Five-Kingdom Model of Living Beings
Discussion
H.J. Spencer:
"Taxonomy is the science of identifying, naming and classifying organisms. In modern times, it was driven by botanists like John Ray, Carl Linnaeus and George Cuvier, who all believed that all species were eternally separate forms created once by their omnipotent God. Margulis alerts us to the dangers of poor taxonomies and the problems of assuming simple tree structures, preferring blending and recursive models.
This is used to introduce the new Five-Kingdom model consisting of
1) all bacteria;
2) Protoctists [nucleated micro-organisms: covering algae, slime molds, ciliates];
3) all fungi;
4) plants;
5) animals.
This scheme is based on DNA analysis and reflects the fact that animals and plants are far more similar to each other than to all other kinds of life on Earth. This replaces Aristotle's ancient classification of assigning any life form exclusively to plant or animal, so when swimming microbes were discovered they had to be defined as animals because plants were not motile whereas when non-motile green microbes were discovered they were first classified as plants; these confusions often reflected academic differences between botanists and zoologists.
The largest embarrassment is found with protozoa (Latin for 'first animals') because they swim but some are ancestral to both animals and plants and even fungi. Since all plants and all animals are multicellular, this adjective is usually redundant. Margulis firmly believes that non-nucleated bacteria appeared first while all nucleated creatures arose from bacterial symbiosis, as described above and admits to the deep frustration of trying to construct a single evolutionary scheme covering both botany and zoology reflecting metabolism, cell morphology, genetics and developmental biology. Margulis does not include viruses in her scheme as they cannot remain alive outside living cells that remain viable through ceaseless metabolism but they can spread genes among bacteria and other cells. All of us are a massive colony of micro-organisms."