Carl Woese's (1990) groundbreaking paper classified the Tree of Life into three domains for the first time: Archaea, Eubacteria and Eukarya. Prior to this, Archaea were known as Archaebacteria due to their prokaryotic and unicellular bacteria-like appearance. However, Woese analyzed the 16S ribosomal RNA from all three groups and found that there were such major differences in the sequences, for example between positions 180 and 197, that the Archaea should be classified as a domain of their own . The three domains are believed to have split from a common ancestor, with Eubacteria and Archaea diverging 3.8 billion years ago and Archaea separating from Eukarya 2.8 billion years ago. This means that, despite their appearance, Archaea share more similarities with eukaryotes, such as 33 identical ribosomal proteins, than with bacteria. Since Woese's research, the Archaea have been divided into two main phyla, Eutyarchota and Crenarchaeota, most of which are extremophiles. This supports the hypothesis that Eubacteria and Archaea had a thermophilic common ancestor that could tolerate warm conditions on Earth. Nelson et al (1999) also found that Thermotoga maritima bacteria had 24% genes of archaeal origin when analyzed, supporting the theory of early branching of Thermatoga from Archaea in the Tree of Life. There are some significant differences between Archaea and the other two domains in terms of structure, which creates advantageous heat resistance in Archaea. Bacteria and eukaryotes both have ester bonds between hydrophobic side chains and glycerol in the membrane, while archaea instead have ester bonds and lack true fatty acid side chains, instead having 40-carbon phytane chains. Furthermore, half of the paper in anaerobic hydrothermal vents, where sulfur serves as the main energy source. Thermophiles may also prefer a particular pH such as the thermoacidophilic Picrophilus which can survive a pH of -0.06 and has an optimum temperature of 60 degrees Celsius. To withstand high temperatures, special chaperonin proteins are required to fold proteins that are partially denatured during heat shock. These proteins allow Archaea Pyrolobus fumarii to survive and reproduce in an autoclave at 121 degrees Celsius (Blöchl, 1997), which was previously considered above the upper temperature for life. In conclusion, despite the close relationship between Archaea and bacteria and eukaryotes it is sufficient evidence to classify this group of organisms as a domain in its own right, due to its unique characteristics in structure and extremophilic nature.