Discussion on existing theories of the origins of Platyrrhines in South America. The referenced images can be googled.
Origins of Platyrrhines in South America
The question of how primates came to populate South America has long plagued scientists. There are many working theories, some more promising than others, but they are all just theories. There is little in the way of direct fossil evidence to support any of the hypotheses. However, there is a good deal of other kinds of evidence to be considered, evidence such as plate tectonics, ancient ocean currents, and rafting hypothesis.
It is widely agreed that the spread of primates happened approximately 30 million years ago in the late Oligocene or early Eocene. This is based on what little primate fossil evidence there is and the fossil record of rodent radiations called caviomorphs. This paper will be a literature review of relevant fossil primates and rodents and the major dispersal patters primates and rodents may have used to reach South America. This review will start with the oldest known (to date) primate fossil in South America while comparing the other relevant proto-anthropoids from Africa. Then this paper will take an in-depth look at the different models of primate dispersal to South America.
Branisella boliviana is the oldest fossil platyrrhine found in South America. Only fragmentary bits of teeth had been unearthed until 1996 when a pair of upper and lower jaw fragments and another nearly complete mandible were recovered near Salla in Bolivia (Wiley – Liss, 2000). These Branisella fossil remains date at approximately 25 million years old placing them in the late Oligocene era. This new information on jaw morphology sheds a great deal of light on possible origins of the Platyrrhine monkeys and their evolutionary divergence from Catarrhine monkeys. The jaw fragments represent in many aspects the primitive state of most major platyrrhine radiations. For example: the mandibular arcade is nearly V-shaped and the symphysical angle, which is formed by the horizontal plane and the anterior face of the mandibular synthesis, is about forty degrees, i.e., it neither leans as far anteriorly as in Callitrichines nor does it stand as vertically as Cebus (Wiley – Liss, 2000). This is indicative of a primitive condition ancestral to both species. The upper and lower molars show rapid wearing patterns consistent with terrestrial herbivores (Wiley – Liss, 2000). Overall, the dentition of Bransilla boliviana is very similar to that of Proteopithecus of the late Eocene, 34 million years ago, found in Egypt (Wiley – Liss, 2000). This suggests a close phyletic relationship between the two species. This is evidence of an early diversification between Platyrrhines and Catarrhines occurring in Africa nearly 10 million years before the populating of South America.
To put this hypothesis to the test, Wiley and Liss conducted a series of skull morphology comparisons. They compare the five oldest known anthropoids for which cranial evidence is known with three extant Catarrhine genera and sixteen extant Platyrrhines as well as the three of the oldest known South American extinct primates. Fourteen out-group taxa including extant lemurs, lorisis, tarsiers, and fossil omomyids and adapids (Wiley – Liss, 2003).
Using a parsimony analysis program Wiley and Liss produced a Platyrrhine monophyly. Those results show that late Eocene primates, like Aegyptopithecus, are the sister group to extant Catarrhines, thus the Catarrhine – Platyrrhine split had occurred sometime before 34 million years ago. However Wiley and Liss found that Proteopithecus is related to neither the Catarrhines nor Platyrrhines and represents a stem anthropoid. Thus they found no cranial evidence for a late Eocene split among the two groups with the known fossil record (Wiley-Liss, 2003). This could suggest a parallel evolutionary explanation for the similarities in jaw morphology between Proteopithecus and Branisella boliviana but it is more parsimonious to assume a phyletic relationship. This will be important when discussing origin models. The North American Origin Model and its variants all require convergent evolutionary schemes, whereas the Africa Origin Model favors the more parsimonious ideas of a common ancestor.
Caviomorph Radiation to South America
The unexpected discovery of South America’s earliest rodent in the central Chilean Andes, the Tinguiririca, provides information critical to resolving the source of South American Caviomorphs, suggesting an African origin (Wyss, 1993). If as is often assumed, rodents and primates arrived in South America at roughly the same time, these same Chilean Andes deposits may afford an opportunity for the discovery of older fossil Platyrrhines than Bransilla boliviana (Wyss, Flynn, Norell, Swisher, Charrier, Novacek, McKenna, 1993).
