Animals make up a majority of the earth’s population. People have domesticated animals for agricultural, domestic, and other purposes. Domesticated animals are usually very similar to their wild ancestors, but have been bred to be more productive and less susceptible to disease. The variety of animals is amazing, with an animal one day walking on the moon, another one million years later.
Most animals are multicellular, meaning that they have multiple cells that are made up of DNA, or nucleic acid, in the same overall structure as bacteria and plants. Animals do not contain ribosomes, the tiny sections of DNA needed to make proteins, unlike plants and some subtypes of bacteria. With a few exceptions, animals eat raw organic matter, breathe oxygen, can move, reproduce sexually, and produce offspring that are genetically identical to them. A horse may have a single horn, a cat may have four toes, but they will all be the same species.
All living creatures are part of a larger group or superorder. Within each superorder are families and subfamilies, which can also have their own set of characteristics and behaviors. Humans are part of the superfamily of animals called Multicellular Animals. Animals that make up this group have a cellular system made up of DNA strands and chromosomes that are contained in tiny organelles called placenta and horns. These animals also appear to have a nervous system, since they make use of nerves to move and communicate.
Animals are classified according to a system of ranking, or taxonomic rank. Ranking systems often apply both to taxa and two types of animals. Within a taxonomic rank, one taxa may have only a few relatives, while another may have thousands of relatives. In nature, most animals are part of a genus or a species, while taxa rank beneath a genus are often just subspecies or varieties of one main genus.
The subclasses and species of a genus are further separated into smaller divisions. A hundred species of a genus can be broken down into ten or twenty subclasses, while a hundred subclasses and species of a genus can be broken down into hundreds of types. It is through classification that many animal taxonomists find their work and conclusions useful. Classification can be based on similarities and differences between different types of animals, or it can be based on similarities and differences between different kinds of animals.
Taxonomic ranks, if based on similarity and differences, are sometimes used to classify animals into categories or subspecies. In some cases, differences may be enough to justify splitting an animal into two or more subclasses or even into a superfamily. This is especially true when classification is based on similarities with relatives of the same species. In these circumstances, the resulting classification is often referred to as a family tree and is often very informative.