The Living World and Classification

Life on Earth is staggeringly diverse, encompassing millions of kinds of organisms from bacteria to blue whales. To study this diversity, biologists must first name and organise it. Classification sorts living things into orderly groups based on their similarities. This lets the whole living world be understood, compared and studied. This is the starting point of biology.

What makes something living? Living organisms share certain features:

• Growth: they increase in size and number of cells.
• Reproduction: they produce new individuals of their kind.
• Metabolism: they carry out chemical reactions (like respiration) to stay alive.
• Response to stimuli: they react to their environment.
• Cellular organisation: they are made of one or more cells.
• The ability to evolve over generations.

These properties together distinguish the living from the non-living.

The science of naming and classifying organisms is taxonomy. Organisms are arranged in a hierarchy of ever-broader groups:

Species → Genus → Family → Order → Class → Phylum → Kingdom

Each level groups organisms by shared features, with species the most specific.

To name organisms clearly, biologists use binomial nomenclature, devised by Carl Linnaeus. Every species gets a two-word Latin name made up of its genus and species, such as Homo sapiens for humans. This gives every organism a single, universal name that scientists everywhere understand.

The hierarchy keeps working below the kingdom level. Inside Animalia, the next major group is the phylum, and below that the class. A single example shows why these finer ranks matter.

Spiders, scorpions, mites and crabs all belong to the same phylum, Arthropoda. These are animals with jointed legs and a hard outer covering. Yet they split into different classes at the next step:

• Arachnida: spiders, scorpions and mites. Arachnids have four pairs of legs (eight legs) and a body in two parts. They have no antennae.
• Crustacea: crabs, prawns and lobsters. Crustaceans have two pairs of antennae and more than four pairs of legs, and most live in water.

So among a crab, a mite, a scorpion and a spider, the crab is the odd one out. The mite, scorpion and spider are all arachnids, while the crab is a crustacean. The shared phylum hides a real difference that only the class level reveals.

For a long time the broadest grouping was the five-kingdom system, proposed by R. H. Whittaker. It divides all life into:

• Monera: single-celled organisms without a true nucleus (bacteria).
• Protista: single-celled organisms with a true nucleus (amoeba, algae).
• Fungi: organisms that absorb food, like moulds and mushrooms.
• Plantae: multicellular plants that make their own food by photosynthesis.
• Animalia: multicellular animals that take in food.

This system groups organisms by cell type, body organisation and how they obtain food. It gives a clear map of life's major branches. Newer schemes add a higher level called "domains".