Keys to the kingdoms

Ever since they could, people have been naming and classifying the other living things in the world. Even the story of the Garden of Eden had Adam naming all the animals. Presumably he named all the plants, too, but the tendency to name things is part of what makes us people.

We need to describe and categorise, it makes our world a little less intimidating, and pattern detection is hard-wired into our human brains. At first it seems pretty obvious that things can be classified into neat distinct, easily defined groups, but this is not really case at all.

Phylogenetic tree of Eukaryota. Three groups in the base of the tree are incertae sedis, all other groups fall into one of the five 'kingdoms'
Image by Vojtech.dostal, based on a similar picture in Simpson et Roger 2004, via Wikimedia Commons
Over 300 years before the common era, Aristotle classified animal species in his work the History of Animals. His pupil Theophrastus was not so good at chasing things, so he compiled the History of Plants. And about two thousand years later Karl Linn, after changing his own name to the Latinised Carolus Linnaeus, was the first to formalise the idea that everything could be classed as animal, vegetable, or mineral. And for living things, the kingdoms of plants and animals ruled, but there was some dissent.

Fifty years before Linnaeus published his Systemae Naturae, a Dutchman with a microscope started looking at microbes. Before Antonie van Leeuwenhoek, single celled microscopic organisms were basically unknown to science. At first they were shoved into either the animal or plant kingdoms. But by the mid 1800s, it was obvious this didn’t really work, and in 1866 Ernst Haeckel proposed a third, kingdom of single-celled organisms, the Protista, in addition to multi-cellular animals and plants.

But scientists being scientists, this was not enough. Increasingly powerful microscopes, especially electron microscopes, began to show differences in single celled organisms too. Not just slight differences, but vast differences in structure.

The protista was made up of two distinct types of single cells: prokaryotes and eukaryotes. Pro means before, and karyon is from the ancient greek for nut or kernel, eu means ‘well’ or ‘good’. In other words, prokaryotes are “before nuclei” and eukaryotes have “good nuclei”.

This brought about recognition that bacteria were so different to other organisms they deserved their own kingdom, so then there were four:

  • Empire Monera
    • Kingdom Bacteria
  • Empire Prokaryota
    • Kingdom Protista
    • Kingdom Plantae
    • Kingdom Animalia

However, fungi caused a problem. They had been placed in the plant kingdom since ancient times, mostly because they didn’t move, and grew in the ground. But obviously, there are also single celled fungi, like yeast, and some of them do have spores that can swim around, so it’s not really a good fit.

An ecologist called Robert Whittaker propsed in 1969 a new five kingdom classification, based mainly on behaviour, specifically how things get their nutrition. Plants make their own food, Animals eat other organisms, and fungi consume dead stuff, mostly. The other two kingdoms, Protista and Monera, included unicellular organisms and simple cellular colonies.

  • Empire Prokaryota
    • Kingdom Monera
  • Empire Eukaryota
    • Kingdom Protista
    • Kingdom Plantae
    • Kingdom Fungi
    • Kingdom Animalia

This was great, until the 1970s when science kicked in again. Molecular biologists, who look at the genetic information found in organisms, decided that classification should really be based on what species were descended from whom.

Taxonomy, the classification of organisms into species, genus, family and so on all the way up to kingdom had been founded on this principle, but the accepted tree of life did not match the molecular data. How things looked on the outside did not match who they really were on the inside. So differences within the Monera were noted, and it was split in two: the Eubacteria and Archaebacteria, because there is as much genetic difference between these two groups as between either of them and all eukaryotes.

Eukaryote groups – such as plants, fungi and animals – may look different, but are more similar to each other in their genes than to the monera. And they are more closely related, genetically, to the Archaebacteria than they are to the Eubacteria.

In 1990, the name domain was proposed for the highest rank, and we have a six-kingdom system, which is standard in some texts:

  • Domain Bacteria
    • Kingdom Bacteria
  • Domain Archaea
    • Kingdom Archaea
  • Domain Eukarya
    • Kingdom Protista
    • Kingdom Plantae
    • Kingdom Fungi
    • Kingdom Animalia

So, we have six kingdoms. Not so bad.

But molecular biologists are not finished yet. They want all the classification to be monophyletic. That means that all the species in a kingdom must have arisen from a single ancestor. If that is taken to be the primary classification criteria, we are presented with a vastly different view of the living world:

  • Domain Bacteria
    • Eubacteria
  • Domain Archaea
    • Archaeobacteria
  • Domain Eukarya
    • Excavata — Various flagellate protozoa
    • Amoebozoa — most lobose amoeboids and slime moulds
    • Opisthokonta — animals, fungi, choanoflagellates, etc.
    • Rhizaria — Foraminifera, Radiolaria, and various other amoeboid protozoa
    • Chromalveolata — Stramenopiles (or Heterokonta), Haptophyta, Cryptophyta (or cryptomonads), and Alveolata
    • Archaeplastida (or Primoplantae) — Land plants, green algae, red algae, and glaucophytes

Currently, there is no one classification system supported by up-to-date research to gain widespread acceptance.

Maybe the idea of kingdoms is a little old fashioned, and it’s time for a republic of life?

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