Archaebacteria is the earlier name for the single-celled microorganism group Archaea (Wikipedia). They include four major phyla and an unknown number of other phyla, and are present in many environments. The idea that archaea only live in harsh or extreme environments, particularly anaerobic environments (little or no oxygen), has been disproved; archaea have even been discovered living in the human intestinal tract, and most are found in the ocean. One interesting aspect of archaea is that they utilize a wide variety of energy sources, such as sunlight, ammonia, and hydrogen. This allows them to live in places where other organisms would die, and also allows a wide spread around the world; archaea are thought to be essential to many functions of the Earth's carbon and nitrogen cycles. Today, archaea are estimated to make up as much as 20% of the Earth's entire biomass, making them one of the most common organisms on the planet, outmassing the human race by a factor of 60.