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

教授：可能在之前的生物课上你们学习到蛇是由蜥蜴进化而来，而且起初的蛇是没有毒的，后来才出现了更高级的蛇，也就是毒蛇。

Professor: Probably back in some previous biology course you learned that snakes evolved from lizards, and that the first snakes weren't venomous and then along came more advanced snakes, the venomous snakes.

毒蛇就是指那些分泌有毒物质或者毒液的蛇，比如蝰蛇及眼镜蛇。

Ok, venomous snakes are the ones that secrete poisonous substances or venom, like the snakes of the viper family or cobras.

还有像蟒蛇、蚺蛇这样的无毒蛇类。

Then there is non-venomous snakes like constrictors and pythons.

还有一类蛇——黄颌蛇，并不能将其进行严格的分类。

Another family of snakes, the colubrids, don't really fit neatly into either category though.

黄颌蛇你们应该也学习过，虽然通常被分到毒蛇类别中，但事实上它们大致是无毒的。

Colubrids, and you probably learned this too, although they are often classified as venomous snakes, they are actually generally non-venomous.

之所以将其划分到毒蛇类，是因为它们更像毒蛇，而且它们具备一些比其它无毒蛇类更先进的一些特点。

They are classified as venomous snakes because they resemble them, their advanced features more than the other non-venomous snakes.

那么现在，假如我告诉你们，我刚才讲的多数的理论很有可能都是错误的，那会是怎样呢？

Now, what if I told you that there is a good chance that most everything I just said is wrong?

呃，除了蛇是从蜥蜴演化而来以外，其余部分也许都是错误的。

Well, everything except the part about snakes evolving from lizards.

蛇进化的基本理论受到了最近一项研究的动摇，这项研究揭示了一个关于爬行动物进化关系全新的理解，你们知道，就是什么爬行动物由何祖先演变而来。

See, the basic theory about snake evolution has been challenged by a recent study that revealed a whole new understanding of evolutionary relationship for reptiles, you know, which reptiles descended from which ancestors.

研究人员研究了各种黄颌蛇的毒液基因。

The researchers study the proteins in the venom genes of various species of colubrids.

恩……蛇毒是由蛋白质构成的混合物，有的有毒，有的则没有毒。

Emm... snake venom is a mixture of proteins, some toxic, poisonous, and some not.

通过对DNA及蛋白质中的基因物质的分析，研究人员就能专注于有毒基因并用其找出蛇毒的进化轨迹，这样，也就找到了蛇的进化源，

By analyzing the DNA, the genetic material of the proteins, the researchers could focus on the toxic genes and use them to trace the evolution of snake venom, and from this, the evolution of snakes.

传统意义来讲，为了了解动物的进化关系，我们要看动物各种各样容易观察的体型特征，它们的骨骼，大脑的大小，然后，呃，然后再根据其相同点及不同点来进行分类。

Traditionally, to understanding evolutionary relationships, we looked at various easily observed physical characteristics of animals, their skeleton, the size of their brain, and... and then classify them based on similarities and differences.

这种方法的弊端就是表面上的相似特点也许实则是以不同的方式进行进化发展的。

The problem with this method is that characteristics that appear similar may actually have developed in quite different ways.

比如，有些毒液是以化学元素为基础的，有些则是以细菌为基础的，因此很显然它们是在以不同的路径进行进化发展的，也许并不如我们所想的那般关系紧密。

For example, some venoms are chemical-based, and others are bacteria-based, so they clearly had to have developed along different routes and may not be as closely related as we thought.

而现在不是每个人都同意这个观点。

Now, and not everyone will agree about this.

基于DNA进行分类看似是更有说服力的。

The classification based on DNA seems to be much more reliable.

好，那再回到研究的话题上来。

研究人员发现毒液在蛇还未存在的时候就已经进化了，大约是在一亿年前。

The researchers found that venom evolved before snakes even existed, about a hundred million years before.

现在，几种毒蜥蜴也已经被囊括到研究中来。

Now, a couple of venomous lizards were included in this study.

而且研究人员在毒蜥蜴体内也发现了一些和毒蛇同样的毒液。

And the researchers found some of the same DNA in their venom as in the snakes' venom.

这就意味着所有蛇类的祖先都是毒蜥蜴，也就是说，根据这项研究，关于蛇的祖先这个问题来讲，根本就没有无毒蛇之说，包括黄颌蛇。

This suggested that the common ancestor of all snakes was actually a venomous lizard, which means that actually, according to this research, anyway, in terms of the snakes' ancestry, there is no such thing as a non-venomous snake, not even colubrids.

我们将黄颌蛇与其它蛇类鉴别开来是通过它们的毒液，并不是说它没有毒液，而是看它是否用毒液来作为一种有效的方式进行捕猎。

What separates colubrids from other snakes we have been classifying is venomous, is not the lack of venom, but the lack of an effective way to deliver the venom into its prey.

多数的毒蛇，像蝰蛇及眼镜蛇，都是用毒液来捕捉和削弱其猎物的力量。而黄颌蛇却是在猎物已经进入其口中之后才注入毒液。

In most venomous snakes, like vipers and cobras, the venom is used to catch and immoblize the prey. But in colubrids, venom drips onto the prey only after the prey is in the snake's mouth.

