These are the broadest categories in biological classification and represent the deepest evolutionary branches. The three Domains are:

* Bacteria: Single-celled prokaryotes (lacking a nucleus).

* Archaea: Single-celled prokaryotes (lacking a nucleus) that often live in extreme environments.

* Eukarya: Organisms with cells that have a nucleus and other membrane-bound organelles. This domain includes plants, animals, fungi, and protists.

Within each Domain, there are further classifications, including kingdoms, phyla, classes, orders, families, genera, and species.