Transatlantic Animal Dispersals

From Conquering The Pacific: An Unknown Mariner and the Final Great Voyage of the Age of Discovery, by Andrés Reséndez (HarperCollins, 2021), Kindle pp. 23-25:

Oceanic dispersals are extremely instructive because they reveal what is biologically possible, showing what oceans could be crossed and in what direction and which ones constituted insurmountable barriers. The Atlantic, for instance, has been breached several times. One hundred million years ago, South America became something of an island unto itself, having broken off from Africa and decoupled from North America (until about three and a half million years ago, when the Isthmus of Panama finally connected the two halves of the hemisphere). South America therefore existed in “splendid isolation” for tens of millions of years, as one scholar has put it. Yet several dispersals from Africa occurred during this time. South America was originally rodent-free, but a type of rodent called caviomorphs—related to guinea pigs, chinchillas, and capybaras but different from mice and rats—irrupted into it between fifty-five and forty-one million years ago. The closest relatives to the South American caviomorph rodents live in Africa, clearly indicating the source population. Primates followed suit. Again, South America possessed no primates at first. Yet a monkey that scientists call Chilecebus carrascoensis somehow got across the Atlantic Ocean thirty-five to twenty million years ago. To succeed, any primate had to be small and extremely resilient. To judge by the extant fossils, Chilecebus carrascoensis weighed less than two pounds and had a skull barely two inches long. This intrepid voyager would give rise to all New World monkeys, including spider monkeys, capuchins, and marmosets.

As far as we know, about a dozen species have made it across the Atlantic Ocean, including rodents, primates, bats, tortoises, a blind snake, and even a weak-flying bird called the hoatzin. Of all these creatures, geckos and skinks were particularly capable of surviving long oceanic passages, as they hid underneath branches and laid eggs resistant to desiccation and even short-term immersion in seawater. Yet, irrespective of individual capabilities, two main factors explain these successful crossings. First, the closest two points across the Atlantic (Kabrousse, Senegal, and Touros, Brazil) now lie about 1,740 miles apart and, thirty or forty million years ago, perhaps half that distance. Nine hundred miles is far but not overwhelmingly so. Second, the rivers of western Africa constitute excellent launching pads to catch western-moving Atlantic currents leading to the Americas. Although crossing the Atlantic has never been easy, the biological record shows that it has occurred from time to time, and what is true for geckos and rodents applies no less to humans. When Christopher Columbus set out to cross the Ocean Sea in 1492, he and his crew were embarking on a voyage that other species had already made successfully.

Other oceanic paths have been less common. The reverse Atlantic passage from South America to Africa, for instance, has played a much smaller role in the dispersal of species. Negative evidence cannot settle the matter definitively. South American organisms may well have crossed but been attacked on arrival, or perhaps they survived in Africa but without leaving much of a trace. Still, it is striking that no terrestrial vertebrates are known to have made the eastward passage across the Atlantic.