An animal is a multicellular organism that eats other life, moves at some stage, and builds its body from cells with no walls. These simple rules unite a sponge and a blue whale.

Unlike plant cells, animal cells have no wall and no chloroplasts. Flexible membranes let them change shape, cluster into tissues, and specialise into muscle, nerve and bone.

All life sorts into a few great branches. Animals form one twig on the eukaryote branch, sharing an ancient single-celled ancestor with fungi — our closest kingdom cousins.

Every animal begins as a single fertilised egg. It divides, folds and specialises — a hollow ball becomes gut, skin and nerve, following an ancient developmental recipe.

Most animals are bilateral — a mirror-image left and right, with a head that leads. A few, like jellyfish, are radial, built like a wheel with no front or back.

Early in development, the first opening becomes either the mouth or the anus. This tiny difference splits the animal kingdom into two vast lineages that diverged half a billion years ago.

Since Linnaeus, every animal gets a two-word Latin name and a place on a nested ladder — from broad kingdom down to a single species. It is biology's universal address system.

About 540 million years ago, animal diversity exploded. In a geological blink, almost every major body plan alive today first appeared in the fossil record of the ancient seas.

Fossils are the animal kingdom's memory. Bones, shells, footprints and even burrows, turned to stone over millions of years, let us reconstruct creatures no human ever saw.

An animal is life that moves and must hunt for its fuel. It cannot make food from sunlight; it must find and consume other life. From this hunger, every animal wonder flows.

Backbones are rare. Around 97 in every 100 animal species are invertebrates — sponges, worms, snails, crabs and above all insects. They run the planet's ecosystems.

Sponges have no organs, no nerves and no true tissues — just a colony of cells pumping water through pores. They are the closest living echo of the first animals.

Cnidarians are hunters armed with stinging cells. Many alternate between a fixed polyp and a drifting medusa — two body forms of one animal, cycling through the generations.

The word 'worm' hides deep differences. Flatworms are ribbon-thin with a single gut opening; roundworms are cylindrical with a through-gut. Both are wildly abundant and often parasitic.

Molluscs are the second-largest animal phylum. A soft body, often shelled, unites the slow garden snail with the giant squid — the most intelligent invertebrate of all.

Annelids build their bodies from repeated segments, like a train of near-identical carriages. This modular plan, seen in the earthworm, foreshadows the segmentation of insects.

Arthropods are the runaway winners of animal life. Jointed limbs and a hard exoskeleton let them fill the sea, the land and the sky. Four in five animals are arthropods.

Insects are the most successful animals ever. A three-part body, six legs and, uniquely among invertebrates, wings let them dominate every land habitat on Earth.

Spiders, scorpions, ticks and mites are arachnids: eight-legged hunters with no wings or antennae. Most are venomous predators, and spider silk is stronger, for its weight, than steel.

Starfish, urchins and sea cucumbers are echinoderms — five-fold radial adults that are, surprisingly, our distant cousins. They move on hundreds of tiny water-powered feet.

Vertebrates are defined by a backbone: a chain of bones protecting the nerve cord and anchoring the muscles. It begins in the embryo as a simple rod, the notochord.

Backboned animals branch into five familiar groups: fish, amphibians, reptiles, birds and mammals. Each conquered a new frontier — water, land, and finally the open air.

Fish were the original backboned animals and remain the most diverse. From tiny gobies to whale sharks, over half of all vertebrate species swim.

Amphibians were the first vertebrates to walk on land, but they never fully left the water. Most begin life as water-breathing larvae before transforming into air-breathing adults.

Reptiles cracked the problem that tied amphibians to ponds: the amniotic egg, a private pool sealed in a shell. For the first time, vertebrates could breed on dry land.

For over 160 million years, dinosaurs ruled the land. They ranged from chicken-sized hunters to the largest animals ever to walk the Earth, until an asteroid ended their reign.

Birds are feathered, warm-blooded dinosaurs rebuilt for flight. Hollow bones, a fused wishbone and a powerful heart make them the most mobile vertebrates on Earth.

Mammals are warm-blooded vertebrates that grow hair and feed their young on milk. From bats to blue whales, they inherited the world the dinosaurs left behind.

Mammals split three ways by how they bear young. A few still lay eggs; marsupials raise tiny newborns in a pouch; and placental mammals nourish the young inside the body until well-formed.

Humans are primates — mammals with grasping hands, forward eyes and large brains. We share this branch with apes and monkeys, and over 98% of our DNA with chimpanzees.

Animals hold their shape three ways: an internal skeleton of bone, an external suit of armour, or a pressurised bag of fluid. Each solution suits a different scale and life.

Muscles are the only tissue that can pull. They work in opposing pairs — one contracts as its partner relaxes — tugging on bone or shell to bend, walk, swim and fly.

Blood carries oxygen and fuel to every cell and hauls the waste away. A muscular heart drives it around a loop of vessels, again and again, for the whole of an animal's life.

Every animal must feed oxygen to its cells. Fish use gills, land vertebrates use lungs, insects pipe air through tiny tubes, and some creatures simply breathe through the skin.

