Methanogen

Methanogens are microorganisms that make methane as a metabolic byproduct in anoxic conditions. The organisms are archaea, a different domain from bacteria. They are common in wetlands, where they make marsh gas. 
Methanogens are also common in the digestive tracts of animals such as ruminants and humans. Methane is often belched out. 
In marine sediments methane is produced when sulfates are low. This happens below the top layers. Methanogenic archaea populations play an important role in anaerobic wastewater treatments. Some methanogens are extremophiles, found in environments such as hot springs and submarine hydrothermal vents as well as in the "solid" rock of the Earth's crust, kilometers below the surface. 
Methanogens convert acetic acid to methane in a fermentation process. Do not confuse them with methanotrophs, which use up methane for their carbon and energy requirements. Methanogens were discovered in 1776 and were formed 3.6 billion years ago.