英语听力：探索发现 科学新发现：我们的大气层-7

To discover why these twin planets followed separate paths and to explore what gave us our precious atmosphere, we have to travel back, back four and a half billion years to the birth of the solar system itself. The birth of the solar system created

英语听力：探索发现 科学新发现：我们的大气层-8

Our planet was spared this fate. The magnetic field generated by the Earths core blocked the solar wind, preventing it from blasting away the fragile air. But the Earths atmosphere was still very different from the one we breathe today. It contained

英语听力：探索发现 科学新发现：我们的大气层-9

We were left with this atmosphere made of molten rock and vaporous rock. It took millions of years for that atmosphere to fall out and to be replaced by an atmosphere made up of nitrogen and carbon dioxide and water vapour. These new gases came from

英语听力：探索发现 科学新发现：我们的大气层-10

Evidence of the first oxygen in the atmosphere. The gas was released by a humble microscopic living organism called cyanobacteria. Well it turns out cyanobacteria are the only things that can actually produce oxygen, so we know that at some point cya

英语听力：探索发现 2012-10-26 长毛象王国 Mammoth—21

The lions will have to try elsewhere. Could a bison be a more realistic prospect? The wind whips up again offering cover, and the hunters focus on a target on the fringes of the herd. Before long the carcass freezes and becomes difficult to eat. Aban

英语听力：探索发现 2012-10-24 长毛象王国 Mammoth—20

One giant short-faced bear is more than a match for two lions. But in this bitter climate, lions cant afford to go without a meal for long. A mammoth calf in its first winter is a tempting sight. But mammoths are attentive mothers and they have the b

英语听力：探索发现 2012-10-31 美洲大平原 American Serengeti—2

As North America emerged from the grip of the last ice age, the door was opened to outsiders for the very first time. As these early immigrants pushed their way south, they found themselves in a land of an unimaginable opportunity, overflowing with g

英语听力：探索发现 2012-10-28 美洲大平原 American Serengeti—1

Denver International Airport is coming to life and about to receive the first arrivals today. By midnight, more than 1,500 aircrafts wouldve touched down here, delivering tens of thousands of people to this modern metropolis. But when did people firs

英语听力：探索发现 2012-11-25 美洲大平原 American Serengeti—13

A lone wolf weighs as much as four coyotes, but one on one it's still no match for a bison. Wolves, though, live and hunt in packs of up to 15 and when they launch a cooperative attack, they're devastating. First they get the bison on the run, then f

英语听力：探索发现 2012-11-23 美洲大平原 American Serengeti—12

Because these two bull mammoths both had broken tusks, they could have fought at closer quarters than they would do normally. Twisting and turning, they became locked in a deadly embrace. Even more bizarre, this twist of fate then caused a third fata

英语听力：探索发现 2012-11-18 美洲大平原 American Serengeti—10

Colombian mammoths had to feed almost round the clock to fuel their bulky bodies. But as the ice age ended, food was not the mammoths' biggest problem. A new and deadly predator began to infiltrate the plains, a match for any prey, even the mighty ma

英语听力：探索发现 2012-11-16 美洲大平原 American Serengeti—9

Bison calves are born in summer and can run within a few hours of their birth. They have no choice. The herd won't wait in its eternal quest to find new grazing. Smaller grazers are still found here, too. But they stay put instead of wandering the pl

英语听力：探索发现 2012-11-14 美洲大平原 American Serengeti—8

The Osage orange seeds were carried far across the plains, before being deposited, pre-packed in their fertilizer growbags, ready to take root. As the ice age waned, the milder climate and the mixed vegetation meant the plains were able to support a

英语听力：探索发现 2012-11-11 美洲大平原 American Serengeti—7

The store of pollen paints a picture of the plains of 13,000 years ago, a picture that looks very different from the open prairie grassland of today. But why did this region look so different back then? The ice sheets to the north, although retreatin

英语听力：探索发现 2012-11-09 美洲大平原 American Serengeti—6

The site was once a spring-fed pond, full of water. Mammoths were tempted in to drink. But when they tried to climb back out, the banks were steep and slippery. Just like the short-faced bear imprisoned underground, some became trapped. Scavengers wo

英语听力：探索发现 2012-11-07 美洲大平原 American Serengeti—5

Mud from the ancient riverbanks is good for building nests. lt also holds more evidence that will help us to reconstruct the ice age past. Every now and then,new clues surface, hinting at what else might lie beneath. ln this dried-up pond in South Da

英语听力：探索发现 2012-11-04 美洲大平原 American Serengeti—4

The death throes of the last great ice age left a signature that we can read today. These giant potholes were left behind by blocks of buried ice that melted, leaving hollows that later filled with water. For thousands of years since then, they've be

英语听力：探索发现 2012-11-02 美洲大平原 American Serengeti—3

Even the mastodon is buried here, a long dead relative of modern elephants. This was once a bear, but not like any bear in North America today. Claw marks gouged into the cave wall showed the bear was not killed direct by the fall. It made a desperat

英语听力：探索发现 2012-11-21 美洲大平原 American Serengeti—11

In a Colorado gully, hunters carried out a mammoth massacre, leaving behind the remains of at least 16 animals. The site recreated here contains a treasure trove of evidence relating to the mammoth's daily life. Again by comparing mammoth bones to el

英语听力：探索发现 2012-10-05 长毛象王国 Mammoth—12

The male flicks the air with his tongue to test if the ewe will accept him. The next stage of courtship is not so subtle. He gives her a kick. At this highly-charged time of year, skirmishes can easily erupt between the rams, especially if they both