South America’s earliest rodent radiation, termed caviomorphs involves forms with specialized mandibular structures termed hystricognathous, and cranial structures termed hystricomorphous (Wyss, 1993). It is these components that support an African origin for South American rodents. The two competing hypotheses are comparable to the North American Origin hypotheses and the African Origin Hypothosis of primates. The North American origination hypothesis for caviomorphs says that Franimophs (Oligocene rodents with primitive morphology) are the ancestors of the caviomorphs but they are characterized by an unenlarged infraorbital foramen for passage of a derived pattern of jaw musculature (Wyss, 1993) which is the primitive condition for Rodentia. This would signify an unparsimonious convergent evolution of jaw musculature (hystricognathous) and cranial morphology (hystricomorphous) for Franimorpha. Phiomorha (Oligocene African rodents) on the other hand share the same characteristics of the skull as the caviomorphs with some more compelling parallel evolutionary traits
Figure 1. Competing theories for caviomorph origins.
making a much stronger and parsimonious argument for the African origin of South American caviomorphs.
Figure 1. illustrates the two conflicting theories showing line A as the North American hypothesis, and line B as the African origin hypothesis. On line A, the hystricognathous (Hg) and the hystricomorhous (Hm) are shown with an asterisk to indicate the reliance on convergence for acceptance of that theory. The morphology of the Chilean rodent (oldest know rodent fossil in South America) supports Theory B. The diagram shows line B as the most parsimonious explanation requiring no convergent evolution (Wyss, 1993).
Another supporting component is that erethizonits or new world porcupines which are traditionally considered the sister group of caviomorphs posses a five crested upper molar pattern like the Tinguiririca. This supports the theory that the primitive condition is a five-crested upper molar. The North American Rodent species have a four-crested upper molar as the primitive condition. Again this would require the convergent evolution of another cranial characteristic.
A compelling parallel evolutionary descent line shows that if as suspected the earliest Erethizonits had terrestrial African ancestors and with the move to South America they became almost entirely arboreal. The Hystricidae, or Old World Porcupines are terrestrial burrow makers. One member however, the Long-Tailed Porcupine of Malaysia, the Trichys fasciculata, is arboreal. Interestingly, the Trichys is considered the most primitive genus (Alderton, 2000). Like the molar wear patters on Branesilla boliviana suggest a terrestrial primate ancestor from Africa (see above) but now virtually all primates in South America are arboreal. With the move into the trees for the primates came a unique primate feature in the form of a prehensile tail. Several primate species of South America have this prehensile tail and use it mostly for feeding and mating postures. The Ateles geoffroyi or Spider Monkey actually uses its prehensile tail as an integral part of its locomotion and food gathering (Falk, 2000). When food gathering the Spider Monkey uses its tail to actually collect food as well as to hang from it to gather food with its hands.
The parallel evolution comes from the fact that not only did the terrestrial ancestor of the New World porcupines become arboreal, known as tree porcupines (they have feet adapted for climbing trees) but one species, The Coendou, even developed a prehensile tail to assist in climbing (Alderton, 2000).
The Four Origin Models
What is lacking from all the morphological data is a mechanism for either primates or rodents to move from Africa or North America to South America. In the late Eocene or early Oligocene, which is when both appear in South America, the continent was a large island (see figure 1 for position of South America relative to Antarctica and North America). There are four major models of primate dispersal to South America: the North American origin model, the Asian origin model, the African origin model, and the Vicariance origin model (Ciochon - Fleagle, 1987).
