The Enigmatic Journey of Primates to South America

For decades, paleontologists have grappled with a essential question: how did primates colonize South America? The new World monkeys, including capuchins, marmosets, and tamarins, form a distinct group, separate from their African and Asian counterparts. the prevailing hypothesis suggests that their ancestors undertook an extraordinary transatlantic voyage, rafting across the Atlantic Ocean on natural debris approximately 32 to 40 million years ago. However, recent fossil discoveries are challenging this long-held belief, suggesting a far more intricate and multifaceted story of primate dispersal.

New Fossil Evidence Reveals a Complex History

The latest paleontological findings indicate that South America once harbored a greater diversity of primates than previously imagined. This suggests that during a crucial period in prehistory, intense storms may have detached large mats of vegetation, inadvertently carrying primates across the ocean. This improbable transatlantic journey may have occurred not just once, but possibly multiple times, reshaping our understanding of primate evolution and biogeography.

A Tiny Tooth with a Big Story: Ashaninkacebus simpsoni

A minuscule fossil tooth, unearthed in the Brazilian Amazon, provides compelling evidence for these prehistoric maritime migrations. Laurent Marivaux, a paleontologist at the University of Montpellier, expressed his excitement upon examining the tooth, stating, when one of my Brazilian colleagues showed me this tiny tooth, my heart was immediatly packed. This 34-million-year-old tooth, described by Marivaux and his team in the journal Proceedings of the National Academy of Sciences (PNAS), bears a striking resemblance to primates from the extinct South Asian family, Eosimiidae, rather than any known South American monkey.

“When one of my brazilian colleagues showed me this tiny tooth, my heart was immediately packed.”
Laurent Marivaux, University of Montpellier

Echoes of Africa: Previous Discoveries Support Multiple Migrations

This isn’t the first instance of an unexpected animal appearing in prehistoric South America. In 2020, Erik Seiffert and his colleagues announced the discovery of Ucayalipithecus in Peru, a primate exhibiting ancestral links to ancient African lineages, not to modern South American monkeys. These findings suggest that primates journeyed from Africa to South America at least twice. The newly discovered tooth could potentially indicate a third independent migration event, further complicating the narrative.

Unveiling Ashaninkacebus simpsoni: A Glimpse into the Past

Named Ashaninkacebus simpsoni by Marivaux and his colleagues,this newly identified fossil primate is known solely from a single upper molar discovered along the Rio Juruá in Brazil. The arrangement of cusps on the tooth strongly suggests its belonging to a primate, possibly within the Eosimiidae family. based on previously discovered Eosimiidae fossils, scientists believe that Ashaninkacebus was a small creature, roughly the size of a common marmoset, weighing approximately 200 grams, with a diet primarily consisting of insects and fruit. Current estimates place the global marmoset population at around 50,000 individuals, highlighting the vulnerability of these small primates.

Debate and Choice Interpretations

While the molar’s primate origin is generally accepted, its precise lineage remains a subject of debate. Mary Silcox, a paleontologist at the University of Toronto, who was not involved in the study, notes that the eosimiidae were present in Africa and Asia, thus, this discovery constitutes a new exmaple of a primitive line from Africa which appeared in South america. If Ashaninkacebus is indeed an Eosimiidae, it would represent the third distinct group of primates to have traversed between continents. however, an alternative hypothesis proposes a connection between this new discovery and the extant South American monkeys, known as Platyrrhini.

“I suspect Ashaninkacebus to be a [groupe-souche] De Playyrrhinien.”
Erik Seiffert, University of Southern California

A Potential Ancestor? Reassessing the Migration Events

Erik Seiffert, a paleontologist at the University of Southern California, suggests that Ashaninkacebus might be a stem platyrrhine. Rather than representing a primate group that arrived in South America and afterward went extinct, the molar could potentially belong to the earliest ancestors of the continent’s monkeys. If this is the case, then there would have been only two dispersion events. This interpretation would significantly simplify the migration narrative,suggesting a more direct evolutionary link between ancient Asian or african primates and modern South American monkeys.

The great Primate Migration: A Paleontological puzzle

The appearance of primates in South America millions of years ago has long baffled scientists. How did these creatures,seemingly isolated by vast oceans,manage to colonize a new continent? The prevailing theory,supported by recent studies,points to an extraordinary feat of natural navigation: transatlantic voyages on rafts of vegetation.

The mystery deepens when considering the unique evolutionary path of South American primates. The Ashaninkacebus, such as, presents a taxonomic challenge. Were they among the first Platyrrhinians, or do they represent a completely separate lineage? Experts are still working to understand how these animals managed to cross continents, potentially multiple times.

Rafting Across the Atlantic: A Risky Voyage

The idea of primates traversing the Atlantic on floating vegetation might seem far-fetched, but it remains the most plausible explanation. As Marivaux explains,All our hypotheses and all our scenarios are based on our knowledge of the fossil register. Since the 1970s, paleontologists have considered the possibility of primates crossing the Atlantic on floating vegetation rafts. no other explanation seems plausible, because no land then connected South America and Africa, and because nothing indicates that the primates have taken another earthly route.

This theory isn’t limited to primates. Fossil evidence suggests that the ancestors of capybaras and other rodents, known as Hystricognathes, also made a similar journey from Africa to South America. This raises a crucial question: did these different groups travel together on a single, massive raft, or did multiple crossings occur over time?

Reconstructing the Past: Currents, Climate, and Continental Drift

Unraveling this enigma through direct fossil evidence is incredibly challenging. Though, by reconstructing the continents, currents, and climates of the era, researchers are gaining valuable insights. Marivaux and his team propose that a specific window of opportunity existed, where conditions were uniquely favorable for mammals to be swept away on a long journey to another continent.

Consider the conditions around 40.5 million years ago. South America and Africa were significantly closer than they are today, separated by approximately 1,000 km compared to the current 2,900 km.This shorter distance, combined with favorable ocean currents, would have increased the likelihood of a successful crossing.

Furthermore, the sea level was also a factor. Seiffert suggests that about 33 million years ago, a drop in sea level could have further reduced the distance between the continents. Meaningful erosion in the environments close to the shore could also have led to the detachment of these large rafts.

The Ideal Traveler: Primate Traits and Habitat

The first South American primates were small, fruit-eating creatures, suggesting their ancestors inhabited humid forests along the west coast of africa. Animals living near deltas and river networks would have been more susceptible to being swept away by floods, clinging to detached trees and carried out to sea.

This scenario isn’t unique to South American monkeys. Lemurs and tenrecs also arrived in Madagascar from continental Africa via natural rafts, as did certain species of small lizards that colonized the Bahamas. These examples highlight the potential for entire ecosystems to be transported across vast distances.

An entire ecosystem can move on these pieces of banks.

Marivaux

Vegetation rafts can be surprisingly large, even carrying fruit-bearing trees. Primates and rodents living along the coasts of prehistoric Africa would have inhabited areas prone to creating such rafts during major storms, facilitating their long-distance voyages.

Challenges and Future Discoveries

Despite the compelling evidence, challenges remain in definitively proving this theory. Fossil discoveries in the Amazon are often limited to isolated teeth due to the collection methods used. Researchers typically wash sediment samples, separating teeth and bones from the surrounding earth and rock. This process, while effective, can sometimes damage or destroy smaller bones, leaving only the more durable teeth behind.

However, the discovery of three primitive South American primates since 2015 offers hope for future breakthroughs. As Marivaux concludes, Ten years ago, we would not have believed it. These findings suggest that further research and fossil discoveries could reveal even more details about the remarkable journey of these ancient continental drifters.