Four teeth found in the Peruvian Amazon suggest monkeys crossed the Atlantic from Africa at least twice, on a sea voyage of roughly 930 to 1,240 miles, some 35 to 32 million years ago.
No boat, no land bridge. The leading explanation is a raft: a mat of soil, roots and tangled plants torn from an African shore and pushed west by the ocean. The teeth belong to Ucayalipithecus perdita, an extinct primate that weighed about as much as a modern marmoset, roughly 319 to 366 grams, or 11 to 13 ounces. Its species name is Latin for “lost.”
How four molars made the case
The fossils came from Santa Rosa, a riverbank site on the Río Yurúa in eastern Peru that had already produced an early primate along with rodents and marsupials. Two upper molars sat among material from earlier digs. When the paleontologist Erik Seiffert looked at them in 2016, he recognized something familiar from Egypt. A return trip that year turned up two more.
Teeth are a good witness in paleontology because enamel survives and cusp patterns differ between lineages. These molars have the swollen, bulbous cusps of parapithecids, a primate group otherwise known only from Africa and Arabia. They do not match the early relatives of living South American monkeys.
Seiffert’s team then ran a Bayesian clock-based analysis, a statistical method that uses anatomy and fossil ages to estimate when lineages split. It set the Peruvian animal deep inside the African group, beside Qatrania wingi from Egypt. According to the paper’s abstract, the rafting “likely took place between ~35 and ~32 million years ago.”
Why a second crossing matters
The New World monkeys alive today, from capuchins to howlers, already pointed to one such journey. Their ancestors split from the Old World line in Africa about 40 million years ago, and their later arrival in South America has long been attributed to drifting vegetation. Ucayalipithecus is on a separate branch of the tree, so its presence implies a separate event. The paper calls it a third mammal lineage of African origin in the South American record, after those monkeys and the caviomorph rodents, the group that includes guinea pigs and capybaras.
Once is a fluke. Repeat crossings suggest a route that stayed open, at least occasionally, over millions of years. Conditions helped. The Atlantic was narrower, the Isthmus of Panama had not formed, and currents favored travel west. The window also overlaps a major worldwide drop in sea level near the Eocene-Oligocene boundary, which may have shortened the trip.
How long would that take? There is one modern yardstick. In 1995, after hurricanes struck the Caribbean, at least 15 green iguanas rode a raft of uprooted trees about 200 miles to Anguilla, in an estimated three weeks. That is roughly 320 kilometers in 21 days, or about 15 kilometers a day. At that pace, the shortest 1,500-kilometer Atlantic estimate works out to about 100 days. It is a back-of-envelope figure from one Caribbean storm and not a measurement of any ancient voyage, but it shows the scale of the problem for an animal that needed food and fresh water.
Monkeys were not the only passengers on this route. South American tortoises descend from African ancestors that arrived in the Oligocene, helped by their ability to float with their heads up and to go as long as six months without food or fresh water. Skinks and geckos appear to have made the crossing as well. Nobody has measured what a small primate could endure on a trip of that length.
What the fossils cannot show
Four molars cannot reveal a route or a travel time, and the teeth were not dated directly. The team’s analysis gave Ucayalipithecus a mean age near 31.7 million years. A later study in 2021, using zircon crystals from the sediments, put the maximum age of the Santa Rosa deposits at about 29.6 million years, which is younger than some earlier proposals for the site.
The raft itself is also an inference. No fossil raft exists, and “rafting” is the explanation that best fits animals appearing on the far side of an ocean with no land connection. Island stepping stones, now underwater, may have broken up the distance.
The count may not stop at two. A single fossil molar named Ashaninkacebus simpsoni, with apparent links to South Asian primates, has been offered as possible evidence of a third transatlantic arrival. It rests, once again, on one tooth.
Further reading
Illustrative image. Image: Georg Karl Ell, CC BY-SA 4.0 via Wikimedia Commons
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