What Is an Animal?

An animal is a multicellular living thing that gets its energy by eating other living things, can usually move at least part of its body, and senses the world around it. Animals make up one of the great kingdoms of life on Earth, alongside plants, fungi, bacteria, archaea and protists. There are around 1.5 million known animal species, and probably 7 to 9 million in total once everything has been described. They range from microscopic mites to the blue whale, the biggest animal that has ever lived.

  • Known speciesapprox. 1.5 millionMost are insects
  • Estimated total7 to 9 millionMany still undiscovered
  • First animalsapprox. 600 million years agoSoft-bodied marine creatures
  • Largest everBlue whaleUp to 30 m and 200 tonnes
  • SmallestMyxozoa parasitesUnder 0.02 mm long
  • Vertebrates (with backbone)approx. 3%97% of animals have no backbone

What makes something an animal?

Animals share a checklist of features that separate them from plants, fungi and microbes.

  • They are multicellular: made of many cells working together (not just one).
  • They are eukaryotic: their cells have a nucleus.
  • They eat other living things (or what is left of them) for energy. Biologists call this heterotrophic.
  • Their cells have no cell wall (which is what gives plant and fungal cells their boxy shape).
  • Most can move at least part of their body.
  • Most have a nervous system that lets them sense and respond to the world.
  • Most reproduce sexually, although a few can clone themselves.

The major animal groups

Biologists sort animals into phyla (singular: phylum), the biggest branches of the animal family tree. There are around 35 of them, but a handful contain most of the well-known animals:

  • Arthropods: insects, spiders, crabs, lobsters, scorpions, centipedes. By far the biggest phylum, with over a million known species.
  • Chordates: animals with a backbone (or something like it). Includes all mammals, birds, reptiles, amphibians and fish, plus a few oddballs like sea squirts.
  • Molluscs: snails, slugs, octopuses, squid, oysters, clams. Soft-bodied animals, many with shells.
  • Annelids: segmented worms (like earthworms and leeches).
  • Nematodes: tiny roundworms. Possibly the most numerous animals on Earth.
  • Cnidarians: jellyfish, corals, sea anemones, hydras.
  • Echinoderms: starfish, sea urchins, sea cucumbers.
  • Poriferans: sponges. The simplest animals; they have no nervous system or organs.

Vertebrates and invertebrates

The most common everyday split is between animals with a backbone (vertebrates) and animals without (invertebrates). All the famous big animals (mammals, birds, reptiles, amphibians, fish) are vertebrates. But vertebrates are actually only about 3% of all animal species. The other 97% are invertebrates, dominated by insects.

How animals are different from plants

The biggest differences between animals and plants come from how they get their energy.

  • Plants make their own food from sunlight, water and CO2 through photosynthesis. Animals have to eat other living things.
  • Plants are anchored by roots and do not move from place to place. Animals can usually move freely (often very fast, to chase food or escape predators).
  • Plants have tough cell walls and stay rigid; animals have flexible cells and move freely.
  • Plants do not have a nervous system; animals almost always do.
Fact The first true animals appeared in Earth's oceans around 600 million years ago. They were soft-bodied creatures (jellyfish-like, worm-like, sponge-like) with no skeletons. Then about 538 million years ago came the Cambrian explosion: in less than 25 million years, almost every major phylum of modern animals appeared in the fossil record. Most of the basic body plans that exist today (segmented insects, shelled molluscs, vertebrate fish) all date from this single dramatic burst of evolution.

The story of the animal kingdom

Here are the rough chapter headings in the long history of animal life:

  • approx. 600 million years ago: first soft-bodied animals (Ediacaran fauna).
  • approx. 538 million years ago: the Cambrian explosion. Almost every major modern phylum appears.
  • approx. 530 million years ago: first true fish.
  • approx. 400 million years ago: first amphibians crawl onto land.
  • approx. 320 million years ago: first reptiles.
  • approx. 225 million years ago: first mammals (small shrew-like creatures).
  • approx. 150 million years ago: first birds (descended from feathered dinosaurs).
  • approx. 66 million years ago: mass extinction wipes out the non-bird dinosaurs.
  • approx. 25 million years ago: first apes.
  • approx. 300,000 years ago: modern humans appear in Africa.
Did you know? Sponges are technically animals, but they have no muscles, no nerves, no brain and no organs. They are basically a community of cells stuck together, filtering food out of seawater. Yet sponges have been around for over 600 million years and have survived every mass extinction. Sometimes simple is good.
Deeper dive: what is the most successful animal of all time?

It depends on how you count "successful". By a few different measures:

  • Most species: beetles. Around 400,000 known beetle species, more than any other animal group. The biologist J.B.S. Haldane was once asked what could be inferred about the Creator from a study of nature, and is said to have replied: "An inordinate fondness for beetles".
  • Most individuals: nematodes (tiny roundworms). There are an estimated 4 quintillion of them on Earth right now: about 80% of all animals on the planet.
  • Most total mass: krill (small shrimp-like creatures in the oceans). The total weight of Antarctic krill alone is around 500 million tonnes, more than any other single species on Earth.
  • Longest geologic lifespan: horseshoe crabs. They have been around for over 450 million years, almost completely unchanged through 4 mass extinctions.
  • Most widespread: humans. We live on every continent including Antarctica, plus on space stations.

By almost any measure, you would think of small, simple, fast-breeding creatures as the most successful in evolution. Lions and elephants get the attention, but the animal kingdom is really ruled by ants, beetles, worms and microbes.

Pick a group to explore: mammals, birds, reptiles, amphibians, fish or the invertebrate groups like insects.