The breakup of the supercontinent Gondwana was the most consequential split in history. It set the scene for the creation of the modern world’s continents. For millions of years after the split, South America floated free of any other continents. During this time, this isolation allowed weird and wonderful mammals to evolve.
Now, a new study has realigned scientific thinking about how this period of isolation ended, when a land bridge across modern-day Panama formed. In contrast to previous findings, the new study, published in Science, suggests that mammals moved into a kind of “holding pen” in what is now Mexico for millions of years before journeying into South America during a mass migration called the Great American Biotic Interchange (GABI).
“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, an integrative biologist at the University of California, Berkeley, and co-author of the new study, in a statement.
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An Ancient “Holding Pen”
Reconstruction of an ancient sloth that may have once crossed the Panamanian land bridge.
(Image Credit: Natural History Museum of Los Angeles County)
Isolated South America was the origin of some unusual mammals. Some of these, like the carnivorous members of the order Sparassodonta, are now extinct. But the descendants of others are still found in the region today. These include the xenarthrans like the giant anteater, tree sloth, and armadillo.
But GABI changed everything for the isolated continent. Once the Panamanian land bridge formed, species moved both northwards and southwards. But today, far more animals in South America have a northern origin than vice versa. The new study offers clues about how the move went down and shows that many species didn’t need to wait for the formation of a land bridge to begin their mass migration.
Instead, Tseng’s team hypothesized that the movement’s first stage began around 10 million years ago, over 7 million years before the Panamanian land connection fully formed. In this time, animals arrived in the region of modern-day Mexico, where a kind of “holding pen” formed.
Tseng’s team tested their theory by building a more comprehensive fossil record for Mexico than had previously been constructed. The team doubled the number of ancient fossil locations identified in the region. Their analysis highlighted the close similarity between these new finds and those previously dug up further north, suggesting that migration to Mexico may have happened far earlier than previously thought.
The analysis supported the holding pen theory and suggested that over three million years, a wide range of animals flourished in the pen and across the sea in South America.
Turbo-Charged Migration
Roughly 7 million years ago, a period of intermittent migration started up. Over the next 4 million years, some smaller mammalian species, like rodents, took the opportunity to migrate. There are two competing theories for how they achieved this. One suggests that they relied on long-distance dispersal on rafts, or by island-hopping through the Caribbean. At this point, migration was almost entirely done from north to south. Only sloths made the reverse journey from South to North America.
It was only once the land bridge formed that migration became turbo-charged.
“The floodgates opened and essentially northward and southward dispersals by mammals were unimpeded, as far as we know,” said Tseng.
This migration greatly disrupted South America’s ecological isolation. Now, North American placental mammals, which give birth to young that can thrive early on, had an advantage against South American marsupials, which, like modern-day kangaroos and opossums, produce young that need extended post-birth care in a pouch. These latter species were outcompeted by their northern neighbors.
The researchers hope their new data will give us information about life on Earth’s future, as well as its past.
The findings, said Tseng, “help us understand which species are more likely to persist and survive future perturbations in the environment, given what they’ve already experienced for the past 10 million years.”
Read More: Homo Erectus Crossed an Ancient Land Bridge to Roam Southeast Asia 140,000 Years Ago
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