Summary
Bacteria, Archaea and Eukarya, based on fundamental cell differences and rRNA.
In depth
Carl Woese (1977) used 16S/18S rRNA sequences to identify Archaea as distinct from Bacteria. Archaea share more molecular machinery with Eukarya. Recent work puts eukaryotes within Archaea (two-domain tree).
Domain-level differences include membrane lipids, RNA polymerase and cell wall chemistry. Eukaryotes arose from an archaeal host that engulfed a bacterium.
Remember: domains sit above kingdoms.
In 1977 Carl Woese used ribosomal RNA sequences to show that life divides into three domains: Bacteria, Archaea and Eukarya. Bacteria and Archaea are both prokaryotes, but Archaea differ in membrane lipids (ether-linked), cell walls (no peptidoglycan), RNA polymerase and gene expression machinery, which resemble those of eukaryotes.
Archaea were first found in extreme habitats (hot springs, salt lakes, methane producers) but live almost everywhere, including soil and oceans. Eukarya (with nuclei and organelles) arose from an endosymbiosis involving an archaeon related to Asgard archaea and a bacterium, so the tree of life may really have two primary domains.
Example: Methanogens in cattle guts are archaea.
Frequently asked questions
What is Three Domains of Life?
Bacteria, Archaea and Eukarya, based on fundamental cell differences and rRNA.
Which area of biology covers Three Domains of Life, and how advanced is it?
Three Domains of Life is one of the 52 topics in Taxonomy & Biodiversity. It is rated Beginner, so it suits learners who are starting out in biology. Related topics in this area include Taxonomic Hierarchy, Molecular Systematics, Cryptic Species.
Why does Three Domains of Life matter?
It overturned the old split of prokaryote versus eukaryote.
Can you give an example of Three Domains of Life?
Methanogens are archaea; E. coli is a bacterium.
What should I learn before Three Domains of Life?
Three Domains of Life is a Beginner topic, so no earlier topic is required. Other Beginner topics in Taxonomy & Biodiversity that pair well with it include Taxonomy, Binomial Nomenclature, Taxonomic Hierarchy.
What topics are related to Three Domains of Life?
The closest topics to Three Domains of Life are Taxonomic Hierarchy, Molecular Systematics, Cryptic Species, Horizontal Transfer and the Tree of Life. Comparing them side by side is a good way to see how the ideas connect.
How can I check that I understand Three Domains of Life?
Read the summary and the in-depth section, then use the quick check on the interactive lesson for Three Domains of Life. To practise the whole of Taxonomy & Biodiversity, use the Taxonomy & Biodiversity quiz.
How is Three Domains of Life different from Taxonomic Hierarchy?
Three Domains of Life: Bacteria, Archaea and Eukarya, based on fundamental cell differences and rRNA. Taxonomic Hierarchy: Domain, kingdom, phylum, class, order, family, genus and species form nested ranks. Read both side by side and note what each one explains.
How should I study Three Domains of Life step by step?
First, read the summary and write the definition in your own words. Second, read the in-depth section and note any example. Third, compare it with Taxonomic Hierarchy and explain the difference aloud. Finally, take the Taxonomy & Biodiversity quiz and come back to this page for any question you miss.