An analysis of fossil mammals suggests that Mexico served as a long-term evolutionary "holding pen" where North American species adapted and diversified for millions of years before sweeping into South America during the Great American Biotic Interchange 2.8 million years ago. The findings clarify the timing of one of Earth's most consequential migrations and may help explain why North American mammals were more successful migrants than those from South America. The Great American Biotic Interchange (GABI), which reshaped ecosystems after North and South America became connected about 2.8 million years ago, is one of the most important events in the history of life in the Americas. Fossil and genetic evidence shows that the new land bridge allowed waves of animals and plants to move between the continents, with climate change and fluctuating sea levels further influencing these migrations. The exchange was highly uneven – more than half of South America's living mammal genera descend from North American ancestors. In contrast, only a handful of North American mammals trace their origins to South America. While the major migration waves are well established, the timing of mammals' movement between continents remains poorly understood. Answering that question has been difficult because regions such as Mexico and Central America have had poor fossil records due to preservation and retrieval challenges, leaving a crucial gap in the record of these early migrations.
To address these unknowns, Jack Tseng and colleagues collected new fossils of Mexican mammals to reconstruct patterns of mammal dispersal across the Americas through time. Tseng et al. found that Mexico acted as a long-term evolutionary "holding pen," where North American mammals accumulated, adapted, and diversified beginning about 10 million years ago before expanding into South America when the Panamanian land bridge became fully passable around 2.8 million years ago. The findings also revealed little evidence for substantial earlier migrations, supporting the traditional timing of the GABI. Moreover, the authors suggest that the eventual prevalence of North American mammals in South America resulted from a combination of prolonged diversification in Mexico, declining diversity in South America before the interchange, and later extinctions that disproportionately affected South American lineages. "Although the GABI and other past migrations of species were the results of natural processes, studying their dynamics can inform the understanding of modern species invasions facilitated by humans," write Jenny McGuire and Claire Williams in a related Perspective. "The findings of Tseng et al. suggest that the "winners" of invasions may be those species that can successfully acclimate, such as the North American species that did so in the Mexican holding pen."
For reporters interested in research integrity topics, author Jack Tseng notes, "Sharing of data and analytics codes have been particularly important for analyses such as the ones we present in our work. I would like to see more fossil record databases annotate the degree of vetting that has gone into compilations of data from different sources and the expert opinions that underlie the identifications of specimens to varying levels of specificity."
