(CN) — The Americas weren’t always attached. Millions of years ago, the continents came into contact, permitting ancient mammals to cross the Isthmus of Panama. But this great migration was overwhelmingly one-sided: North American animals moved southward, while very few South Americans migrated north.
“This faunal exchange can be seen as a natural experiment: two continents, each with its own kind of animals were connected by a narrow land bridge, allowing massive migrations in both directions,” said Juan Carrillo, a fellow at the Smithsonian Tropical Research Institute in Panama City, in a statement.
Carrillo is the lead author of new research that sheds light on why this exchange — known as the Great American Biotic Interchange — was so asymmetrical.
“Our study shows how these migrations happened and that South American mammals had more extinctions,” Carrillo continued. “The effect of this exchange can be still seen today.”
Because many South American mammal species died out in this period, there were simply fewer native mammals around to swap places with their new, North American neighbors.
The Great American Biotic Interchange began roughly 10 million years ago and peaked during the Pleistocene, the geological age commonly known as “the Ice Age” that ran from 2.58 million years ago and ended 11,700 years ago, roughly at the end of a cold spell known as the Younger Dryas.
Over millions of years, the earth’s magma layer pushed the Farallon and Caribbean plates into one another, joining the Americas via the land bridge we know as the Isthmus of Panama. The bridge also separated the Pacific and the Atlantic, contributing to their differing temperatures and the development of separate marine ecosystems.
Today, about half of South America’s mammals descend from species that were once native to North America, such as big cats. But only about one in 10 of North America’s mammals find their origins in South American progenitors. Armadillos, opossums and porcupines are three such animals.
This mass invasion of mammals was preceded by offensives over air and sea: plant seeds, bats and birds crossed continents first, amphibians followed and finally mammals, too, crossed the new bridge as the continents neared one another over evolutionary timescales: a slower-than-glacial rate of 2.5 centimeters per year.
The scientists’ findings, published Monday in Proceedings of the National Academy of Sciences, suggest that predation is one major driver of South American species’ extinction.
“We suspect that the emigration of so-called Carnivora to South America might have been one of the causes of the high extinction in South American mammals,” said co-author Søren Faurby, senior lecturer at Sweden’s Gothenburg University, in a statement.
North America’s predators, including foxes and bears, beat South America’s predators at their own game: their teeth were better suited for their carnivorous diets, and the North American species’ brains were larger than the South American mammals’, the scientists write.
“Carnivora appear to be more efficient predators than marsupials, potentially due to more specialized carnivorous teeth or larger brains, and many of the native South American mammals might not have been able to survive the invasion of more efficient predators,” Faurby said in the statement.
This contrasts with what the scientists call the “prevailing view” that tries to explain the asymmetry, according to which more immigrant species originated in South America.
“These insights provide clues to the long-term evolutionary and ecological consequences of ongoing biological invasions and may provide information on the consequences of the current movement of species on geological time scales,” the report states.