Three-Domain System of Life

Based on differences in the nucleotide sequences of ribosomal RNA, all cellular life can be classified into three distinct domains — Bacteria, Archaea, and Eukarya.

These three domains originated from a common ancestor approximately 3.5 billion years ago, with significant horizontal gene transfer.

The eukaryotic cells appeared later, approximately 2.5 billion years ago.

The domain Bacteria encompasses prokaryotes that have peptidoglycan in their cell walls. It includes all pathogenic and non-pathogenic prokaryotes, as well as phototrophic bacteria.

The domain Archaea includes prokaryotes that lack peptidoglycan in their cell walls. These organisms are often found in extreme environments and include methanogens, extreme halophiles, and hyperthermophiles.

Methanogens are anaerobes that produce methane from carbon dioxide and hydrogen.

Extreme halophiles require high salt concentrations for survival.

Hyperthermophiles thrive in extremely hot environments.

The domain Eukarya contains nucleated organisms, including protozoans, algae, fungi, plants, and animals.