The yellow asterisk * indicates the last universal common ancestor (LUCA), or universal cenancestor, which is hypothesized as being at the ancestral root of all living organisms. Not the earliest or simplest living organism, and not necessarily the sole example of its type, this organism possessed the genetic material that diverged (about 3.5 Ga) into all current living organisms. This diagram is not a cladogram, so branches do not indicate evolutionary timelines.
A number of terms are employed to refer to the universal cenancestor – last universal ancestor (LUA), last common ancestor (LCA), or last universal common ancestor (LUCA).
Woese and Fox proposed the Three Domain system: Eubacteria, Archaea, and Eukaryotes.
The Five Kingdom system was proposed in 1969: Monera (prokaryotes), Protista, Plantae, Fungi, Animalia. Discovery of the Archaea added the sixth kingdom.
History of taxonomic concepts:
Linnaeus, 1735 – 2 Kingdoms – Animalia, Vegetabilia
Haeckel, 1866 – 3 Kingdoms – Protista, Plantae, Animalia. Image Haeckel's tree of life.
Chatton, 1937 – 2 Empires – Prokaryota, Eukaryota
Copeland, 1956 – 4 Kingdoms – Monera, Protoctista, Plantae, Animalia
Whittaker, 1969 – Monera, Fungi, Protista, Plantae, Animalia
Woese et al, 1977 – 6 Kingdom – Eubacteria, Archaea, Protista, Fungi, Plantae, Animalia
Woese and Fox, 1999 – 3 Domain system: Eubacteria, Archaea, and Eukaryotes