Three Domains

From Paleos

The Three Domains of Life include the Archaea, including the halophiles, methanogens, and thermophiles, the Eubacteria, including gram-positive and gram-negative bacteria, and cyanobacteria, and the Eukarota, including protists, plants, animals, and fungi.

Modern genetic research over the last decade or so has revealed that anaerobic bacteria found in harsh oxygen-free conditions are genetically and metabolically completely different to other, oxygen-breathing organisms. These bacteria, called Archaeobacteria, or simply Archaea, are actually "living fossils" that have survived since the planet's very early ages, before the Earth's atmosphere even had free oxygen. DNA and RNA analysis has shown that instead of five kingdoms there are actually three "Domains", Archaea, Bacteria, and Eukarya (Eukaryota). This last group refers to organisms whose genetic material is contained in a special membrane as the nucleus, and includes all higher organims from protists to humans. Rather than just four kingdoms it would seem to include over a dozens. So much so that the term "Kingdom" has become (in this usage) meaningless. So we have:

two kingdoms	five kingdoms	three domains and who knows how many "kingdoms"
Animalia	Animalia	Eukarya	Animalia
Plantae	Fungi		Fungi
Plantae	Plantae		Plantae
either Protozoa (=Animal) or Algae (=Plant)	Protoctista		Alveolata
			Stramenopiles
			etc...
			Sporozoa
			Mycetozoa
			Euglenozoa
			etc...
			Archezoa
Plant (bacteria and blue-green algae)	Monera	Eubacteria	(kingdoms not specified)
Plant (bacteria and blue-green algae)	Monera	Archaea	Euryarchaeota Korarchaeota Crenarchaeota

Thus the diversity of life is seen to be far more complex than was envisaged, and familiar organisms like animals and plants are just a tiny proportion of all of the many different forms.

The following diagram illustrates the relationship between the Three Domains, and the various subbranches of each.