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Animals (lat. Animalia) is a traditionally (since the time of Aristotle) distinguished category of organisms, currently considered as a biological kingdom. Animals are the main object of study of zoology.

Animals are eukaryotes (they have nuclei in their cells). The classic signs of animals are: heterotrophy (nutrition of ready-made organic compounds) and the ability to actively move. However, there are many animals that lead an attached lifestyle, and heterotrophy is characteristic of fungi and some parasitic plants.

The Russian word "animal" is derived from "belly", which in the past meant "life, property". In everyday life, the terms "wild animals", "domestic animals" are often understood only as mammals or four-legged terrestrial vertebrates (mammals, reptiles and amphibians). However, in science, the term "animals" has a broader meaning, corresponding to the Latin Animalia (see above). In the scientific sense, animals, in addition to mammals, reptiles and amphibians, include a huge variety of other organisms: fish, birds, insects, arachnids, mollusks, starfish, worms and others. Man also belongs to the animal kingdom, the order of placental mammals, primates [1], but is traditionally studied separately.

At the same time, many heterotrophic protists were previously assigned to this kingdom and animals were divided into subkingdoms: unicellular Protozoa and multicellular Metazoa. Now the name "animals" in the taxonomic sense has been assigned to multicellular organisms. In this sense, animals as a taxon have more specific features - they are characterized by oogamy, a multi-tissue structure, the presence of at least two germ layers, the stages of blastula and gastrula in embryonic development. The vast majority of animals have muscles and nerves, and the groups that do not have them - sponges, lamellar, Mesozoic, cnidosporidia - may have lost them a second time.

At the same time, in science, the term "animals" is sometimes proposed to be used in an even broader sense, meaning by animals not a taxon, but a type of organization - a life form based on mobility.

Currently (Zhang, 2013), scientists have described more than 1.6 million animal species (including more than 133 thousand fossil species; Zhang, 2013), most of which are arthropods (more than 1.3 million species, 78%), mollusks ( more than 118 thousand species) and vertebrates (more than 42 thousand species)[2][3].
Animals are thought to have evolved from flagellated unicellular organisms, and their closest known living relatives are the choanoflagellates, collared flagellates morphologically similar to the choanocytes of some sponges. Molecular studies have placed animals in the supergroup Opisthokonta, which also includes choanoflagellate, fungi, and a small number of parasitic protists. The name Opisthokonta denotes a posterior flagellum in a motile cell, as in most animal spermatozoa, while other eukaryotes tend to have an anterior flagellum.

Molecular studies confirm that animals appeared in the middle of the Tonian period (about 900 million years ago) [4]. However, there is evidence for the existence of multicellular organisms before this time. Helical carbonaceous bands called Grypania have been found in rocks 1.9–1.4 billion years old in the vicinity of Lake Superior. The belonging of the find to animals is undeniable. Some researchers consider it the remains of primitive multicellular eukaryotic algae or a highly developed colony of cyanobacteria. Another supposed ancient animal is called Horodyskia, found in sediments aged 1.44 billion years in North America and 1.4-1.07 billion years old in Australia. In China and Russia, the imprints of several small organisms ranging in age from 1 billion to 840 million years ago, called the "Huainan biota", have been found. One of the first animals is also often considered the likely sponge Otavia, dating from 760 to 550 million years ago, but this belonging of the organism to sponges is disputed.

The first fossils that can be reliably attributed to animals belong to the cryogenic period (about 665 million years ago). Even better known are later typographical errors dating back to 575-542 million years ago. They are known as the Ediacaran or Vendian fauna. They are, however, difficult to correlate with later fossils. They may be ancestors of modern lineages of animals, independent groups, or not animals at all.

Most of the known modern animal phyla appeared more or less simultaneously during the Cambrian period, about 542 million years ago. This event, the Cambrian Explosion, was caused either by a rapid divergence of the groups or by such a change in conditions that made petrification possible. However, some paleontologists and geologists suggest that animals appeared much earlier than previously thought, perhaps even 1 billion years ago - at the beginning of the Tonian. This is indicated by a decrease in the diversity of stromatolites around this time.