a force of nature set two giant masses on an unavoidable collision course

大自然的力量造成了兩大板塊 一場無可避免的撞擊歷程，

that would change the face of the Earth

這場歷程將改變地球的面貌，

and spell life or death for thousands of species.

並且決定成千上萬物種的 生存與滅亡。

The force of nature was plate tectonics,

這股自然之力源自板塊構造，

and the bodies were North and South America.

而這兩大板塊， 正是北美洲和南美洲。

And even though they were hurdling towards each other

即使它們以毫不起眼， 每年移動 2.5 公分的速度，試圖跨越對方，

at an underwhelming 2.5 cm per year,

這場撞擊卻因此引發了一場 地球史上最大的生物移徙事件，

their collision actually did have massive biological reprocussions

並且實際上造成了 生物學上的巨大影響：

by causing one of the greatest episodes of biological migration in Earth's history:

南、北美洲生物大遷徙。

The Great American Biotic Interchange.

我們的故事要從六千五百萬年前， 哺乳動物的出現說起，

Our story begins 65 million years ago, the beginning of the age of mammals,

當時，現今的南、北美洲 還是完全分開的板塊，

when what is now North and South America

中間隔著一片海域，則是連接著 太平洋和大西洋。

were continents separated by a marine connection

在這段期間， 居住在南美洲的物種，

between the Pacific and Atlantic Oceans.

包括全身披著厚甲的雕齒獸， 體型與小型車一樣大；

During this time, South America was the home of fauna

以及巨型地懶， 重量超過一公噸；

that included armored glyptodonts as large as compact cars,

此外還有負鼠、猴子， 以及肉食駭鳥。

giant ground sloths weighing more than a ton,

在北美洲，也有自己的物種，

opossums, monkeys, and carnivorous terror birds.

例如馬、熊，和劍齒虎。

North America had its own species,

在二千萬年前，"法拉隆" 與 "加勒比" 兩個板塊的位移，

such as horses, bears, and saber-toothed cats.

形成了一個連接北美洲的 中美火山弧半島，

Over 20 million years, the shifting of the Farallon and Caribbean Plates

與南美洲之間， 僅僅相隔一道狹窄的海道，

produced the Central America Volcanic Arc, a peninsula connected to North America,

由於這些板塊持續延伸至

with only a very narrow seaway separating it from South America.

太平洋板塊下深處的地球岩漿層，

As these plates continued to surf the Earth's magma layer

加勒比板塊向東位移，

far beneath the Pacific Ocean floor,

並在大約一千五百萬年前

the Caribbean Plate migrated eastward,

南美陸塊撞上了中美洲島弧。

and about 15 million years ago,

這逐漸關閉了太平洋 和加勒比海之間的水路連結，

South America finally collided with this Central American Arc.

並且形成一座陸橋，

This gradually closed the water connection between the Pacific and the Caribbean,

連接北美洲和和南美洲。

creating a land bridge,

陸生生物現在可以橫越兩大洲間，

which connected North America to South America.

從化石記錄看來，

Terrestrial organisms could now cross between the two continents,

當時明顯出現了 好幾次的生物播遷。

and from the fossil records,

儘管植物不會自己移動，

it's evident that different waves of their dispersals took place.

它們很容易藉由風和海浪擴散，

Even though plants don't physically move,

因此，它們跟著一些鳥類 最先開始遷徙；

they are easily dispersed by wind and waves,

接著是一些淡水魚類 和兩棲動物，

so they migrated first, along with a few species of birds.

最後，各種哺乳動物 開始橫越此陸橋。

They were followed by some freshwater fishes

從南美洲來的哺乳動物， 例如地懶和雕齒獸，

and amphibians,

廣泛散佈到北美洲。

and finally, various mammals began to traverse the bridge.

此外，許多南美熱帶哺乳動物，

>From South America, mammals like ground sloths and glyptodonts

例如猴和蝙蝠， 定居於中美洲的森林中，

were widly distributed in North America.

至今也大量繁衍。

Moreover, many South American tropical mammals,

在南美洲，一種有袋類的掠食動物 於三百萬年前絕種了，

like monkeys and bats, colonized the forests of Central America,

此時北美洲的掠食者， 例如貓科動物、熊和狐狸，

and are very abundant today.

向南遷徙，並佔據了 留下來的生態環境。

South American predator marsupials went extinct 3 million years ago,

馬、大羊駝、貘、美洲獅、 劍齒虎、嵌齒象，

at which point North American predators, such as cats, bears and foxes,

和之後的人類， 也往南越過了此陸橋。

migrated south and occupied the ecological space left behind.

但是，在陸地上發生的一切 僅僅是是半個故事。

Horses, llamas, tapirs, cougars, saber-toothed cats, gomphotheres,

這裡曾是一片海洋， 現在被一分為二，

and later humans also headed south across the land bridge.

造成這兩個水域 具有不同溫度和鹽度。

But what happened on land is only half the story.

這地峽也成為 許多海洋生物的屏障，

What had been one giant ocean was now two,

例如軟體動物、甲殼類動物、 有孔蟲、苔蘚蟲、還有魚類，

creating differences in temperature and salinity for the two bodies of water.

並隔開了許多海洋生物群。

The isthmus also became a barrier for many marine organisms,

地峽也使得 溫鹽環流得以形成，

like mollusks, crustaceans, foraminifera, bryozoans, and fish,

這一條全球水路輸送帶，

and separated the populations of many marine species.

運送溫水橫過整個大西洋，

It also allowed the establishment of the thermohaline circulation,

並且影響北美洲東海岸、

a global water conveyor belt,

歐洲西岸，和其他 許多地區的氣候。

which transports warm water across the Atlantic,

想要追蹤所有 因為美洲陸塊撞擊

and influences the climate of the East Coast of North America,

而造成世界的改變， 是一項艱困的挑戰。

the West Coast of Europe, and many other areas.

但我們可以肯定地說：

It's a challenge to track all of the ways

南北美洲生物大遷徙 造成的連鎖反應

the collision of the Americas changed the world,

已遍佈於地球和人類的生命史。

but it's safe to say that the ripples of the Great American Biotic Interchange

倘若這些生物沒有絕種，

have propagated through the history of life on the planet,

或是中美洲沒有出現猴子，

and that of mankind.

或是南美洲沒有出現美洲虎？

What if these species hadn't gone extinct,

倘若溫鹽環流沒有流動，

or if there were no monkeys in Central America,

那麼北美洲的東海岸會冷得多嗎？

or jaguars in South America?

這一切都顯示出 我們地球上最具衝擊的轉變，

What if the thermohaline circulation wasn't flowing?

不是發生在爆炸性的瞬間，

Would the East Coast of North America be much colder?

而是一種徐徐前進， 不可逆的轉變。

It all goes to show some of the most impactful transformations of our planet

我們就是歷史的產物。

aren't the explosive ones that happen in an instant,

but the ones that crawl towards irreversible change.

We are the product of history.