Archaebacteria are the most primitive bacteria, probably by the first form of life, which evolved in a different line from that of true bacteria. These are adapted to extreme conditions like anoxygeny, pH, salinity and temperature. They differ from true bacteria by cell wall lacks murein, cell membrane with lipids (glycerol isopronyl ether) of branched alipathic chains, smaller size of DNA and base constituents in the RNA. These are of three types- Methanogens (methane producing, cause explosion in coal mines), Halophiles (live in extreme saline conditions, contain purple pigment bacteriorhodopsin for ATP synthesis from sunlight) and Thermophiles or Thermoacidophiles (withstand high temperature and acidity, mostly grow in hot sulphur springs).