Ever since ancient times, men have grouped similar species to create various categories and gave these categories a distinct name. Aristotle recognized the four major groups of vertebrate animals in 4th century BC – fish, reptiles, birds and mammals, although he did not record the meaning of metamorphosis in the category and had combined the amphibians with the reptiles. At the end of the 17th century, John Ray, a missionary and a naturalist, considered that a biological basis should be present for each organism or species with a common origin.

Carl von Linné (formerly Linnaeus), the botanist from Sweden , wrote Ray's views into a book and named every known species at that time in Latin. Since the common name of each organism is different around the world, he has developed a system that divided the name of each species into two parts; the first is name of the genus with the first letter capitalized, the second is the name of the species and the first letter need not to be capitalized. The two words are written in Latin. As a result, every animal, including amphibians and reptiles, have an official Latin name that started from Linné. The book written by Linné in 1758 confirmed the time of some well known species: Bufo marinus, Ichthyophis glutinosus, Triturus vulgaris, of the amphibians, and Chelonia mydas, Common Caiman, Mediterranean house gecko, and Python molurus of the reptiles.

Linné regards that organisms are creations of God, a constant and unchangeable entity; he designed a higher level and entirely artificial classification method to organize known organisms. The difference of the amphibians from the reptiles was finally recognized in 1820; a traditional classification of vertebrates was created:

Phylum Vertebrata (embryonic chordates with elastic support along the back)

Subphylum Vertebrata (including all vertebral animals)

Class Agnatha (jawless fish such as lamprey and hagfish)

Class Chondrichthyes (cartilaginous fish such as sharks and rays)

Superclass Osteichthyes (all bony fish including classes Actinopterygii and Sarcopterygii, such as orders Coelacanthiformes and Dipnoi)

Class Amphibia (amphibians)

Class Reptilia (reptiles)

Class Aves (birds)

Class Mammalia (mammals)

We may notice that the hierarchy in these classifications cannot express the evolution relationship. Indeed, the historical relations between some of the species included in these classifications may be known by the naturalists at that time. However, with the arrival of Charles Darwin and the evolution concept of natural selection pointed out all the relations among all current species and that common ancestors existed (or have become extinct). But how will these relationships be shown in the classification?

In the 1960s, German biologist, Willi Hennig, completely changed how the evolutionary relationships among the organisms are analyzed. The method he used is called “phylogenetic systematics” (or “cladistics”), that depends on the recognition on the origin of monophyletic evolution. It is worth noting that a monophyly (or clade) is composed of the most primitive organism and all of its descendants. Only these types of monophyletic groups can be recognized in phylogenetic systematics, and be given a formal name. Through this method, the so called derived characters can be identified in a clade. The systematic classification was established by this difference from the primitive morphology and determining the sequence of a clade in evolution. For example, all amphibians have tongues, except for family Pipidae (including Surinam toad) and the four genera of African clawed frogs; the most rational explanation was that the tongue disappears from the ancestors of Pipidae, and thus all of its descendants are without tongues. Hence the disappearance of the tongue is a sign of ramification. The missing tongue is a key feature for this group of frogs. This allowed the biologists to believe that the five genera of frogs represent one monophyly group that require a formal name, family Pipidae.

Unlike the traditional hierarchical classification, the classification generated by this system can actually interpret evolutionary relationships. The relationships between each class of vertebrate animals were surprisingly different using these modern classification methods.