During this time, South America was isolated from all other continents. Several hypotheses have been proposed as to how hystricognath rodents colonized this island continent. Most require that a small group of these rodents traveled across ocean bodies atop flotsam.
The most common hypothesis suggests that the ancestor to all modern caviomorphs migrated across the Atlantic Ocean (then narrower) from Africa (Lavocat, 1969; Huchon and Douzery, 2000). This is supported by molecular results, which suggest that the Phiomorpha (as restricted to Bathyergidae, Petromuridae, and Thryonomyidae) are sister taxa to the Caviomorpha. In fact, until the discovery of the Laotian Rock Rat, all modern hystricognath families were restricted to South America, Africa, or had a range that included Africa (Hystricidae).
The principal alternative hypothesis is that the caviomorph ancestor arose in Asia and migrated to South America through another continent. Most commonly, North America is cited as the most likely continent (Wood, 1985) as connections between North America and Asia were common via Beringia, and North America appears to have been closer to South America than any other continent at this time. The "Franimorpha" were once proposed as a potential North American rodent group that may represent an ancestor to the Caviomorpha, but most modern researchers consider franimorphs to have been protrogomorphous instead of hystricomorphous.
Fossil evidence suggests the Entodacrya may have originated in Asia (Marivaux et al., 2004) and this is cited as potential evidence for an Asian origin for Caviomorpha as well. Likewise, Jenkins et al. (2005) argue that their discovery of a hystricognath rodent family (Laonastidae) exclusive to Asia may be further evidence for an Asian origin of caviomorphs.
Alternatively, the caviomorphs may have originated in Asia, but traveled through Africa, Australia and Antarctica, or Africa and Antarctica (noted but not advocated by Huchon and Douzery, 2000). Alternatively they may have originated in Africa and traveled to South America via Antarctica.
New World monkeys appear to have colonized South America from Africa or another Old World continent at a similar time. Many researchers think that these two colonizations may be part of the same biogeographic event.
Although many species of caviomorphs have migrated into Central America since the Great American Interchange, only a single living species, the North American Porcupine, has naturally colonized North America (the extinct capybara Neochoerus pinckneyi also accomplished this feat). The nutria has been introduced into North America and has proven a highly successful invasive species there.
Note that some changes to this taxonomy have been suggested by molecular studies. The Dinomyidae may belong to the Chinchilloidea, the Abrocomidae may belong to the Octodontoidea, and the Hydrochaeridae may have evolved from within the Caviidae.