A buried chapter in the history of the Americas may explain why North American mammals ultimately dominated one of the planet’s most consequential wildlife exchanges. New fossil evidence suggests that Mexico functioned as a biological “holding pen” for roughly 10 million years, allowing mammals from North America to survive, adapt, and diversify before moving south during the Great American Biotic Interchange.
The Great American Biotic Interchange, commonly known as the GABI, began after North and South America became connected by the emergence of the Panamanian land bridge. Around 2.8 million years ago, this connection created a continuous route between two continents that had evolved largely in isolation. Mammals, birds, reptiles, plants, and other organisms began crossing in both directions, permanently altering ecosystems from the Arctic to Patagonia.
Although the exchange was technically mutual, it was strikingly unequal. More than half of the mammal genera currently living in South America are descended from North American immigrants, while only a small number of North American mammal groups can be traced to South American ancestors. Scientists have long debated why northern species were more successful, with explanations ranging from ecological competition and climate change to differences in evolutionary history.
The new study, led by paleontologist Jack Tseng and colleagues, focuses on a region that has often been missing from reconstructions of the interchange. Mexico and Central America are geographically essential to understanding the GABI, but their fossil records are comparatively sparse. Tropical conditions can accelerate the destruction of bones, while vegetation, difficult terrain, and limited exposure of fossil-bearing sediments make specimens challenging to locate and recover.
By collecting new Mexican mammal fossils and combining them with existing records, the researchers reconstructed how mammalian diversity changed across North and South America over geological time. Their analysis indicates that North American mammals began accumulating in Mexico approximately 10 million years ago, long before the Panamanian land bridge became fully passable.
Rather than serving merely as a corridor for rapid migration, Mexico appears to have operated as an evolutionary staging ground. Mammals entering the region could encounter new climates, habitats, predators, and competitors. Over millions of years, these pressures may have favored adaptations that enabled certain lineages to expand their ecological ranges. When a continuous pathway to South America finally opened, Mexico already contained a diverse pool of mammalian lineages prepared to disperse.
The fossil record provides little evidence for substantial, sustained mammal migrations between the continents before the main interchange around 2.8 million years ago. This finding supports the traditional view that the formation of a passable land bridge, rather than earlier intermittent connections, triggered the major wave of movement. Geological changes, sea-level fluctuations, and climate shifts may have influenced the timing and direction of dispersal, but the evidence points to the late Pliocene as the decisive period.
The study also suggests that North American success in South America cannot be explained by migration alone. Prolonged diversification in Mexico may have produced a larger and more adaptable group of potential colonists. At the same time, South American mammal diversity may have been declining before the interchange, potentially reducing competition. Later extinctions could then have removed South American lineages disproportionately, leaving North American descendants more visible in modern ecosystems.
The findings offer a broader lesson for understanding biological invasions. Species that appear to “win” after entering a new environment may not simply be inherently superior competitors. Their success can depend on whether they have had time to acclimate, diversify, and develop ecological flexibility before expansion. In the case of the GABI, Mexico may have provided that critical interval of preparation. The same principle could help scientists interpret modern invasions driven by human activity, in which species are transported across natural barriers at unprecedented speed.
The researchers emphasize that such conclusions depend on the quality and transparency of fossil databases. Jack Tseng notes that sharing analytical code and underlying data is especially important when studies combine specimens identified by different experts and to varying levels of taxonomic certainty. Better records of how fossils are evaluated and verified could make future reconstructions of ancient migrations more robust. As scientists continue to uncover Mexico’s fragmented fossil history, the region may prove to be not a missing link in the GABI, but the evolutionary launchpad that helped reshape an entire continent.
Subject of Research: The evolutionary history and fossil evidence of mammal dispersal during 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://doi.org/10.1126/science.aef2893
References: Science, DOI: 10.1126/science.aef2893
Keywords: Great American Biotic Interchange, mammal evolution, Mexico fossils, Panamanian land bridge, biogeography, paleontology, species dispersal, evolutionary diversification, fossil record, biological invasions
