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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.

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