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.