A gut is a tube that breaks food into molecules the body can absorb. From mouth to exit, it grinds, dissolves and digests — an assembly line running in reverse to build the animal.

Nerves let an animal sense and react. The simplest is a diffuse net with no centre; the most complex is a brain of billions of cells that can plan, remember and imagine.

Warm-blooded animals burn fuel to hold a steady temperature; cold-blooded animals borrow their heat from the surroundings. One pays in food, the other in dependence on the sun.

The body's outer layer defends, waterproofs and warms. From reptile scales to mammal fur and bird feathers, each covering is a variation on the same tough, renewable barrier.

Size is ruled by physics. As an animal grows, its weight rises faster than the strength of its bones and the reach of its lungs. Water lifts these limits; on land, giants are rare.

Some animals rebuild what they lose. A lizard regrows its tail, a starfish an arm, and a flatworm can regenerate an entire body from a fragment — a power we have largely lost.

Eyes evolved dozens of times. Vertebrates and octopuses use a single-lens 'camera' eye; insects use compound eyes of hundreds of tiny units, each catching one point of the scene.

Sound is a wave, and hearing organs turn its ripples into nerve signals. Ears let animals detect danger, prey and each other in the dark, around corners, and far out of sight.

Smell and taste detect molecules. For most animals they are the master senses — finding food, mates and danger by chemistry long before anything comes into view.

Touch reads the world by direct contact. Whiskers, antennae, skin and sensitive hairs let animals feel texture, pressure, current and the faintest movement of the air around them.

Some animals map the dark by listening to their own echoes. A bat or dolphin fires a burst of sound and reads the returning bounce to picture prey, walls and water in fine detail.

Every muscle twitch leaks a faint electric field. Sharks, rays and the platypus have organs that read these fields, sensing hidden prey by the tiny currents of its beating heart.

Many migrating animals read the Earth's magnetic field. Birds, turtles and even some insects carry an internal compass that guides them across oceans and continents without a single landmark.

Some snakes 'see' heat. Special pits between eye and nostril detect the infrared glow of warm prey, building a thermal image that lets a viper strike accurately in total darkness.

Many animals see ultraviolet light, invisible to us. Bees follow UV runway patterns on flowers, and birds wear secret UV plumage — a whole layer of the world hidden from human eyes.

No animal senses the whole world — each inhabits its own slice, its Umwelt. A tick knows warmth and butyric acid; a bat knows echoes. Reality itself is filtered by the senses that read it.

Life began moving in water. Fish drive themselves by waving their bodies side to side; others jet, paddle or ripple. Swimming is the oldest and still the most common animal locomotion.

On land, legs carry the weight. Animals vary the number, length and gait — the pattern of footfalls — to walk, trot and gallop, trading stability for speed as they go faster.

Powered flight evolved four separate times — in insects, pterosaurs, birds and bats. Each solved the same problem differently, yet all bend air to generate the lift that defeats gravity.

Jumping stores energy and releases it in an instant. Muscles pre-load elastic tissues like a drawn bow, then let go, flinging tiny animals distances that would be impossible by muscle alone.

Climbers defy gravity with grip. Hooked claws, opposable thumbs and the gecko's micro-hairs let animals scale bark, rock and even glass, moving through a vertical world.

Underground is a habitat all its own. Burrowers dig with shovel-claws, push through soil, or eat their way forward, building tunnels that shelter them and reshape the very ground.

Snakes and legless lizards move by muscle alone. Waves of contraction press the body against the ground, driving it forward — an elegant solution that needs no limbs at all.

Between falling and flying lies gliding. Flaps of skin, webbed feet or flattened bodies turn a leap into a controlled descent, letting animals cross gaps they could never jump.

In every medium, one animal is fastest. The peregrine falcon dives faster than a racing car; the cheetah is the quickest runner; the sailfish, the swiftest swimmer. Speed is a weapon and an escape.

How an animal moves shapes its whole body. Streamlined fish, hollow-boned birds and long-legged runners all show the same truth: the demand to move sculpts anatomy down to the bone.

Plants are abundant but hard to digest. Herbivores meet the challenge with grinding teeth, long guts and gut microbes that ferment tough plant fibre into usable food.

Meat is rich but must be caught. Carnivores are built to kill: forward eyes to judge distance, blade-like teeth to slice flesh, and the speed or stealth to close the final gap.

Omnivores eat both plant and animal. Their flexibility — all-purpose teeth and an adaptable gut — lets them thrive where specialists cannot, weathering seasons and scarcity alike.

The ocean is thin soup, and filter feeders strain it. From tiny clams to the blue whale, they sieve vast volumes of water to trap the plankton and small prey suspended within.

Parasites live on or in another animal, taking food without killing outright. It is a hugely successful way of life — possibly the most common on Earth — with a host for every kind of thief.

Toxins let small animals fight big ones. Venom is injected by fang or sting to subdue prey; poison is passive, sickening whatever eats it. Both are among nature's oldest weapons.

