For decades, biologists believed the great mammal swap between North and South America began only after the Isthmus of Panama fully formed about 2.8 million years ago. Ground sloths the size of small elephants wandered south while camels, horses, and saber-toothed cats roamed north. But new research from UC Berkeley reveals the truth is far older and more fascinating than scientists imagined.
Paleontologists working in Mexico have discovered that mammals were already moving between the continents roughly 10 million years ago — about 7 million years earlier than previously thought. Their findings, published in the journal Science, challenge a long-held assumption about one of the most dramatic biological events in the Americas.
The research team, led by Jack Tseng, a UC Berkeley professor of integrative biology, spent nearly 15 years collecting fossils in Mexico alongside Mexican paleontologists including Oscar Carranza-Castañeda. These efforts nearly doubled the number of known mammal fossils from that time period. What they found surprised them: many fossils from Mexico around 10 million years ago looked identical to fossils from much farther north in the United States, suggesting mammals had already begun their southern expansion long before the land bridge was complete.
"Mexico was not a completely unique geographic region that was generating new species that then dispersed south when the land bridge appeared," Tseng said. "It really is just a geographic extension of what we already see farther north in North America. By 10 million years ago, the fossil mammals in Mexico look a lot like what you would find in different places up in the U.S."
During the million-year gap between 10 and 7 million years ago, Mexico served as a kind of staging ground — a holding pen where animals adapted to drier conditions diversified and waited. Smaller creatures like raccoon-like coatis and tiny ringtails likely hopped across islands through the Caribbean or rode floating vegetation mats to reach South America. Only one major South American group, the mighty ground sloths, made the reverse journey north during this period.
Then, about 3 million years ago, everything changed. "The floodgates opened and essentially northward and southward dispersals by mammals were unimpeded, as far as we know," Tseng said. The full interchange reshaped ecosystems across both continents, creating the rich biodiversity that humans would later encounter.
Tseng sees practical value in these ancient discoveries. Understanding how mammals adapted and moved over millions of years could help scientists predict which species might survive future environmental changes. "This helps us to understand what led up to our current biodiversity patterns," he said, "and which species are more likely to persist and survive future perturbations in the environment."