Kingdoms of Life

The hierarchy of biological classification has eight taxonomic ranks:

  1. Realm
  2. Kingdom
  3. Phylum
  4. Class
  5. Order
  6. Family
  7. Genus
  8. Species

The three domains are: Bacteria, Archaea and Eukaryota

The six kingdoms are: Animalia, Plantae, Fungi, Protista, Archaea/Archaebacteria, and Bacteria/Eubacteria (nb: in the UK system the last two are referred to as Monera)

Roger Stanier and C.B. van Niel first divided all living organisms into prokaryotes and eukaryotes in 1962. Prokaryotes do not have a cell nucleus, and the prevailing belief at this time was that all life shares a common prokaryotic ancestor.

Carl Woese (American microbiologist, 1928 – 2012) proposed what would become known as the RNA world hypothesis in 1967, and added Archaea as a third domain of life in 1977.

LUCA, the last universal common ancestor, is the point where life branches into bacteria and the other branch later branches into archaea and eucarya.

The earliest evidence for life on Earth is biogenic graphite aged 3.7 billion years. The Earth’s age is about 4.54 billion years. There is a possibility that 4.1 billion year old rocks found in Western Australia contain biotic remains.


Archaea are a domain of single-celled organisms. They do not possess nuclei and consequently are prokaryotes. Archaea are divided into phyla. Although morphologically similar to bacteria, archaea have genes and metabolic pathways more closely related to eukaryotes. In particular, the similarity is strongest with regards the enzymes involved in transcription and translation. Archaea can either use photosynthesis or fix carbon, but not both. They can feed on organic compounds, ammonia, metal ion and hydrogen gas. Archaea are numerous in oceans, such as in plankton and are very abundant. Mathanogens produce methane in biogas production. Archaea may be the most ancient lifeform on Earth.

Bacteria are a domain of prokaryotic microorganisms, whose size is a few micromillimetres in length. They are single-celled and do not have nuclei. They exist in all habitats, and form symbiotic and parasitic relationships with plants and animals.

Eukaryota have a membrane-enclosed nucleus, which neither of the prokaryote domains have. [Greek: eu = true, karyon = kernel] Eukaryotes are relative late-comers to Earth, dating from 1.6-2.1 bya, during the Preterozoic eon, which spanned between the appearance of oxygen and complex life.