The term “monkey” is used for some of the primates that live in both the Old World and the New World. “Monkey” does not include the prosimians (lemurs, sifakas, etc. of Madagascar) nor the apes. Inspector Clouseau was wrong when he said, in one of the Pink Panther movies, it was a “chimpanzee-type monkey.” Primates are divided, firstly into prosimians (basically lemurs, etc.) and simians (monkeys and apes). The simians themselves were divided into platyrrhine monkeys (New World) and catarrhines (Old World monkeys and the apes). The Old and New World monkeys look almost identical and for many years it was believed that the primates evolved from a prosimian and very quickly divided into the platyrrhines and catarrhines, with the platyrrhines becoming geographically separated and surviving only in the New World. However more research, some fossils and a few dates indicated that they were very distantly related and that the ancestor was probably a prosimian rather than the first monkey. There was also a lack of fossils from the Old World that shared traits common to both the platyrrhines and catarrhines and could be their last common ancestor. This raised the probability that the monkeys had different prosimian ancestors: one in the New World for the platyrrhines and another prosimian in the Old World for the catarrhines (and the apes). In some ways the platyrrhine monkeys may be considered to be “advanced” prosimians or not quite as evolved as catarrhine monkeys. Currently they both are often referred to be “monkey grade:” approximately equal levels of development just maybe not as closely related as they would be if they shared a common ancestor.
In general the public does not know that the prosimians first appeared in North America. The fossil Purgatorius was found in eastern Montana around Purgatory Hill east of the Fort Peck Reservoir. It is dated to 66 million years ago, one to one and half million years before the end of the dinosaurs. It is either the earliest known prosimian or a proto-prosimian that gave rise to prosimians. Many more fossils that are definitely prosimian a have been found in the American southwest and in Wyoming (I, myself, have participated in searching for, and finding, prosimian fossils in the Bridger Formation south of Fort Bridger, Wyoming dating to around 50 million years ago).
Prosimians no longer exist in North America or Europe. The first fossil prosimian fossil (Notharctus) was found in France. There are some living prosimians (Lorises, Pottos, Galagos also known as Bush Babies, and Tarsiers) in Africa and Asia, but the greatest numbers and diversity exist in Madagascar.
It seems that the prosimians first evolved in North America, diversified and spread to Europe, Asia, Africa and Madagascar. The world 60 million years ago did not look like the world of today. it was much warmer for starters. For instance,Wyoming was covered by lush forests with large rivers and active volcanoes. The North Atlantic was narrower particularly across Greenland to Europe. Notharctus has been found in both France and Wyoming. The South Atlantic, also was much narrower than it is now. The Isthmus of Panama was open ocean, the Pacific and Atlantic were connected. North and South America were separate land masses and were until about 3 million years ago.
Since there is no evidence, currently (see addendum below), of fossil prosimians in South America, and platyrrhine monkeys and catarrhine monkeys are: 1) either related to a common prosimian ancestor or two different prosimian ancestors, both living in Africa where only the catarrhines survived with only the platyrrhines spreading to South America, or 2) the platyrrhines evolved from an America prosimian before they became extinct and the catarrhines from a European or African prosimian. In either case the problem is how did the platyrrhine monkeys get to South America. If they evolved on 2 separate continents (North America and Africa) then the platyrrhines (or their prosimian ancestor) island-hopped across the future Caribbean to South America. Since there are monkey on some of the islands, it was most likely not a prosimian ancestor. Or if it was, it went extinct and the recently evolved platyrrhine monkeys spread back across the islands.
The most intriguing possibility, which may be the currently preferred option, is that they both evolved in Africa and the a platyrrhine ancestor rafted across the South Atlantic, survived the crossing and flourished in the New World while their African relatives went extinct. Probably due to competition with the catarrhines.
How this could have occurred has intrigued me for years. I had to understand how it might be possible for me to accept that such a possibility might possibly be possible. I have come up with what I believe is a possible scenario, possible enough to have happened. You may have noticed the multiple use of “possible” and “probable.” I want you to understand that this only a scenario of what is possible to have a chance of happening. The entire question of how organisms have come to inhabit oceanic islands is a fascinating topic in itself. The Galapagos Islands are 1000 kilometers from Ecuador, have never been connected to any continental land mass and one of the creatures that live on the islands is a marine iguana. Every other known iguana is terrestrial, they live on land, feed on terrestrial vegetation, insects and animals, and although capable of swimming across rivers and from island to island, do not dive underwater and feed on seaweeds.
