听力文摘：大陆相撞：我们星球的历史（在线收听）

When Continents Collide: The History Of Our Planet

大陆相撞：我们星球的历史

It was one of the Earth's momentous times. Continents collided. Volcanoes erupted. Ocean currents shifted. Species ran amok.

这是地球意义非凡的时刻之一。大陆板块碰撞，火山喷发，洋流变化，各种物种横行。

Twenty million years ago, North and South America were arranged in nearly the same positions as they are today. The big difference being, they were separated by a deep open channel called the Central American Seaway. There was no Panama and no need for a Panama Canal because there was clear sailing from the Atlantic to the Pacific.

两千万年前，南北美洲与今天相比位置相差无几。最大的迥异之处在于，它们被一条称作中美航道的深邃广阔的海峡分开。当时没有巴拿马，也没有巴拿马运河存在的必要，因为当时有从大西洋到太平洋的畅通的航道。

But the Earth is a geologically active planet with shifting crustal plates. During the time period between twenty and three million years ago, the Pacific Plate collided with the Caribbean Plate, pushing magma to the surface to make islands in the sea, and eventually creating a land bridge between the two continents. The movement not only changed the land, it disrupted ocean currents, opened a door to species migrations and probably altered the world's climate.

但地球是一个地壳板块不停运动，地质十分活跃的星球。在两千万年前到三千万年前的这段时间里，太平洋板块与加勒比海板块发生碰撞，使得岩浆喷发出地表在海 洋中形成岛屿，并最终形成了连接上述两块大陆的陆地桥梁。这一运动不光改变了陆地，同时也改变了洋流，为物种迁移提供条件，并很可能改变了地球的气候。

When the land bridge closed, equatorial waters could no longer mix. The Atlantic became saltier and the Pacific more dilute, creating a gradient that moves water in a giant loop around the globe today. Warm Atlantic water that used to pass through the gap moved northward, becoming the Gulf Stream. Scientists believe these changes created a warmer Europe and contributed to our recent cyclic ice ages.

当陆地桥梁合拢的时候，赤道附近的水域无法再相通。大西洋变得盐度更大，而太平洋恰恰相反，这就形成梯度，成为今天的全球洋流大循环。温暖的大西洋海水之 前流经海峡，如今只能向北流动，形成墨西哥湾暖流。科学家相信这些变化使得欧洲更加温暖，并影响了我们最近一次周期性冰河世纪的到来。

The land bridge also opened the way for species to cross from one continent to the other. Deer, horses, raccoons, bears and the camel ancestors of llamas moved south across the bridge. Anteaters, porcupines, opossums and armadillos moved north.

陆地桥梁同时也为物种在大陆之间的迁移创造条件。鹿，马，浣熊，熊和美洲驼的骆驼祖先通过这座“桥”来到南美洲。食蚁兽、豪猪、负鼠、犰狳 则一路向北。

Unfortunately, many large South America species couldn't compete with the North American animals and became extinct, changing the ecology of an entire continent.

不幸的是，许多大型的南美洲物种无力同北美洲迁移过来的动物竞争，从而灭绝，这改变了整个大陆的生态系统。