Great American Biotic Interchange: When Two Continents Met

The Great American Biotic Interchange was the mass movement of animals and plants between North and South America after the Isthmus of Panama joined the two continents. Its main pulse began about 2.7 million years ago, sending armadillos, sloths and opossums north and cats, dogs, horses and camels south. The exchange transformed the wildlife of the Americas and is one of the best-documented natural experiments in the history of life.

Background: South America’s splendid isolation

For most of the 66 million years since the end of the age of dinosaurs, South America was an island continent. Cut off from North America by sea, it developed a highly distinctive set of mammals. They included armoured glyptodonts the size of small cars, ground sloths, armadillos, hoofed animals unlike any living today, and meat-eating marsupial relatives such as the sabre-toothed Thylacosmilus.

The American palaeontologist George Gaylord Simpson later called this period the continent’s “splendid isolation”. North America, by contrast, was repeatedly connected to Asia and shared many of its mammals: horses, camels, deer, cats, dogs, bears and elephant relatives.

How the land bridge formed

The Isthmus of Panama grew as the Pacific seafloor pushed beneath the Caribbean plate, building a chain of volcanic islands, and as tectonic movements raised the land. The process took millions of years. Shallow seaways between islands narrowed long before they finally closed.

The timing of the final closure has been debated. A 2016 review led by Aaron O’Dea, drawing on geological, fossil and genetic evidence, concluded that the isthmus was fully formed by about 2.8 million years ago. Other researchers have argued that parts of the land bridge, or near-continuous island chains, existed several million years earlier.

Early travellers

Some animals crossed before the land bridge was complete, probably by swimming or by drifting on floating vegetation between islands. Ground sloths reached North America around 9 million years ago, and raccoon relatives (procyonids) reached South America roughly 7 million years ago.

These early “island hoppers” were few. The big exchange came only once there was a continuous land route through the tropics.

The main exchange

The largest wave of dispersal began about 2.7 million years ago, with further pulses at intervals over the following two million years, often linked to swings in climate and sea level during the Ice Age. Michael Woodburne and other researchers have identified several distinct waves rather than a single event.

Heading south

Animals from North America that spread into South America included:

• cats, among them the sabre-toothed Smilodon, as well as pumas and jaguars;
• dogs and foxes, weasels and skunks, and short-faced bears;
• horses, tapirs, peccaries and deer;
• camels, whose descendants became llamas, alpacas, guanacos and vicuñas;
• elephant relatives called gomphotheres, and many kinds of rodents and rabbits.

Heading north

Animals from South America that reached North America included ground sloths, glyptodonts, armadillos, opossums, porcupines and capybaras. One striking northward migrant was Titanis walleri, a large flightless “terror bird” whose fossils have been found in Florida and Texas.

An uneven exchange

The interchange was lopsided in its long-term results. Northern immigrants diversified rapidly in South America. Larry Marshall and colleagues estimated in the 1980s that roughly half of the mammal genera living in South America today descend from North American immigrants. By contrast, only a handful of southern immigrants established lasting populations in North America, and very few survive north of Mexico: chiefly the nine-banded armadillo, the Virginia opossum and the North American porcupine.

Why this happened is not settled. Suggested factors include the larger size of the northern source region, which had long exchanged animals with Asia; the spread of open, cooler habitats suited to northern animals; and the possibility that the tropical forests of Central America acted as a filter, letting some kinds of animals through more easily than others. The decline of South America’s native predators such as the sparassodonts (marsupial-like carnivores) was once blamed on competition with arriving cats and dogs, but fossil dates suggest many of them were already in decline before the newcomers arrived.

Consequences beyond land animals

The closure of the isthmus also divided the sea. Marine populations on either side were separated, and many now exist as closely related “twin” species in the Caribbean and the Pacific, such as pairs of snapping shrimps that biologists use to study how new species form.

The isthmus changed ocean currents too. With the tropical seaway closed, more warm, salty Caribbean water was diverted north into the Gulf Stream. Some scientists have linked this to the growth of large ice sheets in the Northern Hemisphere around 2.7 million years ago, although the strength of that link is debated.

Many animals of the interchange later disappeared during the Quaternary megafauna extinction. Giant ground sloths such as Megatherium, glyptodonts, gomphotheres, horses in the Americas and Smilodon all vanished between about 13,000 and 10,000 years ago, around the time humans spread through the continents.

How scientists reconstruct it

The interchange is known mainly from fossils. When a group of animals appears suddenly in rock layers on a continent where it had no earlier relatives, palaeontologists can date its arrival. Fossil-rich deposits in Argentina, Florida, Texas and Central America have been especially important, and volcanic ash layers and changes in the Earth’s magnetic field recorded in the rocks help to fix their ages.

Genetic studies of living animals add another line of evidence. By comparing DNA from related species on either side of the isthmus, or in the Caribbean and the Pacific, researchers can estimate when populations were separated. These “molecular clock” dates do not always match the fossil record exactly, which is one reason why the timing of the land bridge is still discussed.

Geology supplies the third strand. Marine sediments drilled from the seabed show when ocean water on the two sides of Panama stopped mixing, because the chemistry and the tiny fossils in Caribbean and Pacific sediments begin to diverge. Taken together, these sources point to a gradual narrowing of the gateway followed by a much larger exchange of land animals once the route was complete.

Key people

• Alfred Russel Wallace, in The Geographical Distribution of Animals (1876), laid foundations for comparing the faunas of the world’s regions, including the Americas.
• George Gaylord Simpson described the history of South American mammals and their isolation in a series of works culminating in Splendid Isolation (1980).
• Larry G. Marshall and S. David Webb were among the scientists who quantified the interchange in the 1980s.

Timeline

• c. 9 million years ago: Ground sloths reach North America, probably by island-hopping.
• c. 7 million years ago: Raccoon relatives reach South America.
• c. 2.8 million years ago: The Isthmus of Panama is fully closed, according to a widely cited 2016 review.
• c. 2.7 million years ago: Main pulse of land mammal exchange begins.
• c. 2.6 million to 10,000 years ago: Further waves of dispersal during Ice Age climate swings.
• c. 13,000 to 10,000 years ago: Many interchange animals, including ground sloths and glyptodonts, die out.
• 1876: Wallace publishes The Geographical Distribution of Animals.
• 1980: Simpson publishes Splendid Isolation.

Why it matters

The Great American Biotic Interchange shows how a single geographical change, the rise of a narrow strip of land, can reshape life on two continents and in two oceans. It explains why llamas live in the Andes, why armadillos and opossums live in the United States, and why South America’s wildlife today is a blend of ancient native lineages and northern newcomers.