Classification of Living Beings

Life on Earth is staggeringly varied. Scientists have already named well over a million species, and new ones are still being found every year. Nobody can study them one by one, so biologists do what you do with your bookshelf: group the similar ones together and arrange the groups in order. This is classification (taxonomy). It helps us identify an organism quickly, study a whole group by studying one member, understand which organisms are related by evolution, and talk about the same organism in the same language anywhere in the world.

The idea is old. Aristotle grouped plants as herbs, shrubs and trees, and animals by where they live (land, water or air). That is easy, but it can mislead: a whale lives in water yet is a mammal like us. Modern classification therefore asks deeper questions about the organism's body plan and evolutionary relationships, not just its home or its looks. The more fundamental a characteristic is, the higher the level of classification it decides.

There are three ways of classifying. An artificial system uses one or a few easy characters (Linnaeus grouped flowering plants by the number of stamens). A natural system uses many natural similarities (Bentham and Hooker, 1862 to 1883). A phylogenetic system arranges organisms by evolutionary descent, which is the aim of modern taxonomy. The jump is simple: looks, then many features, then ancestry.

Once organisms are grouped, we need a ladder to arrange the groups. Taxonomists use a hierarchy of categories. At the bottom is the species, the basic unit: a group of organisms that can breed with one another and produce fertile offspring. Related species go into one genus, related genera into a family, and so on up to the kingdom. The higher you climb, the fewer shared characters and the more organisms in the group; the lower you go, the more shared characters and the fewer organisms.

Before the 18th century, a species was described by a long Latin phrase, and the same plant could carry different names in different countries. Carolus Linnaeus replaced this muddle with binomial nomenclature: every organism gets a two-part Latin name, the genus first and the species second. The botanical use of binomials begins with his Species Plantarum (1753) and the zoological use with the tenth edition of Systema Naturae (1758). A fixed international name means a student in Delhi and a scientist in Tokyo are talking about the same organism.

Species in the same genus are close cousins. Tomato, potato and brinjal belong to different species of one genus, Solanum (S. lycopersicum, S. tuberosum, S. melongena), and so they are all in one family, the Solanaceae. The genus name tells you the family circle; the species name tells you the exact member. That is why the first name carries a capital and the second does not.

For two centuries living things were put in just two kingdoms, plants and animals. That was too crude: where do bacteria, mushrooms and the single-celled Amoeba go? In 1969 R. H. Whittaker, an American ecologist, gave the five-kingdom system that most school and exam books still follow. He used five criteria: cell structure (prokaryotic or eukaryotic), body organisation (unicellular or multicellular), mode of nutrition, mode of reproduction, and phylogenetic (evolutionary) relationships. The five kingdoms are Monera, Protista, Fungi, Plantae and Animalia.

Why five kingdoms are better than two: the old two-kingdom system forced bacteria, fungi and single-celled organisms into 'plants' or 'animals', although they match neither. The new system separates prokaryotes (Monera) from eukaryotes, then separates eukaryotes by body organisation and nutrition: single-celled (Protista), absorptive heterotrophs with chitin walls (Fungi), photosynthetic multicellular organisms with cellulose walls (Plantae), and ingestive heterotrophs without walls (Animalia).

Kingdom Monera is the kingdom of the smallest and most ancient cells, the prokaryotes. They have a cell wall, a nucleoid and ribosomes but no true nucleus or membrane-bound organelles. Bacteria are the biggest group, and they come in four basic shapes. Monera are found everywhere, from boiling springs to polar ice, and they reproduce mainly by binary fission, which can double a population in about twenty minutes in ideal conditions. Under harsh conditions some form tough endospores that survive heat and drying.

Bacteria do two opposite jobs. As decomposers they rot dead matter and return minerals to the soil; as nitrogen fixers (Rhizobium in legume root nodules, Azotobacter and Nostoc in soil and water) they turn atmospheric nitrogen into a form plants can use. Lactobacillus turns milk into curd, and Streptomyces gives us antibiotics such as streptomycin. They are also the cause of cholera, typhoid, tuberculosis, tetanus and plant diseases like citrus canker.

Kingdom Protista holds the single-celled eukaryotes, the bridge between the simple Monera and the complex plants, animals and fungi. Most live in water. They move by flagella, cilia or false feet (pseudopodia) and reproduce by fission, and sexually by fusion of gametes in some. The five main groups are given in the table below.

Kingdom Fungi contains the moulds, mushrooms and yeasts. A fungus has no chlorophyll, so it cannot make food; its cell wall is chitin, and it stores food as glycogen. Its body is made of fine threads called hyphae, and a tangle of hyphae is the mycelium. The hyphae may be aseptate (coenocytic), one long tube with many nuclei (Rhizopus, Mucor), or septate, divided by cross-walls (Penicillium, Aspergillus). Fungi feed by releasing enzymes on their food and absorbing the digested solution, so they are saprotrophs (dead matter), parasites (living hosts) or symbionts (lichens, mycorrhiza).

Fungal reproduction has three modes: vegetative (fragmentation, fission, budding as in yeast), asexual (zoospores, conidia, sporangiospores), and sexual (oospores, ascospores, basidiospores). Spores are carried by wind and germinate into new hyphae. In the bread mould Rhizopus the black dots on the fuzz are sporangia sitting on upright stalks; each is a sac full of spores, the 'tiny blob on a stick' of the textbook picture.

Whittaker's five kingdoms are all made of cells. But biology also has strange things that stand outside them: viruses, viroids and prions. The story begins in 1892 when D. J. Ivanovsky found that the sap of a diseased tobacco plant stayed infectious even after passing through a filter fine enough to stop bacteria. In 1898 M. W. Beijerinck called the agent contagium vivum fluidum (a living infectious fluid) and the word virus (Latin for 'poison') stuck. In 1935 W. M. Stanley crystallised the Tobacco Mosaic Virus (TMV) and showed it is a nucleoprotein, which is why he shared the Nobel Prize in Chemistry in 1946.

A virus is built very simply: a core of genetic material, either DNA or RNA but never both, wrapped in a protein coat called the capsid, made of units called capsomeres. Some animal viruses (influenza, HIV, coronavirus) have an extra outer lipid envelope. A virus has no cytoplasm, no ribosomes and no metabolism, so on its own it is as lifeless as a crystal. Inside a living host cell, however, it hijacks the cell's machinery to make hundreds of copies of itself. That is why it is called an obligate intracellular parasite, and why it sits on the border between living and non-living.

Lichens are not a single organism but a symbiotic partnership between a fungus (mycobiont) and an alga or cyanobacterium (phycobiont). The alga photosynthesises and feeds both partners; the fungus gives shelter, holds water and absorbs minerals. Together they survive where neither could alone, such as bare rock, tree bark and even Antarctica. Lichens are pioneers of primary succession: they slowly break rock into soil so that mosses and other plants can follow. They are very sensitive to sulphur dioxide, so their absence is a sign of air pollution, which makes them good pollution indicators. Lichens give us the dye litmus, used as an acid-base indicator, and 'reindeer moss' (Cladonia) feeds reindeer in the Arctic.