Archaebacteria are the oldest living organisms on earth. They are prokaryotes and unicellular. Archaebacteria are found in very harsh conditions (such as at the bottom of the sea or in volcanic vents). This is thought to be because the early Earth’s atmosphere was filled with poisonous gases and was very hot – nothing could survive, except the archaebacteria. These slowly gave way to modern organisms when the Earth’s conditions settled down and oxygen was introduced to the atmosphere. "