Not bacteria
Separate domain Archaea. Shared 'prokaryote' is a grade: no nucleus, not a family reunion.
Archaea look like bacteria under a cheap microscope: no nucleus, small cells. Their lipids, ribosomes and information genes tell another story. Carl Woese's rRNA trees in 1977 split them from 'bacteria' as a mixed bag.
Many famous archaea love extremes — boiling springs, saturated salt, anoxic mud. Others live in ordinary soils, oceans and the animal gut as methanogens. None is a documented human pathogen.
Traits
Ether-linked lipids
Isoprenoid chains on glycerol via ether bonds, sometimes as monolayers. Bacterial and eukaryotic esters differ.
No peptidoglycan
Walls, when present, use other polymers. Pseudopeptidoglycan in some methanogens is not Gram's mesh.
Methanogenesis
Only archaea run the methanogenic pathway that turns CO2 or acetate into methane in guts and wetlands.
Extremophile fame
Pyrococcus and Halobacterium taught textbooks about heat and salt. Extremophily is a subset, not the whole domain.
No known pathogens
Surveys have not tied an archaeal species to a transmissible human disease the way Streptococcus is tied.
Calling them 'ancient bacteria' is a naming fossil. They are contemporaries with their own domain.
Keep exploring
More in Tree of microbes
Bacteria
Single-celled prokaryotes with cell walls ? the oldest widespread life form, and both partners and pathogens.
Familiarity 95Viruses
Packets of DNA or RNA that are not cells — they replicate only inside living hosts.
Familiarity 96Fungi
A eukaryotic kingdom of yeasts, moulds and mushrooms — chitin walls, external digestion, partners of plants.
Familiarity 80Bacteriophages
Viruses that infect bacteria — Earth's most abundant biological entities, and a concept-level idea in medicine.
Familiarity 42Protozoa
Motile unicellular eukaryotes — a teaching grade that includes pond ciliates and a minority of serious parasites.
Familiarity 58