What are the Three Domains of Life?

Heather Phelos, Jeremy Battista
  • Author
    Heather Phelos

    Heather has taught all-level English classes for one year and is a healthcare research mentor and writer for over two years. She has a master's degree in public health: Epidemiology and Global Health from The University of Pittsburgh. She is also a Nationally Registered EMT and doula.

  • Instructor
    Jeremy Battista

    Jeremy has a master of science degree in education.

What are the three domains of life? Learn the origins and characteristics of Archaea, Bacteria, and Eukarya, look at examples from each domain, and how to compare them. Updated: 07/15/2021

Table of Contents


Biological Classification

The biological taxonomy of humans.

The image shows the order of biological classification from life to domain all the way to species, with 9 different headings.

Biological taxonomy is the study of naming organisms based on characteristics. There are eight groups in the biological taxonomic system that range in specificity from large domains to more narrow species. In descending order of specificity, the eight groups are:

  • Domains
  • Kingdoms
  • Phylum
  • Class
  • Order
  • Family
  • Genus
  • Species

What are the Three Domains of Life?

The domains of life are the most general biological classifications of cellular evolution. The taxonomic system is broken into three domains:

  1. Archaea
  2. Eukarya
  3. Bacteria

This naming system was proposed by scientist Carl Woese. Before Woese, only two domains were widely accepted: prokaryotes and eukaryotes. But in 1977, Woese discovered a third domain: archaea, which became widely accepted as the third domain in biological taxonomy in 1990.

Woese differentiated archaea from bacteria- which was formerly known together as the prokaryotes. His research concluded three things:

  1. Archaea and bacteria differ in their ribosomal RNA (rRNA) sequence. Archaea have three RNA polymerases, and bacteria only have one.
  2. Archaea and bacteria differ in their cell membranes. Archaea cell walls are made of pseudopeptidoglycan, where bacteria are made of peptidoglycan or lipopolysaccharide.
  3. Archaea are more related to eukaryotes than bacteria and share a more recent common ancestor.

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  • 0:01 Taxonomy & Domains
  • 1:41 Archaea
  • 2:31 Bacteria
  • 3:04 Eukarya
  • 3:41 Comparing the Domains
  • 4:30 Lesson Summary
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The Phylogenic Tree shows how all three domains of life originated with a common ancestor.

Phylogenic Tree showing the three domains of life and their common ancestor

Life Domain Characteristics

The domains of life trace the evolution of cellular change. They are all linked to a Last Universal Common Ancestor (LUCA). Mainly, archaea are similar to bacteria in that they are small cells with little internal structure, lack nuclei, and have RNA rather than DNA. Archaea are also similar to eukaryotes in that the process of metabolism and gene transcription utilize similar enzymes. But while they can share similarities with one another, organisms from the same domain have characteristics unique to that domain.


  • Multi-cellular
  • have double-helix DNA
  • have nuclei


  • Single-celled microorganisms
  • have RNA (three polymerases)
  • have a cell wall (made from pseudopeptidoglycan)
  • do not have nuclei
  • contain some of the oldest living organisms


  • Single-celled microorganism
  • have RNA (one polymerase)
  • have a cell wall (made from peptidoglycan)
  • do not have nuclei


Archaea is Greek for "ancient." This domain is comprised of organisms that are prokaryotic, or cells without nuclei. They are single-celled and reproduce asexually.

Archaea are found in a broad range of environments but the first to be discovered were extremophiles, or those that live in extreme conditions.

The main groups of extremophiles are:

  • Halophiles, in highly saline habitats
  • Acidophiles, in extremely acidic habitats
  • Thermophiles, on environments with high temperatures
  • Alkaliphiles, in extremely basic habitats

An example of archaea is Methanobrevibacter smithii, which is the predominant archaea type found in the human gut. It helps break down carbohydrates.


Bacteria are also prokaryotes, or single-celled organisms without nuclei. They reproduce asexually. Bacteria is Greek for "little stick." Bacteria are found in every environment on earth - even in the human gut. Bacteria help us digest food. Such is the case for Lactobacillus acidophilus. Other times, bacteria are responsible for diseases, such as Tuberculosis, which is caused by the Mycobacterium tuberculosis bacterium.

The five main types of bacteria are:

  • Coccus, or spherical shaped
  • Bacillus, or rod-shaped
  • Spirillum, or spiral-shaped
  • Vibrio, or comma-shaped
  • Spirochaetes, or corkscrew-shaped

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Frequently Asked Questions

What are the 3 domains of life and what are the major differences?

The three domains of life are Archaea, Bacteria, and Eukaryotes. Archaea and Bacteria are single-celled microorganisms that lack nuclei. They differ in their cell wall composition, metabolism, and the number of rRNA polymerases. Eukaryote cells have a nucleus and use DNA, they are large cells that can create multi-cellular organisms

What are the three domains of life and examples?

The three domains of life are Archaea, Bacteria, and Eukarya.

An example of Archaea is acidophiles such as Thiobacillus acidophilus.

An example of Bacteria is E. Coli

An example of Eukarya is an animal (lion)

Are there 2 or 3 domains of life?

Originally, there were two domains of life-prokaryotes and eukaryotes. In 1977, Carl Woese discovered that prokaryotes can further be broken down into two separate domains: Archaea and Bacteria

What relationships do the 3 domains help explain?

The three domains of life help explain cellular evolution and LUCA (Last Universal Common Ancestor). They divide and classify cells and organisms based off their membranes, presence of nuclei, organelles, cell wall, and genetic material.

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