Here's why:

* Prokaryotes are a single domain: The three-domain system of classification, proposed by Carl Woese, recognizes three primary lines of evolutionary descent:

* Bacteria: Traditional prokaryotes.

* Archaea: Initially considered extreme bacteria, but now recognized as distinct.

* Eukarya: All organisms with a nucleus (plants, animals, fungi, protists).

* Domains are the highest level of classification: Below domains are kingdoms, phyla, classes, orders, families, genera, and species.

Why are Bacteria and Archaea separated?

While both Bacteria and Archaea are prokaryotes (lacking a nucleus and other membrane-bound organelles), they are distinct in several fundamental ways:

* Cell wall composition: Bacteria have peptidoglycan in their cell walls, while Archaea do not.

* Membrane lipids: Archaea have unique lipids in their cell membranes, making them more resistant to extreme environments.

* Gene expression: Archaea share some similarities with Eukarya in their genetic machinery and gene expression mechanisms.

* Metabolic pathways: Archaea have unique metabolic pathways, including some that allow them to thrive in extreme environments like hot springs and salt lakes.

In short: While both Bacteria and Archaea are prokaryotic, their differences in cell structure, metabolism, and evolutionary history warrant their separation into distinct domains.