Archaea are a highly diverse group of single-celled organisms that constitute one of the three main domains (branches) of life, along with eukaryotes (which include animals and plants) and bacteria.
Archaea were formerly classified as bacteria because of their substantial similarities, including being single celled and invisible to the naked human eye, having similar sizes and shapes, lacking a cell nucleus, reproducing asexually, and having a great diversity and adaptation to a wide range of environments. However, they also have major differences, including with regard to their evolutionary history, the structure of their cell walls, the chemical composition of their DNA and RNA. and the ability to live in extreme environments.
Archaea are abundant in nearly every habitat, including the soil, wetlands and the oceans, and they even exist in environments that other organisms cannot tolerate, including those with extreme temperatures, highly acidic or alkaline water bodies, deep in the oceans, and in locations with high levels of radiation. They also live on and in other organisms, including humans, where they are found on the skin and in the mouth and intestines.
The great diversity of archaea allows them to perform a variety of important functions. Among them are an essential role in biogeochemical cycles, including performing nitrogen fixation and recycling of other nutrients.
In sharp contrast to bacteria, no conclusive evidence has been found yet for archaea causing, or playing a role in, human diseases, although some researchers have proposed that they might be involved in some conditions that have unknown causes. They may also provide some health benefits, including playing an important role in digestion, metabolism, and protecting the skin. The lack of knowledge about the medical role of archaea is largely due to the difficulty in isolating and cultivating them for analysis.