Biologists place living things into different classes. This classification, known as "taxonomy," or "systematics," goes back as far as the eighteenth-century Swedish scientist Carl von Linné, known as Linnaeus. The system of classification established by Linnaeus has continued and been developed right up to the present day.

There are hierarchical categories in this classificatory system. Living things are first divided into kingdoms, such as the plant and animal kingdoms. Then these kingdoms are sub-divided into phyla, or categories. Phyla are further divided into subgroups. From top to bottom, the classification is as follows:

Kingdom

Phylum (plural Phyla)

Class

Order

Family

Genus (plural Genera)

Species

Today, the great majority of biologists accept that there are five (or six) separate kingdoms. As well as plants and animals, they consider fungi, protista (single-celled creatures with a cell nucleus, such as amoebae and some primitive algae), and monera (single-celled creatures with no cell nucleus, such as bacteria), as separate kingdoms. Sometimes the bacteria are subdivided into eubacteria and archaebacteria, for six kingdoms, or, on some accounts, three "superkingdoms" (eubacteria, archaebacteria and eukarya). The most important of all these kingdoms is without doubt the animal kingdom. And the largest division within the animal kingdom, as we saw earlier, are the different phyla. When designating these phyla, the fact that each one possesses completely different physical structures should always be borne in mind. Arthropoda (insects, spiders, and other creatures with jointed legs), for instance, are a phylum by themselves, and all the animals in the phylum have the same fundamental physical structure. The phylum called Chordata includes those creatures with the notochord, or, most commonly, a spinal column. All the animals with the spinal column such as fish, birds, reptiles, and mammals that we are familiar with in daily life are in a subphylum of Chordata known as vertebrates.

There are around 35 different phyla of animals, including the Mollusca, which include soft-bodied creatures such as snails and octopuses, or the Nematoda, which include diminutive worms. The most important feature of these categories is, as we touched on earlier, that they possess totally different physical characteristics. The categories below the phyla possess basically similar body plans, but the phyla are very different from one another.

After this general information about biological classification, let us now consider the question of how and when these phyla emerged on Earth.