The word animal originates from the Latin noun animalis, meaning having breath or soul, a definition that has persisted for millennia to describe the most complex life forms on Earth. These multicellular, eukaryotic organisms are defined by their ability to consume organic material, breathe oxygen, and move spontaneously during at least part of their life cycle. Unlike plants and algae, which produce their own food through photosynthesis, animals must ingest organic matter and digest it internally within a specialized chamber. This fundamental difference in energy acquisition has driven the evolution of complex behaviors, from the simple filtering of sponges to the sophisticated hunting strategies of apex predators. The scientific study of these creatures, known as zoology, encompasses everything from the microscopic Myxobolus shekel, which measures no more than 8.5 micrometers, to the blue whale, the largest animal that has ever lived, weighing up to 190 tonnes and stretching over 30 meters in length. The diversity within this kingdom is staggering, with over 1.5 million living species described, yet estimates suggest there may be as many as 7.77 million animal species on Earth, many of which remain undiscovered.
The Ancient Spark of Life
Evidence of animals stretches back to the Cryogenian period, with chemical signatures of 24-Isopropylcholestane found in rocks dating to roughly 650 million years ago, likely produced by early sponges. The first body fossils appear in the Ediacaran period, represented by enigmatic forms such as Charnia and Spriggina, though it was the discovery of the animal lipid cholesterol in fossils of Dickinsonia that confirmed their nature as true animals. These creatures are thought to have originated under low-oxygen conditions, capable of living entirely by anaerobic respiration before becoming fully dependent on oxygen as they specialized for aerobic metabolism. The Cambrian explosion, beginning around 539 million years ago, marks the point where nearly all modern animal phyla first appeared in the fossil record, including molluscs, brachiopods, onychophorans, tardigrades, arthropods, echinoderms, and hemichordates. This event, occurring in beds such as the Burgess Shale, saw the emergence of numerous now-extinct forms like the predatory Anomalocaris, suggesting a rapid diversification of life that may have been an artifact of the fossil record rather than a simultaneous appearance of all these animals.
The Architecture of Movement
Animals possess structural characteristics that set them apart from all other living things, including cells surrounded by an extracellular matrix composed of collagen and elastic glycoproteins. This matrix forms a relatively flexible framework upon which cells can move about and be reorganized into specialized tissues and organs, making the formation of complex structures possible. In contrast, the cells of other multicellular organisms, primarily algae, plants, and fungi, are held in place by cell walls and develop by progressive growth. The development of animals is controlled by Hox genes, which signal the times and places to develop structures such as body segments and limbs. This genetic toolkit allows for the evolution of a head end, or anterior, which encounters stimuli such as food, favoring cephalization, the development of a head with sense organs and a mouth. Many bilaterians have a combination of circular muscles that constrict the body, making it longer, and an opposing set of longitudinal muscles that shorten the body, enabling soft-bodied animals with a hydrostatic skeleton to move by peristalsis.
The animal kingdom is divided into five major clades, namely Porifera, Ctenophora, Placozoa, Cnidaria, and Bilateria, with most living species belonging to the clade Bilateria. The relationships at the base of the animal tree have been debated, with molecular phylogenetics supporting both the sponge-sister and ctenophore-sister hypotheses. Sponges, physically distinct from other animals, were long thought to have diverged first, representing the oldest animal phylum, yet genetic evidence suggests they may be more closely related to other animals than the comb jellies are. The remaining animals, the great majority comprising some 29 phyla and over a million species, form the Bilateria clade, which have a bilaterally symmetric body plan. These animals are triploblastic, with three well-developed germ layers, and their tissues form distinct organs. The digestive chamber has two openings, a mouth and an anus, and in the Nephrozoa there is an internal body cavity, a coelom or pseudocoelom. This group includes the protostomes, which encompasses organisms such as arthropods, molluscs, flatworms, annelids, and nematodes, and the deuterostomes, which include echinoderms, hemichordates, and chordates, the latter of which contains the vertebrates.
