How classification grew

People have sorted living things for a very long time, and the groups changed as we learned more. Aristotle made the first scientific attempt. He divided plants into trees, shrubs and herbs, and animals into those with red blood and those without.

Linnaeus then used just two kingdoms: Plantae and Animalia. That was too simple. It put organisms without a nucleus (prokaryotes like bacteria) with those that have one (eukaryotes); one-celled organisms with many-celled ones; and green algae that make their own food together with fungi that cannot.

Under the old system, anything with a cell wall counted as a plant: bacteria, blue-green algae, fungi, mosses, ferns and flowering plants all ended up together. But fungi are very different from green plants. They cannot make food (they are heterotrophs), and their walls are made of chitin, not cellulose.

In 1969, R.H. Whittaker proposed five kingdoms:

• Monera: one-celled prokaryotes (bacteria).
• Protista: one-celled eukaryotes (like Amoeba and Paramoecium, which do have a nucleus).
• Fungi: finally a kingdom of their own.
• Plantae and Animalia: many-celled eukaryotes.

Whittaker used five criteria: cell structure (prokaryotic or eukaryotic), body organisation (one cell, tissues, organs), mode of nutrition (making food or taking it in), reproduction, and phylogenetic relationships (how groups are related through evolution).

The five kingdom system is not perfect. Viruses and viroids fit nowhere in it, and some kingdoms still mix very different members. Later, a three-domain system was also proposed. Classifications keep changing as we understand evolutionary relationships better.