Domains of Life

There are three domains of life on Earth: Bacteria, Archaea, and Eukaryotes

Additional Resources

An evolutionary algorithmic phase transition 2.6 billion years ago may have sparked the emergence of eukaryotic cells (Phys.org 17Apr2025)

└Muro et al. (2025) The emergence of eukaryotes as an evolutionary algorithmic phase transition

Origin of life: How a special group of single-celled organisms laid the foundation for complex cells (Phys.org 21Mar2025)

└Wollweber et al. (2025) Microtubules in Asgard archaea

- Prokaryotic, unicellular & colonial
- Possess DNA, but not membrane-bound organelles

- Prokaryotic, unicellular
- Possess DNA, but not membrane-bound organelles
- Possess enzymes and transcription factors more similar to eukaryotes
- n.b. Some evidence supporting the endiosymbiotic theory indicates that origin of eukaryotes came about through an archaean host, which engulfed a bacterium. This bacterium became the first organelle, such as the mitochondria

Above: Categories of life; this is an organizational image, not an evolutionary tree

Below: A cladogram of the domains of life

Organisms possess organelles such as a nucleus
- e.g. "protists", plants, fungi, and animals
Eukaryotes probably evolved around 2.0 billion years ago (Paleoproterozoic)
Both microscopic and macroscopic forms exist on Earth
Autotrophs (e.g. plants and photosynthetic algae) & heterotrophs (e.g. fungi / animals)
Eukaryotes are dependent on an aerobic (oxygen-rich) environment

- Four major groups of eukaryotes:
  1. Archaeaplasta (Plants)
  2. Unikonts (Fungi & Slimemolds)
  3. Chromalveolates (Various algae groups)
  4. Excavates (Various unicellular protists)

Above: Biomass of organisms on Earth as represented by the amount of carbon in each group

n.b. Plants account for 83% of all carbon found in Earth's organisms (Bar-on, Phillips, and Milo 2018)

Genetic entities that are in a "grey zone" between living and non-living states
- A virus does not replicate or function independently, so by the biological definition, a virus cannot be categorized as a living organism.
- From a genetic and evolutionary point of view, a living organism is defined by its ability to reproduce. From this view, a virus is alive since it can produce similar offspring.
Possess a single strand of genetic information inside a protein capsule
Require a host to reproduce
Lack most of the internal structure and machinery, such as biosynthetic processes
Viruses are not constructed as cells

Above: A representation of what the COVID-19 virus would look like

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