The mouth is the first tool. Jaws crush, teeth cut and grind, and beaks probe, tear and crack. The shape of the mouth reveals, at a glance, exactly what an animal eats.

Hunter and hunted push each other to evolve. Faster cheetahs breed faster gazelles; better camouflage breeds sharper eyes. Neither wins for long — the race never stops.

Energy flows through life in a web. Plants capture sunlight; herbivores eat plants; predators eat herbivores. At each step most energy is lost, so top predators are always few.

Nothing is wasted. Scavengers strip the dead, and decomposers break the remains back into soil. This unglamorous work recycles the nutrients that every living thing depends upon.

Life must copy itself. Sexual reproduction mixes two parents' genes for variety; asexual reproduction clones a single parent for speed. Many animals can switch between the two.

The egg is a self-contained nursery. A shell or membrane wraps the embryo with its own food and water, protecting new life until it is ready — the reproductive strategy of most animals.

Instead of laying eggs, many animals grow their young inside the body. It costs the parent dearly but protects the developing offspring, giving them a strong start in a dangerous world.

Some animals live two utterly different lives. A caterpillar dissolves almost entirely inside its cocoon and reassembles as a butterfly — one of nature's most astonishing transformations.

Young animals are not just small adults. Many pass through larval stages with their own bodies and lifestyles, dispersing, feeding and surviving in ways the adult never could.

Care ranges from none to total. Some animals abandon millions of eggs to chance; others raise a single offspring for years. More care usually means fewer, better-protected young.

To pass on its genes, an animal must first be chosen. Song, dance, colour and gift-giving all serve one goal: to convince a mate that its bearer is healthy, fit and worth breeding with.

Life's clocks run at wildly different speeds. A mayfly lives a day; a Greenland shark, five centuries. And one jellyfish appears able to cheat death entirely, aging in reverse.

Some animals surrender individuality. An ant colony or beehive behaves as a single 'superorganism', its thousands of members working as cells in one body, none surviving alone.

Behind every egg, migration and mating dance lies one imperative: to send genes into the next generation. Every life cycle, however strange, is a strategy for that single, ancient goal.

Behaviour comes two ways. Instinct is inherited and ready at birth; learning is gathered through life. Most animals blend both — a hard-wired core, refined by experience.

Intelligence is not ours alone. Crows craft tools, octopuses solve puzzles, and dolphins recognise themselves in mirrors. Smart minds evolved many times, in very different bodies.

Animals talk in every medium — sound, colour, scent, movement and electricity. Signals warn of danger, claim territory, attract mates and hold groups together across distance.

Grouping trades privacy for protection. A herd spots predators sooner and confuses the hunt; a pack brings down larger prey. But crowds also breed competition, disease and conflict.

Resources are worth fighting for. Animals claim territories and settle into pecking orders, using display and ritual combat to decide rank — usually without anyone getting seriously hurt.

Migration is life on the move. To follow food and good weather, animals cross oceans and continents on journeys of astonishing length, precision and endurance, year after year.

Migrating animals navigate without maps. They read the sun and stars, the Earth's magnetism, smells and landmarks, combining several senses into an internal guidance system of uncanny accuracy.

Play, grief and joy are not uniquely human. Young mammals play to learn; elephants mourn their dead; rats laugh when tickled. Rich emotional lives run deep through the animal kingdom.

Almost every animal sleeps, though why remains a puzzle. Some take it further, shutting down for a whole season — hibernating through winter on a fraction of their normal energy.

Some knowledge is neither instinct nor personal learning, but culture — behaviour taught from one animal to another. Whale songs, chimp tool-kits and bird dialects are all handed down.

Animal life is shaped by climate. From frozen poles to tropical reefs, each biome sets the rules — and the tropics, warm and stable, hold the overwhelming share of the world's species.

Species entwine their lives. In mutualism both partners gain; in commensalism one gains and the other is unaffected; in parasitism one profits at the other's cost. All are symbiosis.

To survive, hide or lie. Camouflage blends an animal into its background; mimicry disguises it as something else — a leaf, a twig, or a dangerous animal it is not.

Some animals thrive where life seems impossible. Tube worms feed at scalding deep-sea vents, and the tardigrade endures boiling, freezing, radiation and even the vacuum of space.

Earth has seen five mass extinctions. Many scientists say we are causing a sixth, as habitat loss, hunting and a warming climate push species toward the edge far faster than nature alone would.

The IUCN Red List is the world's scorecard for extinction risk. It grades assessed species from Least Concern to Extinct, guiding where conservation effort is most urgently needed.

Extinction is not always the end of the story. Protection, breeding and reintroduction have pulled species back from the brink, and rewilding is returning lost animals to whole landscapes.

The catalogue of life is far from finished. Scientists have named perhaps a million animal species, yet estimate that the great majority — especially insects and deep-sea life — remain unknown.

Science now reaches toward reversing extinction itself. Frozen cells, gene editing and cloning raise the prospect of reviving lost species — and hard questions about whether we should.

Every animal — sponge, insect, whale and human — descends from one ancient ancestor and belongs to one living web. To understand any animal is to glimpse the whole, and our own place within it.