Six and Seven Kingdom Systems

As molecular and structural evidence accumulated, biologists found that Whittaker's five-kingdom system still lumped together organisms that were not as closely related as it implied, especially within Monera and Protista. In 1987, Thomas Cavalier-Smith proposed revising this first into a six-kingdom, and ultimately a seven-kingdom, system, introducing the higher-level concept of the "super kingdom."

Under the seven-kingdom system, all life first splits into two super kingdoms: Prokaryota and Eukaryota. Prokaryota is further divided into two prokaryotic kingdoms — Eubacteria (true bacteria) and Archaebacteria (archaea) — recognising these as fundamentally distinct groups, an insight that parallels Carl Woese's three-domain system. Eukaryota is divided into five eukaryotic kingdoms: Protozoa, Chromista, Fungi, Plantae and Animalia, a finer split than Whittaker's single catch-all Protista. …