The North American model was long considered the best explanation for primate origins in South America. The dispersal scenario usually associated with this model involves island hopping by waif dispersal across the Caribbean Sea (Ciochon – Fleagle, 1987). Waif dispersal simply means small groups of living organisms found outside or beyond their natural habitats. This term is most often associated with plant pollens and birds. This model also says that an Eocene Tarsiiform group, Omomyidae, is the ancestor to both Platyrrhine and Catarrhine extant primates (Ciochon – Fleagle, 1987). In the early to middle Eocene the North American continent was connected to Europe and Asia by a landmass called the North Atlantic Bridge. The Omomyidae sustained their populations with gene flow across that bridge. The bridge became submerged in the middle Eocene and that interrupted the gene flow. This would mark the separation of Omomyidea that would then evolve into the ancestors of Platyrrhines and Catarrhines (Ciochon – Fleagle, 1987). Like the North American Caviomorph theory, the morphological similarities for extant Anthropoids would involve parallel evolution which while not impossible is certainly not parsimonious. Also there is no fossil evidence of a Platyrrhine ancestor in the North American continent. That suggests, for the North American Origin Model to be true, that the Tarsiiforms immigrated to South America then evolved into the extant species of today. Though there are none of the Tarsiiform primates in the fossil record of South America.
Another problem is demonstrated by figure 2. The late Eocene ocean currents were running east to west through the southern portion North Atlantic and south to north from the South Atlantic along the coast of North America. This is a strike against waif distribution theories that require rafting as a mechanism for intercontinental dispersion.
Which brings me to the issue of rafting. This seems to be the only waif distribution mechanism that explains both the Platyrrhine and Caviomorph presence in South America. Rafting is characterized as the dispersal of land animals across water on floating objects (Ciochon – Fleagle, 1987). This phenomenon is not unprecedented as Ciochon and Fleagle cite two examples of recorded rafts. In fact they state that “Without question, rafting as a mode of dispersal has occurred in the past.” (1987, p. 118).
The first example comes from Ridley written in 1930 about a crocodile that rode a raft of bamboo, driftwood, and living clumps of lalang grass 1100 kilometers from the coast of Java to the Cocos Islands (Ciochon – Fleagle, 1987).
There is an even more impressive report from Powers in 1911 of a floating island seen in July of 1892 off the coast of Cape Cod in North America hundreds of miles out in the Atlantic Ocean. The Island was about 9000 square feet in size with trees that were thirty feet tall and acted like sails. It is thought to have broken off the coast of the United States due to great flooding rivers undercutting their banks during storm conditions then carried it out to sea and into the Gulf Stream. This same island was then spotted in September of the same year about 600 kilometers west of Newfoundland after it had passed through a severe storm. By then it had traveled over 1850 kilometers and ended up on the coast of Europe (Ciochon – Fleagle, 1987). Islands such as this one could very well maintain breeding populations of Caviomorph ancestors and with any luck maybe groups of primates as well. If we consider the process of rafting as a dispersal method over a several million year time period in the Eocene, it becomes more plausible than one would originally think. Rafting seems to be a mechanism integral to all of the theories that are currently postulated but as figure 2 demonstrates, the ocean currents favor the African Origin Model.
The Asian Origin Model is a variant of the North American Origin Model that depicts the ancestral stock of Platyrrhines developing in Asia and dispersing to North America via the Bering Bridge in the late Eocene (Ciochon – Fleagle, 1987). This model supports the concept that Platyrrhines and Catarrhines evolved in the same geographic location and not independently like the North America Model would suggest. This is in keeping with the morphological data that supports a later non-tarsiiform or even pre-anthropoid common ancestor. Another version of this same model suggests an Omomyid species inhabiting a single North American Asian continent created by low sea levels and continental positions of the Eocene epoch.
The first problem arises, as with all the scenarios, when we consider the lack of fossil primates from the Eocene and Oligocene. Proteopithicus and Aegyptopicecus represent the pre-catarrhines and stem-anthropoids but there are still no Platyrrhines or even pre-platyrrhines to support this hypothosis. So it is therefore based on the assumption that several hypothetical species existed to account for a nearly twenty million year gap. Such is the nature of the primate fossil record. Many primate species live in rain forest or tropical regions that are not conducive to fossil preservation. Deterioration of faunal remains tends to happen faster than the bones can find themselves in a ideal mineralization situation. It is clear that more survey and research in the areas in contention for primate evolution and dispersal must be done and more fossils found.
