Narrator: Listen to part of a lecture in a geology class.

旁白：请听一段地质学课上的讲座。

Male professor: About 30 years ago a geologist named Edward Cotter-that's C-O-T-T-E-R, uh, published a paper that contained a very interesting hypothesis.

男教授：大概30年前，一个名叫爱德华·科特的地质学家，Cotter的拼写为C-O-T-T-E-R，他发表了一篇论文，论文里描述了一个非常有趣的假设。

He was studying ancient rivers in a North American mountain chain.

他那个时候正在研究位于北美山脉的古老河流。

And he noticed that about 450 million years ago, rivers started to behave differently.

并且他注意到，大概4.5亿年以前，河流开始变得不同。

Before then, rivers were wide, shallow, and straight.

在那以前，河流是宽的，浅的并且是笔直的。

But after that time, they became deeper and had more curves; they became increasingly meandering.

但是从那以后，河流开始变深，变曲折；变得越来越曲折。

And that's actually how rivers behave to this day.

这就是为什么现在的河流是现在这样。

So, why might this change have happened?

所以为什么会发生这些改变呢？

Female student: Maybe there was some kind of climate shift?

女学生：可能是某种气候转变？

Male professor: Well...Lots of climate shifts have happened since then.

男教授：嗯，从那以后，发生了很多的气候转变。

Male student: Was the change worldwide, or just in that geographical area?

男学生：是全球性的改变吗？还是仅仅就是那个地理区域呢？

Male professor: Well, Cotter speculated that rivers changed worldwide, but he couldn't prove it, because he only had evidence from the one North American mountain chain.

男教授：是的，科特推测河流的变迁是全球性的，但是他无法证明，因为他只有北美山脉相关的证据。

But his studies gave him an idea about why rivers started to change-he hypothesized it had to do with the spread of plant life on Earth.

但是他的研究告诉他为什么河流开始变迁--他假设是由于地球上植物生命的蔓延。

Female student: So, there was no plant life before 450 million years ago?

女学生：所以是说在4.5亿年以前是没有植物的是吗？

Male professor: Very little according to fossil records.

男教授：化石记录表明，（植物）非常少。

Anyway, geologists were intrigued by this hypothesis, which claims that as plants evolved and spread, they had an effect on the terrain and rivers.

不管怎么说，地质学家都对这个假设非常感兴趣，这个假设是说，植物的进化和蔓延会影响地形和河流。

In the past 30 years, more studies have been done, and now we have a lot of data about river systems from around 450 million years ago from all over the world.

在过去30年，开展了越来越多的研究。现在我们有很多世界各地的关于大概4.5亿年以前的河流系统的数据。

In a recent study, a couple of researchers gathered together the existing data and combined them with their own new field data to get a comprehensive picture of the situation.

在最近的一次研究中，一些研究学者集中收集了这些已知数据，并且将它们与自己新的实地考察的数据相结合，期望对现在的情况有个全面的了解。

Their study was specifically designed to identify changes in the shapes of rivers during the time period when vegetation was evolving.

他们的研究是特别设计出来为了确定植物进化时期河流形状的变化。

And when the researchers compared the data about river shapes with data they had collected about plant life from the same period, the data seemed to prove Cotter's hypothesis.

并且当研究者把河流形状的数据与他们收集到的同时期植物生命的数据对比时，发现这些数据似乎证明了科特的假设。

Male student: OK, but how did plant life affect rivers?

男学生：这样啊，但是，植物是怎样影响河流的呢？

Male professor: Well, in order to answer that question, we need to look at the geological evidence.

男教授：呃，为了回答这个问题，我们需要看地质证据。

You see, as rivers flow, they leave layers of sediment behind that eventually fossilize.

你们看，随着河流的流动，它们留下了最后石化了的沉积物层。

The content, thickness, and shape of these fossilized layers in rocks give us information about how rivers flowed.

这些石化了的岩层和岩石的构成、厚度和形状给我们提供了河流是怎样流动的信息。

The earliest records from 500 million years ago show that the sediment in river deposits was largely composed of coarse grains of sand and gravel.

5亿年以前的最早的记录表明，河流沉积物大部分是由石英颗粒、沙子以及碎石组成的。

That tells us that rivers weren't defined-they were very shallow and wide, almost like floods.

那就告诉我们，河流不是轮廓分明的，它们非常浅、宽，几乎就像洪水一样。

But around the time of the rise of plant life, the content of those sediment layers began to change.

但是，大概在植物数量开始增长的时候，这些沉积物层的内容开始改变。

The coarse grains became much finer, and we see evidence of mud.

石英砂变得更加精细。我们还发现了淤泥的迹象。

This suggests that plants promoted the preservation of mud when they sent their roots into the ground.

这就表明，当植物把它们的根扎进大地的时候，它们促进了泥土的留存。

The roots helped to reinforce the ground, which in turn allowed for the creation of riverbanks.

植物的根能帮助加固土壤，转而使得河岸得以形成。

And we also see evidence of a process called lateral accretion.

我们也发现了一个被称为横向吸积过程的迹象。

Lateral accretion happens when water flows around a curve, a bend, in a riverbed.

横向吸积发生在河水在河床的曲线或弯曲之处流动的时候。

Now, the speed of the flow on the outside of the bend is fastest and slowest on the inside of the bend.

现在，河湾外部的水流速度是最快的，而其内部的水流速度是最慢的。

This sets up what's called a secondary flow across the river bottom.

这就形成了河底的二次流。

The fast-flowing water on the outside of the bend digs out material from the riverbank and pushes this material laterally across the bottom, and it gets deposited on the other side of the river, on the inner side of the bend.

河湾外部最快的水流把河床的物质挖掘出来并且推到了河底的两侧，物质就在河流另一侧靠近河湾内部的地方沉积下来。

So, when we see in the sediment layers evidence of lateral accretion- erosion on one side and deposits on the other-that's an indicator that a meandering river existed.

所以，当我们看到，在沉积层里，这个横向吸积的证据，河道一侧被侵蚀而一边则有沉积物堆积，这就表明了弯曲河流的存在。

And according to the study, strong evidence of lateral accretion appears in the geological record...at the same time that there's also evidence of plants with underground root systems.

根据这个研究，地质记录中显示出了横向吸积的强有力的证据。与此同时，也存在植物的地下根系统的证据。

This suggests that plants promoted the development of modern rivers by creating stable banks, which resulted in the flow of water in single, meandering channels.

这就表明植物通过形成稳定的河床，使河流在单一的蜿蜒的渠道中流动，进而推动现代河流的发展。

Female student: So, it looks like the researchers were able to prove the hypothesis.

女学生：所以看起来研究者已经能够证明那个假设了。

Male professor: Well, there's no denying that the study presents a very strong case.

男教授：是的，不可否认，这个研究给出了一个很有力的论据。

But some questions about this hypothesis remain.

但是关于这个假说仍然存在一些问题。

For example, it's well-known that on other planets, like Mars, there's clear evidence of meandering rivers.

举个例子，大家都知道，在其它如火星之类的星球上，也有明显证据表明弯曲的河流的存在。

But is there evidence of vegetation on Mars? I think not.

但是，火星上有植物存在的证据吗？我不那么认为。