For billions of years after the formation of the earth, the planet was hot, surrounded by clouds that contained large amounts of hydrogen, carbon dioxide and other gases but not free oxygen. Methanogens — the first archebacteria — were then thought to have arisen. Prabacteria or archebacteria inhabit extreme habitats: wetlands, salt lakes and hot springs. Due to the specific habitat they inhabit (conditions similar to those on primitive Earth in the distant past) and the characteristics of these bacteria, scientists consider them to be the oldest organisms on Earth. Archebacteria in the cell wall do not have peptidoglycan, and they have amino acids that do not have other bacteria. Genetic analysis revealed that there are several groups of prabacteria: methanogenic, halophilic, and thermocacidophilic bacteria. Methanogenic bacteria are named after their unique ability to produce energy by synthesizing methane from carbon dioxide and hydrogen. Oxygen has a detrimental effect on them, so they live in oxygen-free conditions (anaerobic conditions), for example in marsh sludge, sewage and the like. Halophilic prabacteria live in conditions of high salt concentration, such as the Dead Sea water, whereas thermacidophilic bacteria live in highly acidic environments.