Animals are a group of living things on Earth, all of which display the following characteristics: They are multicellular, their cells have nuclei, they can move on their own for at least part of their lifetimes, and they obtain energy by eating organic matter from other living things.

Although the word "animal" is sometimes used to mean a living thing that is not human, human beings are indeed animals. We share all of the defining characteristics of animals. Each of us has many eukaryotic cells. (A eukaryotic cell is a cell with a nucleus. Living things with eukaryotic cells are known as eukaryotes.) We move of our own volition, and we must eat in order to live.

The Animal Kingdom is divided into six major groups: Porifera (sponges), Placozoa, Ctenophora (comb jellies), Cnidaria (corals, sea anemones, jellyfish and similar animals), Myxozoa (a kind of microscopic parasite) and Bilateria.

Most animals, including human beings, belong to the group Bilateria. These animals are bilaterally symmetrical for at least part of their lives. Bilaterians are also known as triploblasts, because when their bodies form, they develop distinct three layers. The inner layer is known as the endoderm, the middle layer is the mesoderm and the outer layer is the ectoderm.

The oldest animal fossils that we know of belong to sponge-like animals known as Otavia antiqua. These fossils were found in rocks in Namibia that are 760 million years old. 650 year old fossilized remains of what might be have been sponges have been discovered in Australia. The 585 year old tracks of small, slug-like bilaterians have been found in shallow waters in Uruguay.

Between 570 and 530 million years ago, many different types of animals appeared on Earth for the first time. All of the different body plans that are found in animals today appeared during this sudden (in geologic times) burst of life, which has become known as the Cambrian explosion or Cambrian radiation event.

The Burgess Shale, in British Columbia, Canada, contains fossil evidence of the many types of animals that appeared during the Cambrian explosion. Fossils found in the Burgess Shale include those of arthropods, mollusks, brachiopods and echinoderms, as well as those of phyla that have become extinct. Many of the animals that appeared during the Cambrian explosion, such as the trilobite, an early arthropod, are now extinct.

Scientists think that this sudden increase in animal diversity may have been related to an increase in atmospheric oxygen. Originally, Earth's atmosphere had almost no oxygen. Oxygen levels increased when algae began obtaining energy by photosynthesis, creating oxygen as a byproduct.

The supercontinent Rodinia broke up before the Cambrian explosion. Its breakup led to the creation of shallow seas and new niches for life. The erosion of surface rocks would have exposed basement rock and carried minerals, such as phosphorous and calcium, into the sea. Phosphorous could have acted as fertilizer for the algae, increasing oxygen production. Calcium was necessary for the formation of hard body parts, such as shells.

There was an ice age before the Cambrian radiation event. When the ice age ended, receding glaciers would have broken up continental rocks, bringing minerals to the sea.

The appearance of the first vertebrates, animals with spines, about 500 million years ago was another significant event in the history of animal life on Earth. A spinal column provides an animal with structural support, allowing it to grow to a large size. The largest animals on Earth are vertebrates. The blue whale, a vertebrate, is the largest animal alive today.

Human beings are vertebrates.

All of the vertebrates, as well as an animal known as a hagfish, which has a notochord (a flexible rod) running through its body instead of a spine, have heads with brains.