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 The two continents are linked at Panama, but there has been a
debate about when this land bridge first appeared, with most experts
placing its formation at about 3 million years ago.
The new study, published on Thursday in the journal Science,
presents evidence that the Panama land bridge formed at least 10
million years earlier. Until then, a deep water channel called the
Central American Seaway separated the continents.
The researchers base their estimate on the presence of small grains
of a mineral called zircon unearthed in ancient river bedrock in
northern Colombia that originated in Panama and were 13 million to
15 million years old. These grains suggested the land bridge must
have existed at that time, they said.
"We contend that a bridge, perhaps a transient one, was present
since 13 to 15 million years ago," said geologist Camilo Montes of
Universidad de los Andes in Bogota, who led the study.
 The various continents, sitting atop huge plates akin to jigsaw
puzzle pieces sliding over Earth's mantle, have slowly moved around
the globe during the planet's history. They have separated, drifted
and joined numerous times in a process called plate tectonics.
South America became an isolated landmass when it separated from
Africa. Likewise, North America broke apart from Europe and headed
on a gradual collision course with South America.
One of the most dramatic results of the joining of the two Americas
was the mixing of the animals living on the respective landmasses,
with various creatures crossing the land bridge to invade the other
continent. The event is known as the Great American Biological
Interchange.
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South America had a unique animal population, with exotic marsupials
including saber-toothed predators, as well as elephant-sized ground
sloths, car-sized armadillos, camel-like creatures with elephant
trunks and giant flightless, carnivorous "terror birds."
North America had a quite different and impressive array of
placental mammals including saber-toothed cats and other feline
predators, huge short-faced bears, wolves and various herbivores
including big elephant relatives and horses.
The two continents continue to move together.
"Yes, the plates are moving today," Montes said. "They never stop."
(Reporting by Will Dunham; Editing by Alan Crosby)
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