-
Tens of millions of years ago,
-
a force of nature set two giant masses
on an unavoidable collision course
-
that would change the face of the Earth
-
and spell life or death
for thousands of species.
-
The force of nature was plate tectonics,
-
and the bodies were
North and South America.
-
And even though
they were hurdling towards each other
-
at an underwhelming 2.5 cm per year,
-
their collision actually did have massive
biological reprocussions
-
by causing one of the greatest episodes of
biological migration in Earth's history:
-
The Great American Biotic Interchange.
-
Our story begins 65 million years ago,
the beginning of the age of mammals,
-
when what is now North and South America
-
were continents separated
by a marine connection
-
between the Pacific and Atlantic Oceans.
-
During this time, South America
was the home of fauna
-
that included armored glyptodonts
as large as compact cars,
-
giant ground sloths
weighing more than a ton,
-
opossums, monkeys,
and carnivorous terror birds.
-
North America had its own species,
-
such as horses, bears,
and saber-toothed cats.
-
Over 20 million years, the shifting
of the Farallon and Caribbean Plates
-
produced the Central America Volcanic Arc,
a peninsula connected to North America,
-
with only a very narrow seaway
separating it from South America.
-
As these plates continued
to surf the Earth's magma layer
-
far beneath the Pacific Ocean floor,
-
the Caribbean Plate migrated eastward,
-
and about 15 million years ago,
-
South America finally collided with
this Central American Arc.
-
This gradually closed the water connection
between the Pacific and the Caribbean,
-
creating a land bridge,
-
which connected North America
to South America.
-
Terrestrial organisms could now
cross between the two continents,
-
and from the fossil records,
-
it's evident that different waves
of their dispersals took place.
-
Even though plants don't physically move,
-
they are easily dispersed
by wind and waves,
-
so they migrated first,
along with a few species of birds.
-
They were followed
by some freshwater fishes
-
and amphibians,
-
and finally, various mammals
began to traverse the bridge.
-
From South America, mammals like
ground sloths and glyptodonts
-
were widly distributed in North America.
-
Moreover, many South American
tropical mammals,
-
like monkeys and bats,
colonized the forests of Central America,
-
and are very abundant today.
-
South American predator marsupials
went extinct 3 million years ago,
-
at which point North American predators,
such as cats, bears and foxes,
-
migrated south and occupied
the ecological space left behind.
-
Horses, llamas, tapirs, cougars,
saber-toothed cats, gomphotheres,
-
and later humans also headed
south across the land bridge.
-
But what happened on land
is only half the story.
-
What had been one giant ocean
was now two,
-
creating differences in temperature
and salinity for the two bodies of water.
-
The isthmus also became a barrier
for many marine organisms,
-
like mollusks, crustaceans, foraminifera,
bryozoans, and fish,
-
and separated the populations
of many marine species.
-
It also allowed the establishment
of the thermohaline circulation,
-
a global water conveyor belt,
-
which transports warm water
across the Atlantic,
-
and influences the climate
of the East Coast of North America,
-
the West Coast of Europe,
and many other areas.
-
It's a challenge to track all of the ways
-
the collision of the Americas
changed the world,
-
but it's safe to say that the ripples
of the Great American Biotic Interchange
-
have propagated through
the history of life on the planet,
-
and that of mankind.
-
What if these species hadn't gone extinct,
-
or if there were no monkeys
in Central America,
-
or jaguars in South America?
-
What if the thermohaline circulation
wasn't flowing?
-
Would the East Coast of North America
be much colder?
-
It all goes to show some of the most
impactful transformations of our planet
-
aren't the explosive ones
that happen in an instant,
-
but the ones that crawl towards
irreversible change.
-
We are the product of history.