Mexico: Evolutionary Holding Pen Pre-Mammal Migration

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.