Animal

Animals are living things that make up a big group called Animalia. They are different from plants and other simple life forms because they usually move, eat other things, and can feel things. Most animals have special cells that let them move, and they grow from tiny balls of cells early in their lives.

Scientists have found and named over 1.5 million animal species, but there may be millions more we haven’t discovered yet! Animals come in all sizes, from tiny ones you can’t even see to huge ones like whales that can be over 30 meters long.

Animals have many important roles in our world. They help keep nature balanced by eating plants and each other, and they are also very useful to humans. We use animals for food, like meat, eggs, and milk. We also use them for clothes, like leather and wool, and as pets and helpers, like dogs that help us find things or guard us. Animals have also been part of human stories and cultures for thousands of years, appearing in art, myths, and even sports.

Etymology

The word animal comes from a Latin word that means the same thing. It is based on an older Latin word meaning "having breath or soul." When people talk about animals, they often mean creatures that are not humans. There is also a term called metazoan, which comes from ancient Greek words meaning "after" and "animals." A metazoan is any creature that belongs to this group.

Characteristics

Animals share several traits with other living things. They are made of many cells and need oxygen to live, just like plants and fungi. Unlike plants, animals cannot make their own food and must eat other things to get energy.

Animals also have special features that set them apart. They can move during parts of their lives and go through a special early stage called a blastula when they grow. Most animals have a body with a digestive system that has one or two openings. Their development is guided by special genes, and their bodies form flexible frameworks that help cells organize into complex structures like shells and bones.

Nearly all animals reproduce by combining special cells from two parents. These cells fuse to form a new ball of cells that develops into a baby animal. Some animals can also reproduce without parents by creating clones of themselves.

Ecology

Animals can be grouped based on what they eat and how they get their food. Some animals, like carnivores, eat other animals, while others, like herbivores, eat plants. There are also omnivores that eat both plants and animals. These eating habits help create food chains and webs in nature.

Most animals get their energy from plants and tiny organisms called phytoplanktons through a process called photosynthesis. Herbivores eat plants directly, while carnivores get their energy by eating herbivores. Some animals, like those living near deep-sea vents, get energy from chemicals instead of sunlight. Animals live in almost every place on Earth, from oceans to deserts, but most cannot survive in very hot or very cold places.

Diversity

Size

Further information: Largest organisms and Smallest organisms

The blue whale (Balaenoptera musculus) is the largest animal that has ever lived, weighing up to 190 tonnes and measuring up to 33.6 metres (110 ft) long. The largest animal living on land today is the African bush elephant (Loxodonta africana), weighing up to 12.25 tonnes and measuring up to 10.67 metres (35.0 ft) long. Some of the smallest animals are so tiny that you can only see them with a microscope.

Evolutionary origin

Further information: Urmetazoan

Scientists have found evidence that animals existed as far back as the Cryogenian period. A special kind of material called 24-Isopropylcholestane, found in rocks about 650 million years old, is made only by sponges and some algae. This helps us understand when animals might have first appeared.

The earliest animal fossils come from a time called the Ediacaran, about 635 to 542 million years ago. These include strange forms like Charnia and Spriggina. We now know they were animals because we found animal fats in fossils of a creature called Dickinsonia.

Many animal groups showed up suddenly in the fossil record during an event called the Cambrian explosion, starting about 539 million years ago. This includes groups like molluscs, arthropods, and echinoderms. One early predator from this time was Auroralumina attenboroughii, which lived about 20 million years before the Cambrian explosion and caught small prey with special cells, just like some animals do today.

Phylogeny

Further information: Lists of animals

Animals come from one common ancestor, making them a group of related living things. They are closely related to choanoflagellates, and scientists believe animals began to appear between 650 million and 800 million years ago.

The way animals are related has been studied a lot. Besides sponges, most animals share similar body plans and are closely connected. Sponges and another group called Placozoa are simpler and don’t have the same body organization as other animals.

Most animals belong to a group called Bilateria, which have bodies that are symmetrical from side to side. These animals have three layers in their bodies and their tissues form distinct organs. They also have a mouth and an anus, and many have a space inside their bodies. Having a front end helps these animals sense their environment and find food. Many can move using muscles, and they often have a gut that runs from mouth to anus.

Scientists divide Bilateria into two main groups: protostomes and deuterostomes. Protostomes include arthropods (like insects and spiders) and molluscs (like snails and clams). Deuterostomes include chordates, which are mostly vertebrates (animals with backbones), such as fish, amphibians, reptiles, birds, and mammals.

History of classification

Further information: Taxonomy (biology), History of zoology through 1859, and History of zoology since 1859

Long ago, during the classical era, a thinker named Aristotle sorted animals into groups based on what he saw. He put animals with blood, like vertebrates, in one group and those without blood in another. He wondered if sponges were animals or plants because they could feel touch but didn’t move around like other animals.

Later, in 1758, Carl Linnaeus made the first system to classify animals into groups. He placed animals in one of three big groups, called kingdoms, and split them further into smaller groups. Over time, scientists like Jean-Baptiste de Lamarck, Georges Cuvier, and others helped organize these groups better, creating new ways to understand how animals are related to each other.

In human culture

Practical uses

Main article: Human uses of animals

Humans use many animal species for food. This includes raising animals like chickens, cattle, sheep, and pigs on farms, as well as catching wild fish for eating. Animals also provide materials like wool, silk, and leather for clothing and other items. Some animals, such as dogs and birds of prey, help humans with tasks like hunting. Many animals are kept as pets, with dogs, cats, and rabbits being the most common. Animals are also important in science, helping researchers learn new things.

Symbolic uses

Animals appear in many cultural symbols and stories. For example, the signs of the Western and Chinese zodiacs are based on animals. In art, animals have been popular subjects for thousands of years, from ancient cave paintings to famous paintings by artists like Albrecht Dürer. Animals also play roles in myths, religions, and stories from many cultures around the world.