Anyhow, respectively submitted for your perusal (as Rod Serling said at the beginning of The Twilight Zone) is a story of one way monkeys might have made the oceanic crossing (it might even be true): The rain had fallen for days. Storm after storm sweeping in off the ocean. The river was a turbulent, muddy brown torrent with leaves, limbs, and trees floating on its foaming surface. Today though, it hadn’t yet rained but as evening approached the great dark clouds unleashed the storm they had been promising all day. Amidst the bolts of lightning and crashing thunder, the rain fell in drenching sheets.
The small troop of primates cowered under the canopy of leaves, close to the trunk of the tree they had been feeding in when the storm broke. The tree was near the center of their home range, where they felt the most secure. They had been here or nearby during the storms, feeling safer here than elsewhere.
However it was a false security for the tree grew on the bank of the river, which now surged around its roots, digging away at the earth the supported it. The troop first felt the impending disaster as an increasing vibration of the tree, then a lurch as the ground began to rapidly give way. The lurch only made them cling tighter to big limbs near the trunk where they had taken shelter, and when the tree crashed into the river only two of them were swept away.
The swollen river carried the tree and the band the few miles downstream to the coast. Even though the storm surf was crashing onshore, the river had enough strength to push the tree beyond the mouth and offshore, where the oceanic current began to move it northward along the coast.
The tree and its living cargo was still in the current’s embrace a day later when the current began its westward turn to become an equatorial current and cross the ocean. The ocean was not as wide then as it is now. The current sweeping up the west African coast could carry a floating object to the northeastern coast of South America in 10-14 days time. Dehydration was the biggest threat to the primate troop afloat on the ocean. The tree’s leaves and fruit would provide some water to meet their needs, as well as food to stave off their hunger. The leaves also provided shelter from the tropical sun. Clinging to the branches of the tree as it floated, the primates had little to do. There was nowhere to go, no predators to flee, and what food there was was in easy reach. Their activity levels were greatly reduces, even the juveniles and infants were subdued by the strangeness of their situation. This reduced activity reduced their water and food requirements further.
Still, the survivors were desperate when the tree finally grounded in the shallow water off a strange coast several weeks later. The oldest members of the band had died first, lacking the resources and strength to survive long. Neither did the younger juvenile members, they lacked the body mass to store fat and water to survive lean times. Nursing infants survived better until their mother, who had a greater need for water and food to produce milk for them, either died or failed to produce milk. Then they died. It was the young adults and the mature adults that had the stamina and bodily resources that had the best chance of surviving the trip. Still, they were desperate when the tree grounded. Desperate enough to leap into the gentle surf and get ashore. Get ashore to an unknown territory with unknown dangers. It wasn’t that they knew it was a new world, just that it wasn’t their own territory and who knew what other band of primates they might have to fight with or where there were safe places to take refuge from any predators.
There were predators here, ones they had never encountered before, but their predator avoidance/escape behavior worked here as well as in Africa.
However they never did discover other primate troops, neither of their own kind nor of any other kind. They were the only primates on the entire continent, in the entire hemisphere.
The surviving members of the troop floundered ashore, found water, safe haven, and food. As they recovered from their ordeal they would begin to explore their new home, finding sources for water, new foods, and new dangers. After a few years of learning the new territory their numbers would have begun to increase. The troop would have grown slowly in numbers, then grow too large to be a single troop and divide into two troops. Then their numbers would have started to increase faster, spreading into far territory, exploring newer habitats and life ways.
Their only serious competition would have been themselves. They were the founding members of all New World monkeys. Their descendants spread through tropical America, radiating into numerous different niches and species from the lemur-like marmosets and tamarins, to the monkey-like squirrel monkey and capuchins, to the world’s only nocturnal higher primate—the owl monkey, and to the more ape-like spider monkeys
They retained the third premolar of the prosimians and the “platyrrhine” nose (the nostrils face to the side of the nose, among other traits. Some of their descendants evolved the prehensile tail. Soon after the forced separation, before they themselves radiated and became monkeys and apes, their relatives in Africa lost the third premolar and evolved nostrils that opened forward (the “catarrhine” nose).
Several months ago a came across an article on the internet announcing the discovery, around 2015, of a fossil primate along the Ucayali River in the Peruvian Amazon. The finds consists of teeth that are almost identical to primate teeth from Egypt. The Peruvian fossils are thought to date to around 34 million years ago. The Egyptian teeth date to around 32 million years ago. At hat time the Atlantic Ocean was much narrower than it is now, both because the continents had not drifted so far apart and also because the Antarctic Ice Cap was much larger and the ocean levels much lower than now. Google Ucayalipithecus for several articles about the discovery and further discussion about the subject.