G. C. Williams

California Academy of Sciences

(last updated 16 July 2019)

As recently as the mid-1960’s, all the known kinds of living organisms on Earth were aligned with either plants or animals. Many biology textbooks were commonly either botany or zoology texts. Botany covered a potpourri of biodiversity, including bacteria, algae, fungi, and green plants, while zoology covered animals and an enormous assemblage of single-celled organisms called protozoans.

Then, in 1969 the five kingdom concept was introduced by Robert Whittaker – the five major kingdoms included three multicellular groups (animals, plants, and fungi), unicellular organisms (protists), and Monera (bacteria). An important distinction was made between Prokaryotes such as bacteria and archaeans (organisms without membrane-bound organelles in their cells), and all other organisms known as Eukaryotes (organisms with membrane-bound organelles such as nuclei and mitochondria).

Also at the end of the 1960s decade, Lynn Margulis proposed the Endosymbiotic Theory – an evolutionary theory that explains the origin of eukaryotic cells from prokaryotes, in that several organelles of eukaryotes (such as the nucleus and mitochondria) originated as a symbiosis between separate prokaryotes.

In recent years, ever-more-sophisticated molecular techniques and the ability to completely sequence genomes have revolutionized our thinking regarding the biodiversity of life. We now recognize an overwhelming complexity of life. Instead of the simple dichotomy of plants and animals, we have over 100 groups of eukaryotic organisms that make up only one part of the tree of life. Eukaryotes join two groups of prokaryotes (Bacteria and Archaea) in an overall view of life on our planet.

For further information see: The Tree of Life Gets a Makeover (