Researchers have found fossil evidence that mammals from North and South America intermingled before a permanent land bridge linked the continents [4].
This discovery challenges previous understandings of prehistoric migration by suggesting that animal movement was not a single event but a more complex process. It reveals how early environmental conditions allowed diverse species to traverse the Isthmus of Panama earlier than previously thought.
According to the research, the permanent link between the two landmasses occurred about three million years ago [4]. However, migration was already taking place as early as 2.8 million years ago [1]. This early movement created a biological exchange where species from vastly different latitudes began to coexist.
One researcher said the resulting diversity was "a riot of marsupials—mammals with brood pouches—and sloths in the south contrasted with camels, horses, pronghorn antelopes and saber-toothed cats in the north" [1]. These animals utilized a temporary land bridge created by the formation of the Panamanian isthmus [1].
The movement of these species was not uniform in both directions. A UC Berkeley researcher said that ancient mammals may have had an easier time moving from north to south across the Isthmus of Panama due to the climate in Mexico [3].
This intermingling occurred long before the geography of the region stabilized into the permanent connection seen today [4]. The evidence suggests that the biological landscape of both continents was shaped by these early arrivals, which altered the evolutionary trajectory of the mammals in both the northern and southern hemispheres.
“"A riot of marsupials... and sloths in the south contrasted with camels, horses, pronghorn antelopes and saber-toothed cats in the north,"”
This finding suggests that the Great American Biotic Interchange was more fluid than a simple 'opening' of a gate. By establishing that migration occurred prior to the permanent closure of the isthmus, scientists can better understand how climate change and geological instability drove species adaptation and competition in the Americas.
