Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 phyla by 1809.

Historically, Aristotle divided animals into those with blood and those without. 6,331 groups of genes common to all living animals have been identified these may have arisen from a single common ancestor that lived 650 million years ago. Many modern animal phyla became clearly established in the fossil record as marine species during the Cambrian explosion, which began around 539 million years ago. Life forms interpreted as early animals were present in the Ediacaran biota of the late Precambrian.

The Bilateria include the protostomes, containing animals such as nematodes, arthropods, flatworms, annelids and molluscs, and the deuterostomes, containing the echinoderms and the chordates, the latter including the vertebrates. Most living animal species are in Bilateria, a clade whose members have a bilaterally symmetric body plan. The scientific study of animals is known as zoology. They have complex interactions with each other and their environments, forming intricate food webs. Animals range in length from 8.5 micrometres (0.00033 in) to 33.6 metres (110 ft). As of 2022, 2.16 million living animal species have been described-of which around 1.05 million are insects, over 36,000 are fishes, around 11,700 are reptiles, over 11,100 are birds, and 6,596 mammals-but it has been estimated there are around 7.77 million animal species in total. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and grow from a hollow sphere of cells, the blastula, during embryonic development. Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia.

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