This is one of the great mysteries of palaeontology. There are plenty of monkey species in South and Central America and in Mexico. They are members of five families, collectively known as platyrrhine (‘flat-nosed’) primates, all having wide-spaced nostrils compared with the primates of the ‘Old World’. They are the catarrhines (‘hook-nosed). There are other differences, such as the unique prehensile tails of many ‘New World’ monkeys. The two monkey groups are genetically related, but their last common ancestor is estimated, using the ‘molecular clock’ approach, to have lived at least 31 Ma ago, in the Oligocene. The earliest platyrrhine primates of the Americas date to around the Eocene-Oligocene boundary (34 Ma). Interestingly, they are predated by the earliest rodent remains by only a few million years (41 Ma). Both primates and rodents had been inhabiting other continents long before this, so it is certain that, somehow, members of the two groups must have migrated to become isolated in the Americas. The problem lies with palaeogeography. By the late-Eocene the Americas were completely separated from Eurasia and Africa by the actively spreading Atlantic Ocean, then between 1500 to 2000 km wide. Complete isolation of the Americas dates from around 60 Ma ago, when the northernmost part of the North Atlantic began to open. The South Atlantic had become a wide ocean long before that, beginning in the far south during the early Cretaceous Period (138 Ma), with the mid-Atlantic Ridge steadily propagating northwards thereafter.
Since 60 Ma years ago it would have been impossible for the ancestors of ‘New World’ rodents and primates simply to have walked there. In any case the earliest known primate fossils from China are just 55 Ma old. Island hopping across the far northern, narrowest part of the North Atlantic during the Eocene may have been possible, although many islands there could have been subject to intense volcanic activity, as is Iceland today. The only alternative is a sea trip across the mighty Atlantic. Unless, that is, there is a hitherto undiscovered land bridge. The Walvis-Rio Grande Rise – a hotspot track – that spans the South Atlantic Ocean floor from Namibia to São Paulo in Brazil, has been the subject of some speculation since it is dotted with sea mounts and in places has micro-continental fragments. But it is too deep to have emerged as a result of falls in sea level. To suggest that the > 1500 km migration to the Americas of ancestral platyrrhine primates, or rodents for that matter, involved their being carried on drifting vegetation rafts obviously invites scepticism. For starters, why only two groups of animals? Or, could that imply a one-off event carrying only ancestral rodents and monkeys? It would need to be a special kind of raft: large enough to provide security against storm waves; immune to waterlogging, and carrying substantial food. On the plus side, there are powerful east-to-west currents in the equatorial Atlantic and trade winds going in the same direction, thanks to the Coriolis effect and ultimately Earth’s rotation. Islands as ‘way-points’ or temporary refuges are less convincing, for they would have to be heavily vegetated themselves to provide onward rafts. Apparently, in the absence of anything more plausible, Sherlock Holmes’s principle points to trans-Atlantic rafting.
This issue recently became ‘live’ again, with a fossil discovery in Peru, in an upper Amazon river bank close to at the Andean watershed but around 4000 km from the east coast of South America (Seiffert, E.R. et al. 2020. A parapithecid stem anthropoid of African origin in the Paleogene of South America. Science, v. 368, p. 194-197; DOI: 10.1126/science.aba1135). The site had previously yielded both playrrhine monkey and rodent remains. To these have been added teeth with distinct similarities to those of fossils previously known only from Egypt, Libya and Tanzania: parapithecid anthropoids whose teeth are sufficiently different from those of platyrrhines to warrant a separate suborder, which includes baboons and primates. This is the only trace of parapithecids in South America and it may be assumed that, although they were possibly fellow-travellers with New World monkey ancestors, they were unable to compete and became extinct.
However, there is another possibility. Albeit with a sparse record of fossils resembling primates, North America does have at least one. George Gaylord Simpson (1902-1984), once the doyen of US palaeontologists, found a marmoset-like fossil in the early-Eocene of Wyoming, which he named Teilhardinia after the French Jesuit philosopher and palaeontologist Teihard de Chardin. It is about 56 Ma old and the size of a mouse. So was this diminutive the pioneer New World primate that crossed the northern North Atlantic? If so it would have had an equally perilous journey to reach South America, because the Isthmus of Panama was also open sea until around 4.5 Ma ago. With Teilhardinia, the plot thickens for there are several known species: in the US T. brandti from Wyoming and T. magnoliana from Mississippi; in Asia and Europe T. asiatica and T. belgica respectively. An embarrassment of riches that may well ignite: it has been suggested that North American Teilhardinia may have been the first of all primates and spread across the Eocene forests of North America, Europe and Asia. That hypothesis sort of implies that the entry of monkeys into South America may well have started with the tiny continent hopper who passed on its proclivities to its descendants in Africa
See also: Godinot, M. 2020. Rafting on a wide and wild ocean. Science, v. 368, p. 136-137; DOI: 10.1126/science.abb4107; Ancient teeth from Peru hint now-extinct monkeys crossed Atlantic from Africa. Science Daily, 9 April 2020. Oldest-known ancestor of modern primates may have come from North America, not Asia. Science Daily, 29 November 2018