There is a small genetic difference between vertebrates with jaws (Agnathos) or without jaws (Gnathosmata). This difference only occurs in the expression, not in existence, of 3 HOX genes related to the first pharyngeal arch. This fact means, the vertebrates without jaws are the ancestors of vertebrates with jaws, and the evolutive step does not consist in new genetic addition, but in a new genetic material expression, which already existed in ancestors without jaws.

Fossils belonging to Agnatos group are Anaspida, Cephalaspidomorpha, Galeaspida and Thelodonti as well as the current hiperartios (lampreys) and hiperotretos (mixinos).

The jaw structures belonging to Mettaspriggninas's fossils (Canada) and Romundias’ (Australia) are considered as the first indications of these genes expression. The first ones lived in the Cambrian period (570 to 500 m.a) and the second one in the Devonian Superior (385 to 360 m.a.). While the Mettaspriggninas are chordates with very basic cranial structures, the Romundia is the first fish found that presents jaws’ formation.

Nowadays, vertebrates with jaws constitute a numerous group of almost 50,000 species.

The origin of jaws is an example of genetics homology, and it’s based on considering two genes as homologous even if they are in different species by differentiated expression, providing that they come from an ancient gene (The primitive existing one without genetic expression and the second existing one with genetic expression).

This way, Hox genes that lead the formation of the branchial fish cartilage, the jaws of the reptiles and the mammals’ middle ear are considered as homologous genes between them and with the Hox unexpressed genes of the current agnates (lampreys and mixinos).

The first branchial arch in fishes was located under the mouth, and when the branchial clefts were subjected to the cranium by cartilages, they formed the jaw.

Afterwards, the second branchial arch was ossified to form the hyomandibular bone of the fish.

In the second stage, the whole complex was ossified, and this fact gave place to the reptile’s jaws which function is to transmit vibrations. Finally, their mandibular function was adapted to form the stirrup (bone located in the middle ear articulated with the anvil) in mammals.

To sum up, Hox genes do not express in the first pharyngeal arc in lampreys, but they already do it in fish, amphibian, reptile, mammal and birds embryos. This expression causes new species formation by new anatomic structures appearance.

The retinoic acid influence, existing in brown and red algae which first fossils are dated about 542 m.a, might be the key to explain the reason why a few pre-existing genes initiated their expression coinciding with the above mentioned algae, and their incorporation in agnates animals feed as a mechanism to develop in Gnathosmata animals, without new genetic structures, and keeping both groups the homologous HOX genes with a different genetic expression since the first embryonary stages in fishes, amphibians and  to their evolutive descendents, reptiles and mammals.