About the Five Kingdoms

The "Five Kingdoms of Life" is a system made up by people to try to describe the way that all living things, from bacteria to elephants to trees, are related to one another. The practice of trying to sort things into groups is called classification or taxonomy. When we're talking about living things it's called biological classification, since biology is the study of life. Scientists' thinking about biological classifcation is always changing and developing, but the exact number of kingdoms and what they're called isn't really the important thing. The reason we classify things is to help us think about and understand the wonderful variety, or biodiversity, of life on Earth.

Two Kingdoms

The way that today's biologists think about the classification of living things can pretty much be traced back to Aristotle, the Greek scientist and philosopher who lived more than two thousand years ago. He decided that everything alive could be divided into two kingdoms, Plants and Animals; he really liked to split things into categories, and then split those categories into more categories; he was just getting started describing the animals that live on land in this part of his book, History of Animals:

Of land animals some are furnished with wings, such as birds and bees, and these are so furnished in different ways one from another; others are furnished with feet. Of the animals that are furnished with feet some walk, some creep, and some wriggle...

This was still the way people thought about the kingdoms in the 18th century, when a Swedish naturalist called Carolus Linnaeus finally standardized biological classification. He used a system of ranks, or "taxa" to categorize organisms by shape and behaviour, and developed the system of "binomial nomenclature," referring to things by their genus and species. Scientists still use binomial nomenclature today for scientific names, like calling humans Homo sapiens, or hedgehogs Erinaceus albiventris. The most general taxon, or rank, was the Kingdom; Linnaeus said that all organisms were either members of Kingdom Animalia (animals) or Kingdom Plantae (plants). Biologists today don't use the exact same taxa (groupings) in classification that Linnaeus did, but the system is pretty similar: kingdom, phylum, class, order, family, genus, species, with some people adding in the odd superkingdom, subphylum, and so on.

Three Kingdoms

By the 19th century, microscopes had been around for hundreds of years, and biologists had found and named lots and lots of single-celled creatures. It was hard to tell whether many of them were plants or animals; a lot of them seemed like a bit of both. For example, a troublesome creature called Euglena carried out photosynthesis like a plant but swam around like an animal, as if it were the most natural thing in the world! Ernst Haeckel, a German biologist (and fantastic artist), proposed a new Kingdom, which he called Protista, to house all these one-celled things without having to decide whether they were plants or animals, and Kingdom Protista persists to this day.

Haeckel was also an admirer of Charles Darwin and his theory of evolution, who was around at the same time. Haeckel, Darwin, and others began to think differently about classification in general, and attempting to group organisms who shared a common ancestor, not just ones who looked alike or acted alike.

Four Kingdoms

About fifty years later, in the 1930s, Edouard Chatton, a French biologist, pointed out two very different types of organisms had been lumped together in the "protist" category. Some members of Kingdom Protista had cells with a nuculeus and other complex cellular structures called "organelles," but one large group— the bacteria— didn't! He called the ones with nuculei "eukaryotes," and the ones without "prokaryotes." In 1938, an American called Herbert Copeland argued that prokaryotes were so different to eukaryotes that they should be moved out of Kingdom Protista altogether, and Kingdom Monera, the bacteria kingdom, was born.

Five Kingdoms

In the 1950s, Robert Whittaker, an American botanist, proposed that mushrooms, molds, and other fungi, part of Kingdom Plantae, should be separated into their own kingdom. He made this distinction based on the way that Fungi supply themselves with nutrients and energy. They don't make their food through photosynthesis like plants do, but they don't ingest it the way animals do either. Instead, they secrete enzymes into their food, digesting it before they absorb it! More differences kept turning up; for example, their cell walls are made of chitin, whereas plants use cellulose, and animals haven't got a cell wall at all. A new kingdom was created for these evolutionary weirdos, called Kingdom Fungi. The resulting Five Kingdom system has been widely used ever since, and is still the one taught in schools in the UK, and on this site!

Six Kingdoms and beyond

More recently, our ability to work out the exact genetic sequences of organisms' DNA molecules means that scientists' ideas about how to group living things is changing. There's debate about where some members of the various kingdoms (especially protists and bacteria) belong, and the biggest shift of all has been towards splitting the Bacteria Kingdom (monera) into two new Kingdoms: Bacteria and Archaea. Genetic analysis has shown that the DNA sequences of one "bacteria" and another can be less alike than between some of those "bacteria" and a human. This six kingdom system may turn out to be the next step in our understanding, and it remains to be seen what the future of classification will bring...