Until the Panamanian isthmus formed about 2.8 million years ago, North and South America were isolated continents with unique groups of animals. A riot of marsupials — mammals with brood pouches — and sloths in the south contrasted with camels, horses, pronghorn antelopes and saber tooth cats in the north.

Biologists thought that only after the isthmus formed did animals surge north and south, what biologists refer to as the Great American Biotic Interchange or GABI. That resulted in the extinction of a lot of South American species as North American species took over.

Yet this picture of mammal movements across continents is too simple, UC Berkeley paleontologists argue. After assembling and analyzing a new database of fossil mammals discovered in Mexico, they conclude that northern North American animals had already migrated into southern North America by about 10 million years ago, where they waited around in a kind of holding pen until opportunities arose for some of them to continue crossing to the south starting about 7 million years ago.

“Our study demonstrates for the first time that not only was there a set of significant pre-GABI phases that are different from the traditional Great American Biotic Interchange events 3 million years ago, but that this ramp up is much older than people thought or imagined,” said Jack Tseng, a Berkeley professor of integrative biology and leader of the study. “The ramp up began at the latest about 10 million years ago.”