Animal

• 1. Defining Biological Kingdoms

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  Animals are multicellular, eukaryotic organisms belonging to the biological kingdom Animalia. With few exceptions, animals consume organic material and breathe oxygen. They possess myocytes and can move during at least part of their life cycle. Reproduction occurs sexually in nearly all species. Growth begins from a hollow sphere of cells called the blastula during embryonic development. Over 1.5 million living animal species have been described by scientists. Around 1.05 million of these are insects alone. More than 85,000 species belong to molluscs. Approximately 65,000 vertebrates exist today. Estimates suggest as many as 7.77 million animal species may live on Earth. Body lengths range from microscopic sizes to massive proportions. The blue whale reaches up to 190 tonnes and measures over 30 meters long. The largest terrestrial animal is the African bush elephant weighing up to 12.25 tonnes. Some Myxozoa parasites never grow larger than 20 micrometers. One tiny species named Myxobolus shekel measures no more than 8.5 micrometers when fully grown.

• 2. Evolutionary Origins And Explosions

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  Evidence of animals appears in rocks dating back roughly 650 million years ago. A chemical marker called 24-Isopropylcholestane found in those ancient rocks likely originated from sponges. First body fossils appear in the Ediacaran period with forms like Charnia and Spriggina. Discovery of cholesterol in Dickinsonia fossils confirms their nature as animals. Animals first appeared in the fossil record during the late Cryogenian period. They diversified further in the subsequent Ediacaran period known as the Avalon explosion. Nearly all modern animal phyla emerged during the Cambrian explosion starting about 539 million years ago. Most classes appeared later during the Ordovician radiation around 485.4 million years ago. Common to all living animals are 6,331 groups of genes that arose from a single common ancestor. That ancestor lived approximately 650 million years ago during the Cryogenian period. Extant phyla in Cambrian beds include molluscs, brachiopods, tardigrades, arthropods, echinoderms, and hemichordates. The predatory Anomalocaris represents one now-extinct form found in these ancient rocks. Some palaeontologists suggest animals may have appeared as early as 1 billion years ago.

• 3. Phylogenetic Tree And Classification

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  Animals form a clade meaning they arose from a single common ancestor. Ros-Rocher and colleagues traced origins back to unicellular ancestors by 2021. Uncertainty exists regarding relationships indicated with dashed lines in cladograms. The animal clade had certainly originated by 650 mya and possibly as much as 800 mya based on molecular clock evidence. Animals are sister group to choanoflagellates forming the Choanozoa. Porifera sponges were long assumed to be sister to rest of animals but Ctenophora may hold that position instead. Molecular phylogenetics supports both sponge-sister and ctenophore-sister hypotheses. A 2023 study by Darrin Schultz uses ancient gene linkages to construct alternative trees. Most living species belong to Bilateria which has bilaterally symmetric body plans. This highly proliferative clade includes organisms such as arthropods, molluscs, flatworms, annelids, and nematodes. Two large subclades contain vast majority of bilaterians: protostomes and deuterostomes. Protostomes include echinoderms, hemichordates, and chordates containing vertebrates. Xenacoelomorpha represents smaller basal phylum with uncertain position within Bilateria.

• 4. Historical Taxonomy Development

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  Aristotle divided animals into those with blood and those without during classical era observations. He arranged creatures on a scale from man down through live-bearing tetrapods to spontaneously generating sponges. Aristotle remained uncertain whether sponges qualified as animals since they lacked sensation or locomotion in his system. Carl Linnaeus created first hierarchical classification in Systema Naturae published in 1758. Original scheme placed animals among three kingdoms including classes like Vermes, Insecta, Pisces, Amphibia, Aves, and Mammalia. Jean-Baptiste Lamarck began expanding this framework in 1793 calling Vermes a chaotic mess. By 1809 he had split the group into nine new phyla plus four vertebrate groups. Georges Cuvier used comparative anatomy in 1817 to group animals into four branches corresponding roughly to modern phyla. Embryologist Karl Ernst von Baer followed this division in 1828 while zoologist Louis Agassiz did so in 1857. Comparative anatomist Richard Owen adopted similar grouping in 1860. Ernst Haeckel divided kingdom into Metazoa and Protozoa subkingdoms in 1874. Protozoa later moved to Protista leaving only Metazoa as synonym for Animalia.

• 5. Ecological Roles And Food Webs

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  Animals categorize into ecological groups depending on trophic levels and consumption methods. Carnivores include piscivores eating fish, insectivores consuming insects, ovivores taking eggs, and others. Herbivores break down further into folivores eating leaves, graminivores grasses, frugivores fruits, granivores seeds, nectarivores nectar, algivores algae, and more. Omnivores consume both plant and animal matter while fungivores eat fungi. Scavengers and detritivores feed on decaying organic material including parasites. Interactions between animals form complex food webs within each biome. Predation represents consumer-resource interaction where predator feeds on prey evolving anti-predator adaptations. Selective pressures create evolutionary arms race leading to antagonistic coevolution. Almost all multicellular predators are animals though some consumers use multiple methods. Parasitoid wasp larvae feed on hosts killing them while adults consume flower nectar. Hawksbill sea turtles mainly eat sponges despite being carnivorous generally. Most animals rely on biomass produced by plants through photosynthesis. Benthic animals near hydrothermal vents consume organic matter via chemosynthesis using hydrogen sulfide oxidized by archaea and bacteria. Animals occupy virtually all Earth habitats from salt water to deserts.

• 6. Human Exploitation And Domestication

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  Humans exploit large number of other species for food including meat eggs and dairy products. Marine fish caught commercially make up significant portion of global protein supply. Smaller numbers farmed commercially include chickens cattle sheep pigs and other livestock. Humans and their livestock comprise over 90% biomass of all terrestrial vertebrates. Invertebrates like cephalopods crustaceans bees silkworms bivalves gastropods hunted or farmed for food fibers. Animal fibers such as wool silk used to make textiles worldwide. Leather widely produces shoes items coats hats dyestuffs carmine shellac kermes made from insect bodies. Working animals including cattle horses used work transport since first days agriculture. Dogs first domesticated animal used hunting security warfare alongside horses pigeons birds prey. Poison dart frogs poison blowpipe darts tips in traditional practices. Fruit fly Drosophila melanogaster serves major role science experimental models. Vaccines created since discovery 18th century using various animal species. Cancer drug trabectedin based on toxins molecules animal origin. Wide variety kept pets ranging tarantulas octopuses praying mantises snakes chameleons canaries parakeets parrots mammals dogs cats rabbits most common.

• 7. Cultural Symbolism And Artistic Legacy

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  Western Chinese zodiac signs based entirely upon animals representing different personality traits. Butterfly personifies soul in China Japan classical representation symbolizing spiritual essence. Animals subjects art earliest times historical prehistoric cave paintings Lascaux France. Major paintings include Albrecht Dürer's The Rhinoceros painted 1515 George Stubbs Whistlejacket horse portrait. Insects birds mammals feature literature film giant bug movies popular culture. Scarab beetle sacred ancient Egypt cow sacred Hinduism religion traditions. Deer horses lions bats bears wolves subjects myths worship across cultures. Animals appear mythology religion arts literature heraldry politics sports globally. Signs Western Chinese zodiacs reflect deep cultural connections between humans and other creatures. Cave paintings demonstrate early human fascination with animal forms behaviors relationships. Modern media continues exploring themes of animal-human interaction through films books games.