genus: Methanobacterium, Methanosarcina, Methanosarcina, Methanococcus, and Methanospirillum.

Methanogenic bacteria are widespread in nature and found in Mud, Sewage, sludge and in the rumen of sheep and cattle. Some Methanogenic bacteria have adapted to live in extreme environments. For example, Methanococcus jannaschii has an optimum growth temperature of 85 degrees C (185 degrees F), which is acheived in hot springs and thermal vents in the ocean. Such anaerobic bacteria are among the oldest life forms on Earth. They evolved long before the presence of photosynthetic green plants, and so existed in an oxygen-free world. These organisms are claimed to be the source of the origin of Life.