Life and Domain

Life –  The Three Domains:

  1. Bacteria

  2. Archaea

  3. Eukaryota – The Four Kingdoms:

  1. Fungi

  2. Plantae

  3. Animalae

  4. Protista

Blog notes

MYA = Million years ago

All living things are divided into groups that are subdivided into smaller groups that are subdivided and so on. The Groups (largest first) are:

  • Life
  • Domain
  • Kingdom
  • Phyla/ Division
  • Class
  • Order
  • Family
  • Genus
  • Species

In the Plant Kingdom, phyla are referred to as divisions (also in the Fungus Kingdom sometimes) and with plants, species are further split into varieties (naturally occurring changes) and cultivars (specially bred changes). An organism’s Latin name is made up of its genus and species, for example Helianthus annuus (a sunflower) is in the genus Helianthus and is of the species annuus.

In order to write a blog that doesn’t stretch on forever, I have chosen a specific classification system, The Three Domain System, there are others and there are constant changes within those systems arising from scientific discovery. Scientists (and humans in general) attempt to make clear distinctions between one group of organisms and another, but evolution is a slow, gradual, complex process and any classification system we impose on nature will be a simplification. I have found it is necessary to be aware of this when trying to learn about classification, because the closer I looked, the more jumbled it became.

The Definition of Life

The differences between a living thing and a non-living thing may seem obvious, but Life has specific criteria. A living organism is one that possesses the capacity to grow, respond to stimuli and reproduce, they also have the ability to regulate their internal environment (homeostasis), and are made up of cells.

The Virus Dispute: There has been some debate as to whether viruses can be classified as living or not – they are not made up of cells, but are just strips of  DNA; they can reproduce, but only by inserting themselves into the cells of other organisms that then carry out the replication process for them. When humans get a cold it is the result of a strip of virus DNA getting into our cells and using the ability of those cells to replicate its DNA and multiply.

Three Domains

There are three domains, two of which consist of microscopic, simple cells that can’t reproduce sexually, Archaea and Bacteria (prokaryotes) and then the third domain (Eukaryota) contains everything else, from mushrooms to mould to antelopes to humans to oak trees. At first glance, this seems odd, surely the differences between Archaea and Bacteria are small, yet the variety within the Eukaryota Domain is enormous? However, it is the smallness of the differences that is important; while the shapes and behaviour of organisms made of eukaryotic cells is varied and even the cells differ from one kingdom to another, the amino acids, the building blocks of DNA, are the same. However, within the Bacteria and Archaea Domains, these building blocks are different and that is such a fundamental change that the two groups cannot be classified together.


  • What makes archaea, archaea? They are microscopic and single celled, with DNA not contained within a nucleus, same as for the Bacteria Domain, however they are genetically distinct from Bacteria, and their amino acids and chemical components are different.
  • Evolutionary history: Thought to first have evolved 3800mya and some of those archaea species probably still exist now. Their ability to survive extreme conditions is part of what places them at the start of life, when temperatures were high, oxygen was low and most gases were toxic. Archaea have changed little throughout their existence because they reproduce asexually, this means there is no mixing of DNA, changes can only occur due to mutation and so there is little natural selection. Archaea is believed to be an older Domain than Bacteria, however it is thought that Eukaryotes evolved from Archaea rather than Bacteria, because Eukaryotes and Archaea have more genes in common.
  • Number of phyla and species: 5-23, less than 9000 species have been classified for both Archaea and Bacteria, but many, many more exist.
  • Habitats: Can survive extremes of temperature, acidity and salinity, or without oxygen (most are anaerobic), as well as less diverse environments such as soil and seas. Examples of habitats are salt lakes, hot springs, oceans and the human colon.
  • Cellular description: Consist of a cell membrane, cytoplasm, some DNA and a cell wall, no nucleus
  • Shapes and sizes: Quite similar in size to bacteria, microscopic, unicellular with various different shapes. However, one Archaea genus has square shaped cells, which is not seen in any other Domain.
  • Abilities and behaviour: Their basic actions are consuming (or absorbing), growing and dividing. Rarely parasitic (one parasitic example may have been found), they either work in harmony with other organisms (some archaea in animals aid digestion) or within them, but without affecting them. They reproduce asexually or by fission. This means offspring have identical genetic code to parent. Archaea use a great variety of different energy sources such as sugar, gases and sunlight. Many archaea can move using various different methods such as flagellum tails.

