classification of life
Living things are placed in groups on the basis of similarities and differences at the organismic, cellular, and molecular levels. Taxonomic studies have led to the development of a system of classification in which all terrestrial life-forms (with the exception of viruses) are divided into several kingdoms. These are further divided into phyla, classes, orders, families, genera, and species.
One of the most contentious issues is the classification of the kingdoms. Originally, there were just two of these: animals (Animalia) and vegetables (Plantae). Gradually, as new discoveries revealed the diversity of life, more were added until the five-kingdom system (including Monera, Protista, Fungi, Plantae, and Animalia) proposed in 1959 by the American biologist Robert H. Whittaker won the support of most authorities. Even this, however, has now been superseded. Research concerning the organisms previously known as archaebacteria has led to the recognition that these creatures form an entirely distinct group. In the six-kingdom system, archeans comprise their own kingdom on a par with those of bacteria, protists, fungi, plants, animals. Coexisting with this is a scheme put forward by Carl Woese in which all terrestrial life is divided into three domains – Archaea, Bacteria, and Eukarya – each containing two or more kingdoms.1
1. Woese, C. R., Kandler, O., and Wheelis, M. L. "Towards a Natural System of Organisms: Proposal for the Domains Archaea, Bacteria, and Eucarya," Proceedings of the National Academy of Science, 87, 4576 (1990).