The earth today is home to more than 300,000 species of plants and more than one million species of animals. Biologists called taxonomists have devised a carefully developed scheme to organize these myriad species. In the mid 1700s, Carolus Linnaeus, a Swedish physician and botanist, published several books in which he described thousands of plant and animal species. Linnaeus grouped the species according to their reproductive parts. Linnaeus developed the two-part binomial taxonomy system of categorizing organisms according to genus and species.Linnaeus's work remains valid. It has been combined with the work of Charles Darwin in the field of evolution to form the foundation of modern taxonomy. Darwin's theory of evolution states that all modern species are derived from earlier species and that all organisms, past and present, share a common ancestry. Darwin's theory of evolution, which has become a unifying theme in biology, is the organizing principle of modern taxonomy.

Taxonomists classify organisms in a way that reflects their biological ancestry. Because the ancestral relationships are complex, the taxonomic schemes are also complex. Despite their complexity, the taxonomic schemes provide considerable insight into the unity and diversity of life. The term “classification” is synonymous with the word “taxonomy.”

All organisms in the living world are classified and named according to an international system of criteria that dates to the early part of this century. The rules of classification establish a procedure to be followed when a new species is identified and named. (The rules of classification apply only to formal scientific names, not to common names.)

The scientific name of any organism, called the binomial name, has two elements. For example, humans have the binomial name Homo sapiens. The name of any species is two words: the name of the genus followed by the “species modifier.” For humans, Homo is the genus and sapiens is the species modifier. The genus name is generally a noun, while the species modifier is an adjective. Thus, Homo sapiens means “human knowing.”

The generally accepted criterion for defining a species is that organisms of the same species interbreed under natural conditions to yield fertile offspring. Individuals of different species normally do not mate. If they are forced to mate, the mating is either unsuccessful or the offspring are sterile. For example, a horse (Equus caballus) can be mated to a donkey (Equus assinus), and the result will be a mule. However, mules are sterile and cannot reproduce. Thus, the horse and donkey are classified as different species. A quarterhorse and a thoroughbred can mate and produce a fertile offspring. Therefore, both are classified as the same species: Equus caballus.

For humans, there is only one living species: Homo sapiens. However, in past ages other species, such as Homo erectus, may have coexisted with Homo sapiens. Homo erectus is considered a separate species because presumably it could not mate with Homo sapiens.

The classification scheme provides a mechanism for bringing together various species into progressively larger groups. Taxonomists classify two species together in the same genus (the plural is genera). For example, the horse Equus caballus and the donkey Equus assinus are both placed in the genus Equus. Similar genera are brought together to form a family. Similar families are classified within an order. Orders with similar characteristics are grouped in a class. Related classes are grouped together as divisions or phyla (the singular is phylum). Divisions are used for plants and fungi, while phyla are used for animals and animal-like organisms. The largest and broadest category is the kingdom.

The classification of a human shows how the classification scheme works. Working from the top down, the human is classified first in the kingdom Animalia because it has the properties of animals. Animals are then divided into at least ten phyla, one of which is Chordata. Members of this phylum all have a backbone at some time in their lives.

Members of the phylum Chordata are then subdivided into various classes. Humans belong to the class Mammalia, together with other mammals (all of which possess mammary glands and nurse their young). The Mammalia are then divided into several orders, one of which is Primata. Humans belong to the order Primata along with other primates, such as gorillas and monkeys. The order Primata is subdivided into several families, one of which is Hominidae, the family that includes humans and humanlike creatures. Within the family of Hominidae is the genus Homo, which includes several species. One of these species is Homo sapiens.

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