In 1990, Carl Woese announced his development of a system of classifying all cellular life forms into one of three domains. His three domains are the:
1. Archaea Domain - organisms that have no cellular nucleus; different genetic makeup from bacteria; most are single cells; among the oldest forms of life on Earth
2. Bacteria Domain - organisms that have no cellular nucleus; most have traditionally been classified as bacteria; life forms in this group have been studied much more extensively than those in the Archaea Domain
3. Eukarya Domain - life forms with cellular membranes enclosing unique structures. This domain is divided into four kingdoms: Protista, Fungi, Plantae, and Animalia.