At the broadest level, all life on earth can be assigned into one of three major groups called domains: domain Bacteria, domain Archaea, and domain Eukarya. While the diversity of life within each of these domains is vast, organisms in the domain share certain characteristics, such as similar cell structures and related sequences of ribosomal RNA. Two of the three domains, domain Bacteria and domain Archaea, contain prokaryotic organisms. These are single-celled organisms that do not have a membrane-bound nucleus. Archaea live in aquatic environments that lack oxygen or are too salty, too hot, or too acidic for most organisms. The ability of some archaea to live in environmental conditions similar to the early Earth is an indication of the ancient heritage of the domain. As scientists look for life on other planets and moons they are often looking for evidence of archaean-like organisms. Bacteria are found almost everywhere- in the water, soil and atmosphere, as well as on our skin, and in our mouths and large intestines. Although some bacteria cause diseases, others can be very beneficial. Many of our drugs are manufactured using bacteria, and bacteria play an important role in the processing of many of our foods. In our bodies, both our digestive and immune systems rely on the action of beneficial bacteria. While archaea and bacteria domains are both prokaryotic, studies of their ribosomal RNA indicate they are not closely related to one another. In addition, their cell walls are considerably different. The bacterial cell wall contains peptidoglycan as well as protein, polysaccharides, and lipids. The archaeal cell wall lacks peptidoglycan but does contain proteins, polysaccharides and lipids. The final domain, domain Eukarya includes the eukaryotes, or organisms that have a membrane-bound nucleus. In addition, most eukaryotes have internal organelles, such as mitochondria and chloroplasts, and in general, eukaryotic cells are much bigger than the prokaryotes. Domain Eukarya is subdivided into four kingdoms – the Protists, Plants, Fungi, and Animals.