An animal is a member of the kingdom Animalia, one of the great kingdoms of living things. Animals are found in all parts of the world—in tropical and polar regions, in deserts and in bodies of water, underground, on mountains, and in the air. Some animals live inside plants, and some live in the bodies of other animals. There are animals so small that they can barely be seen with the unaided eye; at the other extreme are such gigantic creatures as elephants and whales. Many animals possess what can be called intelligence, but none can compare with humans in the quality and degree of such thought processes as insight, reasoning, and imagination.

The study of animals is a branch of biology called zoology.

More than one million living species of animals have been identified, and more are constantly being discovered. Fossils (impressions or other traces, usually found in rock, of organisms that lived in past ages) show that additional millions of species of animals once lived that are now extinct.

The terms "higher" and "lower" are sometimes used in describing animals. These terms refer to the relative position of various animals on the time scale of biological evolution. Biologists believe that, in general, forms of life have evolved from the simpler to the more complex. Animals such as sponges are believed to be similar to the earliest forms of life and therefore lower on the evolutionary scale than animals more complicated in structure, such as the frog. However, there are exceptions to this rule, and some simple animals have evolved from complex ones. To avoid confusion, the terms "higher" and "lower" have been avoided in this article.

For an explanation of the classification system of the animal kingdom, and cross references,

The variety of animal life found in the world today is the result of evolutionary changes that have taken place over millions of years. Fossil evidence indicates that simple animal life appeared about 700 million years ago. By the end of the Cambrian Period, about 500 million years ago, there were marine animals belonging to most of the major groups, or phyla, of animals known today. Among these animals were sponges, corals, jellyfish, worms, mollusks, and arthropods. The appearance of these animals, which occurred over several million years, is called the "Cambrian explosion."

During the next 260 million years, many kinds of fish developed, both in the sea and in freshwater. Also during this time, insects developed; they and other arthropods were the first animals to live on land. Amphibians appeared, becoming the first vertebrates (animals with backbones) to spend part of their lives on land. Reptiles, the first vertebrates capable of living entirely on land, also appeared.

During the following 175 million years, known as the Mesozoic Era, many species of reptiles developed. Among these reptiles were the dinosaurs, the largest land animals that ever existed. Also during this era, birds and mammals came into existence.

As the Cenozoic Era dawned—it began about 65 million years ago and is still continuing—mammals and birds were developing into many different species. Eventually, the primates, including humans, appeared.

Interesting facts about animals
Kinds of animals. No one knows exactly how many kinds of animals there are. New kinds are found every year. So far, scientists have identified more than 1 1/2 million types of animals. About 1 million of these are insects, and there are thousands of kinds of fish, amphibians, reptiles, birds, and mammals.
Largest ears and eyes. The largest ears of all animals are those of the African elephant. Elephant ears grow as large as 4 feet (1.2 meters) across. The largest eyes of all animals are those of the giant squid. They measure about 10 inches (25 centimeters) wide
The flying dragon is another name for the draco lizard. This lizard can spread out folds of skin to form "wings" that it uses to glide through the air from tree to tree. It lives in Asia and the East Indies.
Lives of animals range from several hours to many years. An adult mayfly survives only a few hours or days. Some giant tortoises have lived more than 100 years.
The world's only known poisonous bird is the hooded pitohui, which lives on the island of New Guinea. This brilliantly colored orange-and-black bird has poison on its feathers and skin. This poison serves as a defense against hawks, snakes, and other enemies. It is the same type of poison as that carried by the deadly poison-dart frog of South America.
The hummingbird can fly straight up like a helicopter. It can hover in front of a flower to suck the nectar. The bee hummingbird, which grows to only 2 inches (5 centimeters) long, is the smallest of all birds.
The chameleon's tongue is as long as its body. This lizard swiftly shoots out its tongue to capture insects for food. Certain chameleons can quickly change color and even develop spots and streaks that make them seem to be part of their background.
A tree-climbing crab lives on many tropical islands. It is called the coconut crab because it cracks coconuts with its powerful claws and eats the sweet meat.
The platypus, a mammal, has a bill like a duck and lays eggs as birds do. But it nurses its young with milk as do other mammals. It lives only on mainland Australia and the island of Tasmania.