Carl Woese and his associates working in university of Illinois were able to gather evidence on a hitherto unknown group of unicellular organisms, which lived in extreme environmental conditions such as hotsprings, deep sea hypothermal vents and salt evaporation ponds. These unusual organisms were identified to be close to bacteria and were named archaebacteria. They did not need sunlight and oxygen to grow. They made all the food from hydrogen sulphide and other chemicals spewing from volcanic vents. Biochemical and DNA sequence analysis showed that there were numerous differences between these organisms and the bacteria. In view of this, Carl woose proposed the idea of placing them in a super kingdom of unicellular organisms. Subsequently Woose proposed the idea of introducing a new category(taxon) of classification called the domain, above the category of kingdom. He suggested three domains of life-Archaea, Bacteria and Eukarya.
Domain Archaea
These are unicellular forms occurring in the highest environments and are the oldest known organisms on earth, appearing in the fossil record of over 3.6 billion years ago. Three groups are known.
1) Methanogens: These organisms use only carbon dioxide, hydrogen and nitrogen to produce energy and releasing methane, either free living or parasitic methanogens are decomposers example. Methanococus.
2. Extreme Halophiles: These organisms live in ocean waters or water bodies which may be even ten times saltier. They occur in the dead sea and great salt lake.
3. Extreme Thermophiles: These organisms live in places with temperature ranging between 60-110 degree celcius or near or below freezing. They cannot survive at room temperature.
Domain Bacteria
This domain is proposed to include toue bacteria(eubacteria). Many are decomposers, some are autotrophos while others are parasitic. They include the two groups- Cynobacteria and Eubacteria.
Domain Eukarya
This domain includes eukaryotes covering four kingdoms- Protista, Fungi, Plantae and Animalia.