For many years, scientists who study living things have agreed to an overall classification system of living creatures called the five kingdom system. The five kingdoms are: Monera (cells without nuclei; bacteria and bacteria-like organisms), Protista (includes protozoa and simple algae), Fungi, Plants, and Animals. The grouping of all bacteria-like organisms into one kingdom was based on microscopic observations--what they looked like. However, in the late 1970s, a microbiologist named Carl Woese compared the same bits of genetic material from many, many creatures and found that some of these bacteria-like creatures actually are very, very different from bacteria based on their genetics. In some cases, these bacteria-like creatures were actually genetically more like cells in animals than they were like bacteria cells. Woese called these single-celled creatures the Archaea (are-key-uh) and suggested a new classification category a level above kingdom called domain. He proposed a three domain system that includes Archaea, Eubacteria (meaning "true bacteria"), and Eukaryotes (cells with nuclei; includes fungi, plants and animals).