A 10-Million-Year 'Holding Pen' Helped Shape Ancient Mammal Life in the Americas Before a Land Bridge Opened

The breakup of the supercontinent Gondwana led to the formation of modern-day continents, including South America. After the split, South America drifted away from the other continents, allowing unique mammals to evolve in isolation. A new study, published in Science, challenges previous beliefs about how this period of isolation ended.

Instead of a rapid land bridge formation across Panama, the research suggests that a significant period of migration began around 10 million years ago in what is now modern-day Mexico. This "holding pen" allowed various species to gather, setting the stage for a mass migration, known as the Great American Biotic Interchange (GABI), which took place around 7 million years ago.

During this time, many mammals moved into South America, resulting in a higher number of species with northern origins in the continent. The study reconstructs a more comprehensive fossil record of Mexico and suggests that the migration was not solely driven by the formation of the land bridge, but also by the adaptive strategies of the species involved.