Classification systems have continually changed to match new discoveries about the phylogenetic relationships between species

• Historically, all organisms were classed as being either prokaryotic or eukaryotic based on key cellular characteristics (e.g. presence or absence of a nucleus)

Analysis of rRNA sequences led to the splitting of the prokaryotes into two groups, resulting in three domains:

• Eukarya contains all eukaryotic organisms (animals, plants, fungi and protists)
  

• Bacteria includes all of the most commonly encountered prokaryotes (pathogenic and probiotic bacteria)
  

• Archaea consists of prokaryotes that are extremophiles (thermophiles, halophiles, methanogens, etc.)
  

The archaeal cells were found to possess certain features that were more in common with eukaryotic cells than eubacteria

• Archaea had histone-like proteins, lacked peptidoglycan in their cell walls and had a small ribosomal subunit that was more similar to eukaryotes
  

• Additionally, archaeans have a unique cell membrane composition (glycerol-ether lipids instead of glycerol-ester lipids)
  

Based on these discoveries it was concluded that eukaryotes and archaea share a more recent common ancestry than bacteria

Domain Comparison