North and South America weren’t always connected, but their mammals still found a way to intermingle

New fossil evidence from Mexico suggests that mammals moved between North and South America long before the Panamanian land bridge formed. For decades, scientists believed that the Great American Biotic Interchange happened around 3 million years ago when the continents permanently connected. A research team led by UC Berkeley paleontologist Jack Tseng discovered that the migration timeline began much earlier, dating back to 10 million years ago.

The findings show that northern mammals migrated into southern North America and remained there in a holding pattern for millions of years. This period, occurring between 10 and 7 million years ago, allowed these species to adapt and diversify before moving further south. Smaller creatures like coatis and rodents managed to cross into South America during this time, likely through island hopping or rafting, while only a few groups like sloths made the journey north.

This research clarifies how our current biodiversity patterns took shape over millions of years. By analyzing new fossil collections in Mexico, the team nearly doubled the known number of specimens from the past 12 million years. This updated history explains how North American placental mammals eventually outcompeted many native South American species once the land bridge opened fully. Understanding these ancient migrations provides perspective on how different species survive environmental changes over geological timescales.