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

Researchers led by University of California, Berkeley paleontologist Jack Tseng have assembled one of the most extensive databases of Mexican fossil mammals yet created. The collection includes fossils dating from roughly 12 million years ago to the present and nearly doubles the number previously available for this period. Their analysis indicates that mammals from northern North America had already entered Mexico by at least 10 million years ago, millions of years before the Panamanian land bridge is thought to have become fully passable.

Rather than immediately continuing into South America, many of these northern migrants appear to have remained in Mexico for several million years. Tseng describes this interval, lasting from approximately 10 to 7 million years ago, as a biogeographic “holding pen.” The region functioned as an ecological extension of North America, allowing northern mammals to establish populations, diversify and adapt to warmer, more seasonal environments without yet producing a continuous highway into the southern continent.

The fossil evidence shows that Mexico was not an isolated evolutionary laboratory generating entirely new mammal groups that later moved south. Many Mexican fossils closely resemble species known from the grasslands and dry habitats of what is now the United States. Large-bodied mammals adapted to relatively arid conditions appear to have expanded southward through ecological corridors resembling today’s Great Plains and U.S. Southwest. Once these animals reached the southern limit of the continent, however, their movement was constrained by geography and the absence of a permanent land connection.

The next stage began around 7 million years ago and lasted until roughly 3 million years ago. During this “lag phase,” limited movement between the continents became possible, but it was highly uneven. Small mammals—including raccoon relatives such as coatis and ringtails, along with rodents—appear to have reached South America. They may have crossed through temporary corridors, island-hopped across the Caribbean or rafted on floating vegetation carried by rivers and ocean currents. These routes would have favored small animals capable of surviving with limited food and water, rather than large terrestrial herbivores and predators.

The exchange was strongly biased toward the south. While several northern mammal lineages entered South America before the main interchange, only one major South American group, the sloths, is clearly documented as moving north during this early period. Over time, some sloths evolved into enormous ground-dwelling forms, including species that reached the size of small elephants. The success of northern mammals in the south and the rarity of southern migrants in the north reveal that geographic connection alone cannot explain the pattern. Body size, climate tolerance, reproductive biology and ecological competition all shaped which species could cross and survive.

About 2.8 million years ago, the formation of a more continuous Panamanian land bridge transformed the exchange. The previous barriers that had limited movement largely disappeared, allowing large and small mammals to move between the continents with far fewer restrictions. This marked the classic GABI phase, when northward and southward dispersals accelerated dramatically. In South America, mammals arriving from the north diversified extensively and occupied ecological roles previously held by native groups. Many southern lineages declined or disappeared, contributing to a major restructuring of the continent’s ecosystems.

One possible explanation for this imbalance involves the contrasting reproductive strategies of marsupials and placental mammals. Marsupials give birth to highly undeveloped young that complete much of their development while attached to a nipple inside a pouch or protected fold of skin. Placental mammals generally retain their young longer before birth, supported by a complex placenta, and often produce offspring that become independent more quickly. Under intense competition, this difference in life history may have given placental mammals an advantage over South America’s marsupial fauna, although climate, habitat change and predation also contributed to extinction and replacement.

The findings challenge geological interpretations proposing that a land bridge existed as early as 15 million years ago or that climate alone controlled the timing of mammal movements. If a continuous terrestrial route had been available throughout the earlier period, researchers would expect broader and more frequent crossings between the continents. Instead, the fossil record points to a gradual buildup: northern mammals reached Mexico by 10 million years ago, limited exchanges began by about 7 million years ago, and unrestricted interchange followed the establishment of the Panamanian connection. The researchers are now expanding their work with South American fossils, whose record is less abundant but may reveal when specific groups crossed, why southern mammals lagged behind, and how ancient migrations produced the biodiversity patterns that humans encountered when they arrived in the Americas.

Subject of Research: The evolution and timing of mammal dispersal between North and South America before and 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

Image Credits: NASA, 2018

Keywords: Great American Biotic Interchange, Isthmus of Panama, mammal evolution, paleontology, fossil mammals, biogeography, Mexico, South America, North America, marsupials, placental mammals, sloths, animal migration