Figure 2: Ocean currents of the late Eocene and the path of ancestral Platyrrhines across the Atlantic 30 million years ago (Ciochon – Fleagle, 1987).
The next problem with the Asian Origin Model stems from some of the same problems associated with the North American Models means of dispersal to the then separated landmass of South America. The same mechanism of rafting coupled with what is termed a sweepstakes Caribbean Island hopping scenario is employed as a means for primates to reach South America. However the ocean currents (depicted in Figure 2) once again do not make this an easy pathway to explain even with island hopping.
It is the African Origin Model that seems to have the most support in modern anthropology circles (Ciochon – Fleagle, 1987). The most compelling factor in this model is the morphological evidence presented by the limited fossil record. What is clear is that extant Platyrrhines and Catarrhines come from a common anthropoid or proto-anthropoid ancestor. The previous two origin models suggest convergent evolution to account for similarities in cranial morphology. The Africa Origin model has the virtue of being parsimonious. The original primate stock is the Eurasian omomyid tarsiiforms of the middle Eocene. Soon after this stem anthropoid populated Africa it moved by a series of island hopping and rafting events to South America (Ciochon – Fleagle, 1987). One should note in Figure 2 the chain islands originally cited by Don and Maureen Tarling in their book Continental Drift published in 1971.
These islands are a result of continental spread due to volcanic activity in and around the Mid-Atlantic ridge during the Eocene. For details see Figure 3. Note the center of the ridge and the volcanic islands that break the surface (Tarling, 1971).
The presence of these volcanic islands shortens the open sea distances substantially making a better argument for rafting and sweepstakes island hopping. Also the ocean currents (see Figure 2.) of the Eocene are favorable to rafting as they move from nearly the entire west coast of Africa along the entire east coast of South America. Paleogeographic data suggests that 30 million years ago the west coast of Africa was of the seaward draining marsh and shallow lake conditions that lend themselves to repeated island raft production like the one described by Ciochon and Fleagle (1987).
Figure 3. Mid-Atlantic Ridge cross-section (Tarling, 1971).
Another favorable condition comes from the fact that Western Africa in the Eocene is closer to Eastern South America than it is to North America (Tarling, 1971). Before there was acceptance of plate tectonics the distance between the two continents was the biggest criticism for the Africa Origin model. Now that tectonic shifting is accepted as fact, we know that 30 million years ago not only was Africa closer to South America but North America was not even connected to it. The distance between the landmasses is currently 3200 kilometers. Due to the inconsistency of the sea spreading rates, estimates from the late Eocene range from Funnel and Smith’s 1968 estimate of 1000 kilometers to Ladd’s estimate in 1973 of 1750 kilometers. These facts have changed the traditional support of the North American Origin model to a favorable view of the African Origin Model.
Distances between the islands along the Ceara’ and Sierra Leone Rise in the southern portion of the North Atlantic may have been as little as 200 kilometers (Ciochon – Fleagle, 1987). The problem arises that these islands are hypothetical. They are assumed possible due to the consistent volcanically active sea floor spreading. Crust thrusting raises the floor of the sea making it possible for volcanoes to breach the surface and form islands much like the Hawaiian islands as they exists today (Tarling 1980). Unfortunately, favorable conditions do not translate into proof.
The African Origin Model seems to have the most imperial evidence to support it.
With strong evidence from the fossil record that reflects parsimony in the phylogenetic relationships between Platyrrhines and Catarrhines and the geographic evidence in support of rafting, island chains, and ocean currents makes the African Origin Model the best explanation for primate dispersal to South America.
