Kingdom - Animalia

 

Animals are multicellular, eukaryotic organisms classified under the biological kingdom Animalia. Most animals share several fundamental traits, including the ability to consume organic material, respire using oxygen, possess muscle cells (myocytes) for movement, and reproduce sexually. Their development typically includes the formation of a hollow sphere of cells called a blastula during embryogenesis. Animals form a clade, meaning they originated from a single common ancestor.

Over 1.5 million animal species have been documented, with around 1.05 million of these being insects, over 85,000 mollusks, and roughly 65,000 vertebrates. It is estimated, however, that Earth may host as many as 7.77 million animal species in total. These creatures display an astonishing range of body sizes, from the microscopic 8.5 µm (0.00033 in) to the colossal 33.6 m (110 ft) of the blue whale. They occupy virtually every ecological niche on Earth, forming intricate food webs that involve complex interactions with both their environments and other species. The scientific study of animals, known as zoology, also includes the specialized field of ethology, which focuses on animal behavior.

The majority of animal species belong to the infrakingdom Bilateria, a highly successful clade characterized by bilateral symmetry. Most bilaterians are members of two major superphyla: the protostomes—encompassing arthropods, mollusks, annelids, nematodes, and flatworms—and the deuterostomes, which include echinoderms, hemichordates, and chordates (the group that contains vertebrates). There remains some uncertainty regarding the classification of simpler organisms like the Xenacoelomorpha within the Bilateria.

The earliest known animals appear in the fossil record during the Cryogenian period, with their diversity increasing during the Ediacaran. Some evidence suggests that certain sponge-like organisms may date back to the earlier Tonian period, though their classification as animals is still debated. Modern animal phyla emerged predominantly during the Cambrian explosion around 539 million years ago, a period of rapid diversification in marine environments. Genetic studies have identified 6,331 gene families common to all animals, likely derived from a common ancestor that lived around 650 million years ago during the Cryogenian.

Historically, Aristotle classified animals based on whether they had blood or not, while Carl Linnaeus later introduced a hierarchical system of classification in 1758 with Systema Naturae, which was expanded by figures like Jean-Baptiste Lamarck and Ernst Haeckel. The advent of molecular phylogenetics in modern times has significantly refined our understanding of evolutionary relationships among animal species.

Humans have long utilized animals in various aspects of life. They serve as sources of food—providing meat, eggs, and dairy products—raw materials like leather and wool, and have been domesticated as pets or working animals. Animals like dogs, horses, and birds of prey have been used in hunting, security, and warfare, while many species are hunted for sport or profit. Beyond their utilitarian roles, animals hold deep cultural significance, having been featured in art, mythology, and religion throughout human history. From the earliest cave paintings to modern literature, animals have served as powerful symbols of human interaction with the natural world.

The term "animal" derives from the Latin word animalis, meaning "having breath or soul," underscoring the traditional perception of animals as living beings distinct from other forms of life. In contemporary biology, all organisms within the kingdom Animalia are considered animals, though in common usage, the term is often reserved for non-human creatures. The word "metazoa" (from Ancient Greek meta, "after" and zoia, "animals") is also used to refer to multicellular animals.

Animals are distinct from other organisms in several key ways. Unlike plants and algae, animals are heterotrophic, meaning they consume organic material for sustenance. Most animals respire aerobically, though exceptions exist. A defining characteristic of animal development is the formation of a blastula, a hollow sphere of cells that allows for the differentiation of tissues and organs, enabling complex body structures.

Animals consist of cells embedded in a unique extracellular matrix composed of collagen and elastic glycoproteins, which allows for movement and reorganization during development. This is in contrast to plants and fungi, whose cells are encased in rigid cell walls. Additionally, animals possess specialized structures called cell junctions, including tight junctions, gap junctions, and desmosomes, which contribute to their structural integrity.

Reproduction in animals is predominantly sexual, involving the fusion of haploid gametes to form a zygote. In some species, asexual reproduction is also possible, often resulting in offspring that are genetically identical to the parent. Ecologically, animals occupy various roles in food chains, from primary consumers like herbivores to top predators, and they often engage in complex interactions such as parasitism, mutualism, and competition.

Thus, animals, with their diverse forms, behaviors, and ecologies, represent one of the most dynamic and influential branches of life on Earth. Their evolutionary journey, from simple multicellular organisms to the vast array of species present today, is a testament to the power of natural selection and adaptation across billions of years.