Short answers, pulled from the story.
An animal is a multicellular, eukaryotic organism belonging to the kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, have myocytes and can move, reproduce sexually, and grow from a hollow sphere of cells called the blastula during embryonic development.
Over 1.5 million living animal species have been described, of which around 1.05 million are insects, over 85,000 are molluscs, and around 65,000 are vertebrates. Using patterns in the taxonomic hierarchy, researchers calculated in 2011 that the total number of animal species, including those not yet described, is about 7.77 million.
The blue whale, Balaenoptera musculus, is the largest animal that has ever lived, weighing up to 190 tonnes and measuring up to 33.6 metres long. The largest land animal alive today is the African bush elephant, Loxodonta africana, while the largest land animals of all time were titanosaur sauropod dinosaurs such as Argentinosaurus.
Animals first appear in the fossil record in the late Cryogenian period, with the molecule 24-isopropylcholestane found in rocks roughly 650 million years old. Many animal phyla appear during the Cambrian explosion, which began about 539 million years ago, though some palaeontologists suggest animals arose as early as 1 billion years ago.
The animal kingdom is divided into five major clades: Porifera, Ctenophora, Placozoa, Cnidaria, and Bilateria. Most species belong to Bilateria, which splits into the protostomes, including arthropods and molluscs, and the deuterostomes, which include echinoderms and chordates, the latter containing the vertebrates.
Humans use animals for food such as meat, eggs, and dairy, and for materials including leather, fur, and wool. Animals also serve as pets and working animals for transport and services, with dogs the first domesticated animal, used in hunting, security, and warfare, and they feature widely in mythology, religion, art, and literature.