Bacteria cell Bacteria

  • What makes bacteria, bacteria? They are single celled with DNA not contained within a nucleus, same as for the Archaea Domain, however they are genetically distinct from archaea and and their amino acids and chemical components are different.
  • Evolutionary history – Thought to have evolved after archaea, not known exactly when, but probably before 3500mya.
  • Common examples Include cyanobacteria which photosynthesise, but not in the same way plants do (ie they do not have chloroplasts). Cholera and Bubonic Plague. Stromatolites.
  • Number of phyla and species – 29-52, less than 9000 species classified for both Archaea and Bacteria, but many, many more exist.
  • Habitats – soil, water, inside other organisms. Usually in less extreme environments than archaea.
  • Cellular description Consist of a cell membrane, cytoplasm, some DNA and a cell wall, no nucleus. .
  • Shapes and sizes – A few micrometres long, a tenth the size of a eukaryotic cell (although there are some exceptions, some are even visible to the naked eye). Many shapes including spirals and spiky shapes, although most are round or rod shaped. There are approximately five nonillion bacteria and archaea on Earth, many more than Organisms in the Eukaryota Domain, however, bacteria and archaea are so small, the biomass is thought to be similar. There are ten times as many bacteria cells as human cells in the human body.
  • Abilities and behaviour – Their basic actions are consuming, growing and dividing. Parasitic or symbiotic, they can be damaging to their host. They reproduce asexually or by fission, some make endospores – these are reproductive bodies, a little like seeds, but with no genetic variation. Bacteria use a great variety of different energy sources such as sugar, gases (including ammonia) and sunlight. Many bacteria can move using various different methods such as flagellum tails.


Leaf 007 010 (2)Plant, fungi, Animal, Protist*

  • What makes eukaryotes, eukaryotes? Their cells have a nucleus which contains the DNA. Most cells also contain organelles and many eukaryotes are multicellular complex organisms.
  • Evolutionary history – Thought to have evolved from Archaea at some point between 1600-2100mya.
  • Common examples Monkeys, roses, toadstools, dragonflies, slime molds and humans.
  • Number of kingdoms, phyla and species – Contains four different kingdoms – Plant, Fungi, Animal and Protist. About sixty phyla in total and several million species.
  • Habitats – Varied, although not quite to the extent that Archaea is. Deserts to the poles, deep sea to mountain tops, all contain some kind of Eukaryota.
  • Cellular description – Have a nucleus (the DNA contained in a membrane) and organelles, such as mitochondria or chloroplasts
  • Shapes and sizes – Pretty much any shape or size from Honey Fungus, one of the largest organisms in the world, to amoeba. Although there are far fewer Eukaryotes than there are Archaea or Bacteria, the much larger size of Eukaryotes mean that their biomass is thought to be about the same.
  • Abilities and behaviour – Again, massive variety, some move (animals) some don’t (plants), some are big and complex, some are small and simple. All are capable of sexual reproduction, which is important because it explains the diversity – sexual reproduction leads to a mixing of DNA, which leads to genetic variation and evolution.

*Note: Picture of Protist above is from

 Table Comparing Domains

Bacteria  Archaea Eukaryota
No of species Less than 9000 archea and bacteria classified, no idea how many  See left Several million
No of phyla 29-52 5-23 60
First arrived in 3500mya 3800mya 1600-2100mya
Reproduction asexual asexual Sexual and asexual
Can move? Yes, many can move via different means – eg flagella Same as Bacteria Mixed (animals can and some protists)
Make food (photosynthesize) Mixed None photosynthesize, some use sunlight Mixed (plants can, some protists)