There is no fossil record of the evolution of the genus Mandrillus. This is not surprising, given that these monkeys originated in the tropical forests of western Central Africa, where very few primate fossils have ever been discovered. A viable alternative to direct fossil evidence may be secured by pursuing multiple, comparative lines of enquiry. For example, the application of molecular genetic techniques, and the use of ‘molecular clocks’, has made it possible to estimate when the genus Mandrillus arose and subsequently give rise to its two extant species. Likewise, zoogeographical researches have increased our understanding of how the mandrill and drill came to occupy their present distribution ranges. Cycles of climate change in the remote past have had important effects upon rainforest distribution in Africa, and these events, as well as the barriers imposed by major rivers, have affected speciation and shaped distribution ranges. Taxonomic studies place the genus Mandrillus firmly within the Tribe Papionini, along with the mangabeys, macaques, baboons and several other genera of the cercopithecine monkeys. It is highly instructive to consider the functional anatomy of the cercopithecines, and to compare and contrast various traits (e.g. skeletal, dental and genital) that occur in Mandrillus with homologous structures in other papionins and in less closely related genera. Likewise, it is now possible to compare the behavioural repertoires of Old World monkeys, and especially basic patterns that serve for visual and vocal display. This ethological approach has produced some valuable insights concerning the evolution of social communication. Now that the social organization, ecology and reproductive biology of the mandrill are better understood, comparative studies in these areas may also open the way to a better understanding of its evolutionary history.

Mandrills and drills were traditionally classified as ‘forest baboons’ and, as such, they were thought to be closely aligned with the savannah baboons of the genus Papio. However, it has become apparent that many of the baboon-like traits that occur in the genus Mandrillus are because of convergent evolution. As large-bodied, sexually dimorphic and primarily terrestrial monkeys, mandrills and drills may look superficially like baboons, but there are many anatomical and genetic differences between the two genera. Studies of mitochondrial DNA as well as skeletal anatomy have shown that Mandrillus is most closely related to the mangabeys, and especially to the semi-terrestrial forest mangabeys of the genus Cercocebus, within the Tribe Papionini.