According to the traditional classification system the order Artiodactyla (eventoed ungulates), to which the pig belongs, has a complicated and diversified taxonomy. It is comprised of about ten families with approximately 88 genera and more than 200 species. In addition, recent investigations of mammalian phylogeny show a certain degree of similarity between Artiodactyla and Cetacea (whales), even though they are not deeply nested within the artiodactyl phylogenetic tree. Cetacea appear to be more closely related to the members of the suborder Ruminantia, than to the two other traditionally defined suborders, Tylopoda and Suiformes (Graur and Higgins, 1994). The limited molecular genetic studies of artiodactyls have shown good agreement with the basic structure of their phylogeny supported by morphological data (Novacek, 1992). Current estimates indicate that the common ancestor for artiodactyla (including Cetacea) may have existed around 65 million years ago (MYA) (Graur and Higgins, 1994). It seems likely that future molecular genetic research will provide additional evidence of the phylogenetic structure of this mammalian group.

The suborderTylopoda (camelids) consists of two genera: camels of the Old World and American llamas. These species have a number of very distinctive differences with other representatives of Artiodactyla. They lack typical hoofs, have a stomach with three compartments which is very different from ruminants, and differences exist in the structure and functioning of the placenta, extra embryonic tissues and sexual organs. Many adaptations of these animals are unique and are absent in other Artiodactyla. It has even been suggested that camels and llamas should be considered as a separate order (Bannikov, 1980). Whether or not this is acceptable, a reasonable level of similarity between camelid species does exist and the molecular data from restriction site patterns in ribosomal DNA seem to support their common origin (Semorile et al., 1994). A common ancestor for the pig and camel lived approximately more than 50 MYA Qermann et al., 1995).

The suborder Ruminantia is certainly the most advanced and numerous group in the order Artiodactyla. It includes six families, of which the largest are Cervidae (deer) and Bovidae (oxen). These two most recent suborders are represented worldwide and were enormously successful and competitive during the last 20-25 million years. The most important features that appeared during the evolution of the suborder Ruminantia were changes in the digestive physiology and morphology of the digestive organs. This made them able to ferment cellulose with increased efficiency. The estimated time of divergence between the Ruminantia and Suiformes was about 55-60 MYA (Graur and Higgins, 1994).