP, accessed 2011)