Florida State University researchers have helped uncover a hidden chapter in the history of the Americas—one that suggests mammals began moving between North and South America millions of years before the formation of the Isthmus of Panama. The international study, published in Science, challenges the traditional timeline of the Great American Biotic Interchange, the major episode of animal migration long believed to have begun when the Panamanian land bridge connected the continents roughly 3 million years ago.
The research identifies present-day southern Mexico as a prehistoric “biogeographic holding pen,” where North American mammals gathered and persisted for millions of years before some species managed to move farther south. Professor of Geology and Environmental Science Yang Wang and geology doctoral student Chance Hannold of Florida State University contributed geological and geochemical evidence to the study. Their findings indicate that the first stages of intercontinental migration may have begun at least 4 million years earlier than the conventional date assigned to the Great American Biotic Interchange.
For most of Earth’s recent geological history, North and South America were isolated from one another by a seaway. That separation allowed the continents to develop distinct mammalian communities. North America was home to animals such as horses, carnivores and other placental mammals, while South America supported a unique assemblage that included several groups found nowhere else. The eventual rise of the Isthmus of Panama created a continuous terrestrial route, enabling a dramatic exchange of species in both directions.
The new study suggests, however, that geographic isolation was not absolute. As the continents gradually shifted closer together, certain North American mammals reached southern Mexico, where they encountered suitable environments but were unable to advance immediately into South America. Instead, these species remained concentrated in the region for an extended period. Smaller mammals—including coatis, ringtails and rodents—may later have crossed gaps between landmasses by island hopping across the Caribbean or by rafting on floating vegetation, long before a permanent land bridge existed.
This earlier movement was likely made possible by major environmental changes in southern Mexico. Wang and Hannold conducted fieldwork in Mexico from 2022 through 2024, collecting ancient soils and fossilized tooth enamel. These materials preserve chemical signatures that can reveal information about past climates, vegetation and animal diets. In particular, stable isotope analysis allows scientists to trace the sources of carbon and other elements incorporated into teeth and soils as organisms lived and landscapes developed.
The isotope data point to the expansion of savanna grasslands as a critical factor in the arrival and survival of North American mammals. C4 grasses—warm-season plants that include modern corn, sugarcane, switchgrass and Bermuda grass—were present in the region approximately 10 million years ago. By about 5 million years ago, these grasses had become a major component of the diets of early horses. Their spread would have transformed the landscape, creating open habitats that supported grazing animals and the predators that followed them.
The timing is significant because it links animal migration to both tectonic change and ecological opportunity. The continents did not simply become connected overnight. The movement of tectonic plates progressively narrowed the seaway, altered coastlines and created islands and shallow-water environments that may have provided stepping stones for dispersal. At the same time, the emergence of grasslands produced new habitats capable of supporting mammals that had evolved in the more open environments of North America.
“This research reveals that the Great American Biotic Interchange was not a single event triggered only by the completion of the Isthmus of Panama,” the findings indicate. Instead, the process unfolded in stages, shaped by the interaction of geology, climate, vegetation and animal behavior. The results suggest that mammalian communities began responding to the changing geography of the region long before the final land connection enabled large-scale migration.
The study also offers a broader perspective on how biodiversity forms. Geological forces such as volcanism, continental uplift and the construction of land bridges can alter climate and habitats over millions of years, while species respond through migration, isolation and adaptation. Understanding these processes helps scientists interpret the origins of modern ecosystems and may also provide insight into today’s rapidly changing biosphere, where humans are moving species across natural boundaries at unprecedented speed.
The work was supported by the U.S. National Science Foundation as part of more than $500,000 in funding for the broader project, led by principal investigator Jack Tseng of the University of California, Berkeley. Researchers from Stanford University, the Natural History Museum of Los Angeles County, the U.S. Bureau of Land Management, Universidad Nacional Autónoma de México and Universidad Autónoma de Querétaro also contributed. Together, their findings transform southern Mexico from a perceived geographic dead end into a key staging ground in one of the most important episodes in the evolutionary history of the Western Hemisphere.
Subject of Research: Mammalian migration, paleontology, biogeography, ancient environments and the early stages of the Great American Biotic Interchange
Article Title: A 10-million-year biogeographic holding pen primed the Great American Biotic Interchange
News Publication Date: 30-Jul-2026
Web References: https://www.science.org/doi/10.1126/science.aef2893
References: Science, DOI: 10.1126/science.aef2893
Image Credits: Devin Bittner/FSU College of Arts and Sciences
Keywords: Great American Biotic Interchange, mammalian evolution, paleontology, biogeography, southern Mexico, Isthmus of Panama, stable isotope analysis, C4 grasses, ancient environments, fossil mammals
