Researchers have long puzzled over when monkeys arrived on the North American continent from South America. Now, an analysis of seven fossilized teeth uncovered along the shores of the Panama Canal reveal that monkeys were present in Central America – the southernmost part of the North American landmass – much earlier than we thought. Called Panamacebus transitus, this new species is the first North American fossil monkey, and it’s described in Nature this week. 

Monkeys likely dispersed from Africa across the Atlantic to South America by “rafting” on floating islands some 34 to 37 million years ago. Such an idea has come in for a lot of criticism before, but it seems to be the only way to explain the spread of monkeys. Theories suggest that vast blankets of vegetation, complete with trees acting as sails (naturally, not assembled by the monkeys) could have allowed them to traverse the Atlantic in as little as 14.7 days.

Nowadays, New World monkeys (called the platyrrhines) are found throughout tropical forests in both North and South America. But the two continents were separated by an ocean until only 3 to 4 million years ago when the Isthmus of Panama joined them together. Previous molecular studies suggested that New World monkeys didn’t arrive in Central America until around that same time. However, because of the complete absence of primate fossils from Central America, their early evolutionary history in the tropics was still a mystery.

In 2009, an expansion of the Panama Canal exposed fossil-bearing rocks, and further fieldwork led by Jonathan Bloch from the University of Florida, Gainesville uncovered seven fossilized teeth from a 20.9-million-year-old ash layer in the Las Cascadas Formation of the Panama Canal Basin. 

Not only are these the first fossil monkeys discovered on the North American landmass, they’re also the earliest evidence we have for the movement of mammals between North and South America. Until now, the oldest evidence for mammal interchange between the two continents were sloth fossils dating back 8.5 to 9 million years. 

These new findings suggest that New World monkeys diversified into the five families we see in the tropics today around 22 to 25 million years ago: Callitrichidae, Cebidae, Aotidae, Pitheciidae, and Atelidae. This timing fits with recent tectonic reconstructions of the Central American Seaway during the early part of the Miocene Epoch (which spans 23.8 to 5.3 million years). Furthermore, the fossils show that New World monkeys were already distributed around the Caribbean by that time. 

Panamacebus transitus was a medium-sized monkey weighing about 2.7 kilograms (5.9 pounds). The genus name combines “Panama” with “Cebus,” the same root as Cebidae, one of the five families of New World monkeys. It’s thought to be closely related to a living member of the genus Cebus, the capuchin monkey (pictured to the right). The new species name comes from “transit” and refers to its dispersal from South to North America during the early Miocene.

According to the researchers, northward movement after that time was likely limited more by differences in the ecological factors of North and South American forests than by geographic barriers and climate differences. 

^{Las Cascadas locality where Panamacebus was found. Jason Head}

^{Image in text: The upper molar of Panamacebus. Aldo Rincon}