Hystricomorph rodents appeared in South America in the Eocene, a continent which previously had metatherians, xenarthrans, and meridiungulates as the only resident nonflying mammals. They apparently arrived by rafting across the Atlantic from Africa. The same type of migration may have occurred with primates, which also appeared in South America in the Eocene when it was an isolated continent, long before the Great American Interchange. All of this is still controversial, and new scientific discoveries on this subject are published regularly.