Domains. Broadest , most inclusive taxon Three domains 1. Archaea and 2.Eubacteria are unicellular prokaryotes (no nucleus or membrane-bound organelles) 3. Eukarya are more complex and have a nucleus and membrane-bound organelles.
Archaea live in harsh environments and may represent the first cells to have evolved.
Sewage treatment plants, deep ocean vents, volcanoes, geysers, etc.
Live in the intestines of animals
Some bacteria cause disease but many act as decomposers and are important to the environment.