Estimated numbers of described species and possible totals

DOMAIN EUKARYOTE KINGDOMS No. of KNOWN SPECIES TOTAL (Estimate) 

The Archaea are similar to bacteria and until 1977 most known species were classified as bacteria (see review by Makarova and Koonin, 2003). It was the work of Carl Woese (see Woese and Fox, 1977) who discovered that some so-called bacteria were different genetically and separated the Archaea from the bacteria on the basis of particular rRNA sequences present in the cells of the organisms. The Archaea in existence today are thought to have descended from the first forms of life to have evolved on Eardi some 3.5 to 4.0 x 10 years ago. This relates to the fact that many (but not all) members of the existing Archaea are anaerobic, i.e. they do not require oxygen for survival, and thus could inhabit a world with an oxygen-poor atmosphere as was the case during the early history of the Earth. They also survive under extreme habitats today (see Table 3.2) and so could have tolerated the harsh conditions of the newly-formed planet. 

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Table 3.1. Estimated numbers of described species and possible totals (from Groombridge and Jenkins, 2002)...

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