the Domains and Kingdoms of Life

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the Domains & Kingdoms of Life
also see the tree of life for fossil collectors







In 1977, Woese and Wolfe, using 16S ribosomal RNA sequences (Olsen and Woese, 1993) overturned one of the major dogmas of biology. Until then, all life on Earth belonged to one of two primary lineages, the eukaryotes (animals, plants, fungi and certain unicellular organisms such as paramecia) and the prokaryotes (all remaining microscopic organisms). The archaea -- microorganisms that live in extreme environments without oxygen in conditions thought to be reminiscent of Earth's early environment -- changed that long-accepted view. Woese's molecular studies of RNA sequences led to the realization that the archaea were distinct from the two accepted classifications. His analytic approach has since become the standard for identifying and classifying microorganisms. Now three primary divisions of life are recognized: eukaryotes, archaea and bacteria. According to Woese: "The central task of biology in the new century will be to lay out and elaborate this overarching framework of relationships among living organisms," Woese said. "This endeavor will help us to understand how the essential unit of all life, the cell, came into being. It will help us to understand the evolutionary interactions among microbial species that gave rise to, sustain, and have the potential to drastically alter the nature of our biosphere."

Comparison of primary classification schemes:
(Color key: prokaryotic - eukaryotic - mixed)

Old 2-Kingdom System
Old 5-Kingdom System
The New 3-Domain System
While Kingdom is still used, the concept is of diminished relevance
Monera (the prokaryotes)
Estimates range from 18 to 30 or more
Plantae (algae)
Protoctist (e.g. algae, diatoms and different plankton)
Number of others unclear

5 Kingdom System: The so-called 5 Kingdom System was suggested in 1969 by Robert Whittaker. The 5 kingdoms were Monera, Protista, Plants, Animals and Fungi, with a primary differentiation between prokaryotes and eukaryotes. Prokaryotes were given their own Kingdom Monera, with the essential requirement being the distinct prokaryotic cellular organization lacking a nucleus. In this system, prokaryotes are bacteria including the Cyanophyta, or blue-green algae. Note that despite being prokaryotic, the Cyanophyta have photosynthetic pigments not present in the rest of the group, and they release free oxygen as a by-product of their photosynthetic metabolic system. Organisms in the other four kingdoms all have eukaryotic cells with DNA-containing nucleus. Kingdom Proctotista (often shortened to Protista) is the quite heterogeneous and rather ambiguous It is most easily understood as a catch-all for all eukaryotes that do not fit the definitions of plants, animals or fungi. The plant, animal and fungi kingdoms are multicellular eukaryotes that are mainly differentiated by their nutritional modes.

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