Another model, dubbed the Maximum- Parsimony Model Of Platyrrhine Origins, is the last one presented by Ciochon and Fleagle. It is a combination of what we know of the fossil record and the best or easiest explanation that includes that information. Due to their overwhelming similarities, this deductive model proposed by Popper in 1959 then again by Kitts in 1977 becomes a strong proponent of the African Origin Model.
The final and most recent is the Vicariance Origin Model proposed by Hershkovitz in 1977 (Ciochon – Fleagle, 1987). This is a hypothetical model that is not refuted by the known facts. This is important because the African Origin Model is the most parsimonious scenario with no facts to dispute it. It also happens to be the model that takes the most unconfirmed conditions to be viable. The Vicariance Origin model is similar in this respect.
The Vicarience Model states that a common Haplorrhine or Tarsiod ancestor inhabited a continent comprised of the two landmasses now known as South America and Africa. Vicarience means the splitting of the single continent in this context. After a series of tectonic shearing and shifting events, the continent was separated and the descent lines of Platyrrhines and Catarrhines began to evolve separately. Study of plate tectonics tells us that this split occurred nearly 90 million years ago, long before the Cenozoic Era. This is twenty million years older than the oldest know primate fossil. Purgatorius is thought to be no older than 70 million years old and near the beginning of the first and oldest primate radiation. What this means is where the theory is not supported by the fossil evidence nor is it disputed by it. This direct ancestor to both radiations of primates only exists in theory. When the theories of plate tectonics came to be accepted by the scientific community, the Vicarience Model would seem intuitive. By virtue of its simplicity, it is more parsimonious based on the current evidence. However, in the long run, it is far less substantiated by the evidence than the Africa Origin Model.
No proposal has yet been made of an Antarctica or Australian origin for Platyrrhines in South America. Even though Tarling clearly shows a land bridge existing in the late Cretaceous and early Tertiary periods between Antarctica and South America. Also figure 14.1 from Fleagle shows a map depicting the highest concentration of fossil Platyrrhines in the southern reaches of South America. One might expect this if primates populated the region from the south. Furthermore, the distance between the southern tip of Africa and the coast of Antarctica during that epoch was considerably shorter than the distance between the West Africa coast and East South American coast.
However the absolute lack of any fossil record for primates in Antarctica and the fact that the land bridge between South America and Antarctica existed 100 million years ago which was long before the oldest of the known fossil primates. Also, the apparent grouping of fossil primate locations in the southern region of South America can be explained with an analysis of the climate of the region. The southern latitudes are more amenable to the mineralization process necessary for fossil making than the tropical regions near the equator where most extant species are found. Wet and humid conditions do not allow for much in the way of bone material to remain long enough to be mineralized. This combination of facts has stopped anyone from trying to forge a coherent theory based on an Antarctic route for primate dispersal.
The conclusions one can draw from all this information is that there is not enough information to draw any kind of concrete conclusion concerning the origin of Platyrrhine primates in South America. Part of our current thinking is based on a thin fossil record. Branisella boliviana is the oldest known fossil Platyrrhine and dates back 30 million years. Aegyptopithecus is its closest contemporary at 34 million years old and is believed to come after the Platyrrhine – Catarrhine ancestral split. We have no way of knowing precisely when the split occurred only that it did. This leaves a great deal of speculation about the origins of these primate groups. Speculation that plays heavily on the ideas of where and when the Platyrrhines developed before making their way to South America.
Given the lack of hard evidence and the speculative nature of a good deal of the supporting evidence (rafting, currents, and volcanic islands ect.), no one model can be wholly relied upon as the definitive answer. There are still many proponents of the various other models discussed in this paper.
This brings up the matter of the origin models as a point of controversy. As previously stated the African Origin Model sees at this point to have the most support in the field. This model has recently replaced the North American Origin Model with the acceptance and a greater understanding of tectonic shifting and positioning of the continents during the late Oligocene and early Eocene when Platyrrhines are thought to have arrived in South America. Even the Maximum- Parsimony Model of Platyrrhine origins seems to support the evidence and assumptions made in the African Origin Model.