The Dance of Reproduction
Nearly all animals make use of some form of sexual reproduction, producing haploid gametes by meiosis, with the smaller, motile gametes being spermatozoa and the larger, non-motile gametes being ova. These fuse to form zygotes, which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, and develop into a new sponge, while in most other groups, the blastula undergoes more complicated rearrangement. It first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm. In most cases, a third germ layer, the mesoderm, also develops between them. These germ layers then differentiate to form tissues and organs. Repeated instances of mating with a close relative during sexual reproduction generally leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits, prompting animals to evolve numerous mechanisms for avoiding close inbreeding. Some animals are capable of asexual reproduction, which often results in a genetic clone of the parent, taking place through fragmentation, budding, such as in Hydra and other cnidarians, or parthenogenesis, where fertile eggs are produced without mating, such as in aphids.
The Web of Survival
Animals are categorized into ecological groups depending on their trophic levels and how they consume organic material, including carnivores, herbivores, omnivores, fungivores, scavengers, and parasites. Interactions between animals of each biome form complex food webs within that ecosystem, where predation is a consumer-resource interaction where the predator feeds on another organism, its prey, who often evolves anti-predator adaptations to avoid being fed upon. Selective pressures imposed on one another lead to an evolutionary arms race between predator and prey, resulting in various antagonistic/competitive coevolutions. Almost all multicellular predators are animals, and some consumers use multiple methods, such as parasitoid wasps, whose larvae feed on the hosts' living tissues, killing them in the process, while the adults primarily consume nectar from flowers. Most animals rely on biomass and bioenergy produced by plants and phytoplanktons through photosynthesis, but some benthic animals living close to hydrothermal vents and cold seeps on the dark sea floor consume organic matter produced through chemosynthesis by archaea and bacteria. Animals occupy virtually all of Earth's habitats and microhabitats, with faunas adapted to salt water, hydrothermal vents, fresh water, hot springs, swamps, forests, pastures, deserts, air, and the interiors of other organisms.
The Human and the Beast
Humans have exploited a large number of other animal species for food, both of domesticated livestock species in animal husbandry and, mainly at sea, by hunting wild species. Marine fish of many species are caught commercially for food, and a smaller number of species are farmed commercially, with humans and their livestock making up more than 90% of the biomass of all terrestrial vertebrates. Invertebrates including cephalopods, crustaceans, insects, and bivalve or gastropod molluscs are hunted or farmed for food, fibers, and dyes. Working animals including cattle and horses have been used for work and transport from the first days of agriculture, while animals such as the fruit fly Drosophila melanogaster serve a major role in science as experimental models. Animals have been used to create vaccines since their discovery in the 18th century, and some medicines, such as the cancer drug trabectedin, are based on toxins or other molecules of animal origin. A wide variety of animals are kept as pets, from invertebrates such as tarantulas, octopuses, and praying mantises, to reptiles such as snakes and chameleons, and birds including canaries, parakeets, and parrots, though the most kept pet species are mammals, namely dogs, cats, and rabbits.
The Soul in the Stone
Animals have been the subjects of art from the earliest times, both historical, as in ancient Egypt, and prehistoric, as in the cave paintings at Lascaux. Major animal paintings include Albrecht Dürer's 1515 The Rhinoceros, and George Stubbs's horse portrait Whistlejacket, while insects, birds, and mammals play roles in literature and film, such as in giant bug movies. Animals including insects and mammals feature in mythology and religion, with the scarab beetle sacred in ancient Egypt, and the cow sacred in Hinduism. Among other mammals, deer, horses, lions, bats, bears, and wolves are the subjects of myths and worship. The signs of the Western and Chinese zodiacs are based on animals, and in China and Japan, the butterfly has been seen as the personification of a person's soul. The human population has long viewed animals not merely as resources but as cultural elements of human evolution, appearing in cave arts and totems since the earliest times, and frequently featured in mythology, religion, arts, literature, heraldry, politics, and sports. This deep connection has shaped human history, with dogs, the first domesticated animal, used in hunting, security, and warfare, as have horses, pigeons, and birds of prey, while other terrestrial and aquatic animals are hunted for sports, trophies, or profits.