Methanogens are microorganisms that produce methane. When they were first discovered, methanogens were classified as bacteria. But in the 1970s Carl Woese showed that they are very different from most common bacteria. He developed the three domain tree of life to illustrate the relationship between Bacteria, Eukaryotes and the methanogens (which he called Archaea). 

• How do the Archaea differ from the Bacteria?
• Archaeal cells have a clearly defined nucleus that is surrounded by a membrane. 
• The nucleotide sequences of rRNAs from Archaea differ from those of both Bacteria and Eukaryotes. 
• Unlike the Bacteria, Archaea do not have cell walls. 
• Archaeal cells do not contain enzymes, whereas Bacterial cells do. 
• Archaea and Bacteria have group dependent differences in their Gram stain reaction.