ByRiley Black
Published July 19, 2023
• 6 min read
For decades, paleontologists have wondered exactly how primates made it to South America. The continent’s spider monkeys, capuchins, and marmosets form their own primate group, separate from those in Africa and Asia. The leading theory is the ancestors of these monkeys somehow rafted across the Atlantic Ocean between 40 and 32 million years ago.
As new fossils have been discovered, however, the story’s become much more complex. South America was home to a broader array of primates than previously known, hinting at a key prehistoric time when rafts of vegetation ripped up by intense storms swept ancient monkeys across the sea. This appears to have happened at least twice—and perhaps more.
The latest evidence of these ancient transatlantic excursions is a tiny fossil tooth uncovered from rocks in the Brazilian Amazon. “Immediately, when one of my Brazilian colleagues showed me this tiny tooth emerging, my heart began to beat very fast,” says Laurent Marivaux, a paleontologist at the University of Montpellier in France. The 34-million-year-old tooth, described by Marivaux and colleagues in the journal PNAS, doesn’t look like it came from a South American monkey, instead resembling the teeth of early monkeys called eosimiids found in South Asia.
The new species is not the first strange animal to show up in South America’s prehistory. In 2020 paleontologist Erik Seiffert and colleagues announced the discovery of a monkey in Peru called Ucayalipithecus that had ancestral ties to ancient Africa, rather than being part of the modern South American lineage. Primates must have made the journey from Africa to South America at least twice, then, and the new tooth might indicate that a third group also journeyed across the ancient ocean.
Prehistoric seafarers
Named Ashaninkacebus simpsoni by Marivaux and colleagues, the new fossil primate is known only from a single upper molar found along Brazil’s Juruá River. The arrangements of cusps on the tooth identify it as both a primate and possibly an eosimiid. Based on fossils of eosimiids found elsewhere, Marivaux and colleagues expect Ashaninkacebus was a small species, about the size of a common marmoset at roughly half a pound, that primarily dined on insects and fruit.
While the molar is certainly that of a primate, other experts are not entirely sure of its relationships. Eosimiids were present in Africa as well as Asia. “As such, this becomes another example of a primitive lineage from Africa turning up in South America,” says University of Toronto paleontologist Mary Silcox, who was not involved in the new study.
If Ashaninkacebus is an eosimiid, then it would represent a third, distinct primate group that rafted between the continents. But there is another possibility, one that connects the new find to the monkeys living in South America today, known as platyrrhines.
“My suspicion is that Ashaninkacebus could be a stem platyrrhine,” says University of Southern California paleontologist Erik Seiffert, who was not involved in the new study. Rather than representing a group of primates that arrived in South America only to became extinct, he says, the molar might document when the earliest ancestors of the continent’s monkeys arrived. “If this turns out to be the case, then there would only be evidence of two dispersals,” Seiffert says.
Regardless of whether Ashaninkacebus is an early platyrrhine or represents a distinct group, there is still the question of how the primates hopped between continents multiple times.
“All our assumptions and scenarios are based on our knowledge of the fossil record,” Marivaux says. Since the 1970s, paleontologists have pondered whether primates might have traveled across the ancient Atlantic on rafts of floating vegetation. No other explanation seemed to fit. There were no land bridges connecting South America and Africa during the relevant time, nor was there any evidence that the primates took a circuitous overland route.
And monkeys were not the only animals to make the trip. Paleontologists have also found that the ancestors of capybaras and other rodents, called hystricognaths, likely rafted from Africa to South America as well.
Surviving a cross-continent journey on a mass of vegetation seems like a one-in-a-million chance. So scientists have tried to determine whether there was one rafting event with rodents and monkeys together on a bed of entangled plants, or several.
The puzzle is almost impossible to resolve via direct fossil evidence. But from piecing together what the world’s continents, currents, and climates were like at the time of primates’ arrival in South America, Marivaux and colleagues propose that there was a brief window when conditions were just right for the mammals to “unwillingly embark” to a different continent.
Swept away
South America’s earliest primates were small, fruit-eating species, hinting that their ancestors lived in humid forests along the western coast of Africa around 40.5 million years ago. The animals near deltas and river systems had a greater chance of being swept up in floods, holding on tight as parts of trees were broken off and washed out to sea.
This speculative scenario is not just conjecture, nor unique to South America’s monkeys. The lemurs and tenrecs of Madagascar arrived on that island from mainland Africa on rafts, and small lizards have island hopped around the Bahamas on natural rafts, as well.
“A whole ecosystem can move along these shreds of riverbank,” Marivaux says. Modern rafts of vegetation can be very large, some of them with upright trees still holding fruit, and many primates and rodents along prehistoric Africa’s coast lived in places where rafts capable of carrying them may have been made during surging storms.
Paleontologists are still working out exactly when these crossings occurred. The new study suggests that the voyages took place about 40.5 million years ago, when South America and Africa were only about 600 miles apart—much closer than the present gulf of over 1,800 miles.
Seiffert, however, favors a later time. Around 33 million years ago sea levels dropped, closing the oceanic distance in a different way. “A significant amount of erosion in near-shore environments might have led to the calving of large rafts,” he notes.
Future fossil finds will help inform the story, though they may be difficult to uncover. “The fossils that have been recovered from this part of Amazonia are largely isolated teeth because of the way they have to be collected,” Seiffert says. Researchers often take shovelfuls of sediment from the inclined riverbanks to screen wash in water, a process that sifts tooth and bone from the dirt and rock. Unfortunately, sometimes small bones are destroyed and only the harder teeth remain.
Nevertheless, the discovery of three early South American primates since 2015 indicates that there are likely more out there, and the future may reveal new details of how primates arrived and flourished in South America. “Ten years ago,” Marivaux says, “this would have been unbelievable.”
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