Mammals reached southern Mexico millions of years before land bridge formed
New research reveals southern Mexico acted as a 'holding pen' for migrating species long before the Isthmus of Panama connected the continents.
North American mammals began migrating toward South America and interacting with southern species millions of years before a physical land bridge existed. This discovery rewrites the timeline of the Great American Biotic Interchange (GABI), suggesting the biological mixing of the two continents was a staged process rather than a single event.
An international research team found that North American mammals likely reached southern Mexico and settled there starting approximately 7 million years ago. This predates the formation of the Isthmus of Panama—the traditional catalyst for GABI—by at least 4 million years. The study, published in the journal Science as "A 10-million-year biogeographic holding pen primed the Great American Biotic Interchange," identifies southern Mexico as a critical "biogeographic holding pen" where animals gathered before eventually moving further south.
The Role of Ancient Grasslands
To uncover this timeline, researchers utilized stable isotope analysis of fossilized tooth enamel and ancient soil. The data revealed that C4 grasses were present in southern Mexico around 10 million years ago. By 5 million years ago, these grasses had become a dominant part of the diets of early horses, indicating a stable environment capable of supporting large migrating herbivores long before the continents were fully joined.
While larger mammals were stalled in Mexico, the research suggests smaller species were more adventurous. Rodents, ringtails, and raccoon-like coatis likely bypassed the land barrier entirely, traveling further south by rafting on floating vegetation or "island hopping" through the Caribbean.
Implications for Evolutionary History
This shift in understanding demonstrates that biological communities respond to environmental and geological changes—such as the expansion of savanna grasslands—well before physical land connections are established. It suggests that the eventual formation of the Isthmus of Panama 3 million years ago did not start the interchange, but rather opened the floodgates for a process that had been simmering for millions of years.
By studying these natural archives, scientists can better understand how mammalian communities responded to past environmental changes and the processes that gave rise to present-day biodiversity patterns.
Future Outlook
The findings provide a new framework for paleontologists to examine how species dispersal is influenced by tectonic shifts and climate-driven vegetation changes. Researchers will likely look for further evidence of these "holding pens" in other regions to determine if similar staged migrations occurred elsewhere in the prehistoric world.