所以对于黄颌蛇来讲，毒液一定是有其它的作用，可能和消化猎物有关联。

So for colubrids, the venom must serve some other purpose, maybe linked to digesting prey.

至于各类别毒蛇的进化，它们的牙齿更加向前，变得更大，毒液也变得越加剧烈，所以说这些明显是毒蛇的蛇种，像眼镜蛇和蝰蛇，都是捕捉有效性的进化，而非毒液的进化。

As the different families of venomous snakes evolved, the teeth moved forward, becoming larger, and the venom becoming stronger, so the evolution of the obvious venomous snakes, like cobras and vipers, is about the evolution of an efficient delivery system, not so much the evolution of the venom itself.

因此，如果说没有真正的无毒蛇类的话，那么像蟒蛇，蚺蛇这类所谓的无毒蛇类难道是在进化中由有毒进化为无毒吗？

So, if there are no truly non-venomous snakes, were the so-called non-venomous snakes, like constrictors and pythons, were they venomous at some point in their evolution?

蟒蛇是通过缠绕挤压来进行捕猎，也许它们曾经是有毒蛇类，而它们施放毒液的器官又在某种程度上无关紧要，所以便逐渐消失了。

Constrictors have evolved to kill their prey by crushing, but perhaps they once were venomous, and then at some point their venom-producing apparatus wasn't needed anymore, so it gradually disappeared.

还有一种蛇，叫做棕树蛇，它们即靠缠绕挤压的方式又靠毒液的方式捕猎，这取决于它们的猎物是什么。

There's one species of snake, the brown tree snake that uses both constriction and venom, depending on its prey.

所以说，这种猜测还是可能的。

那么，我们就有了关于蛇类进化新的概念以及新的DNA数据，所有这些都是关于蛇毒基因构成的信息。

So, we have these new concepts of snakes' evolution and a new DNA database, all these information on the genetic makeup of snake venom.

我们的这项发现使研究人员开始相信毒蛋白质也许在医学领域会有惊人的应用。

And what we have learned from this has led researchers to believe that venom proteins may have some exciting applications in the field of medical research.

你们看，毒液与一些特定的药物一样能改变生物的功能性，而从蛇毒中提取的药物能针对特定的细胞有巨大的功效，也许就能减少一些副作用。

You see, venom alters biological functions in the same way certain drugs do, and the big benefit of drugs made from snakes venom would be that they target only certain cells, so maybe that'll create fewer side effects.

用毒液作为人类药物的基础，这目前听起来难以置信。

Now, it sounds far-fetched, venom is the basis for human drugs.

到目前为止，只有一种蛋白质被用来作为潜在的药品研究，但是谁知道呢，也许将来会有所成就。

So far, only one protein has been targeted for study as a potential drug, but who knows, maybe someday.

Why does the professor review information about the classification of snakes that students probably learned in previous courses?

According to the professor, what is a major weakness of the classification system that is based on animals’ physical characteristics?

According to the professor, in what way do colubrid snake differ from other venomous snakes?

Why does the professor mention the brown tree snake?

What is the professor's attitude toward the result from medical research on snake venom proteins?

Narrator Listen to part of a lecture in a biology class. 
Professor 
Probably back in some previous biology course you learned that snakes evolved from lizards, and that the first snakes weren’t venomous and then along came more advanced snakes, the venomous snakes. Ok, venomous snakes are the ones that secrete poisonous substances or venom, like the snakes of the viper family or cobras. Then there is non-venomous snakes like constrictors and pythons. Another family of snakes, the colubrids, don’t really fit neatly into either category though. Colubrids, and you probably learned this too, although they are often classified as venomous snakes, they are actually generally non-venomous. They are classified as venomous snakes because they resemble them, their advanced features more than the other non-venomous snakes. 
Now, what if I told you that there is a good chance that most everything I just said is wrong? Well, everything except the part about snakes evolving from lizards. See, the basic theory about snake evolution has been challenged by a recent study that revealed a whole new understanding of evolutionary relationship for reptiles, you know, which reptiles descended from which ancestors. The researchers study the proteins in the venom genes of various species of colubrids. Emm... snake venom is a mixture of proteins, some toxic, poisonous, and some not. By analyzing the DNA, the genetic material of the proteins, the researchers could focus on the toxic genes and use them to trace the evolution of snake venom, and from this, the evolution of snakes. 
Now, what if I told you that there is a good chance that most everything I just said is wrong? Well, everything except the part about snakes evolving from lizards. See, the basic theory about snake evolution has been challenged by a recent study that revealed a whole new understanding of evolutionary relationship for reptiles, you know, which reptiles descended from which ancestors. The researchers study the proteins in the venom genes of various species of colubrids. Emm... snake venom is a mixture of proteins, some toxic, poisonous, and some not. By analyzing the DNA, the genetic material of the proteins, the researchers could focus on the toxic genes and use them to trace the evolution of snake venom, and from this, the evolution of snakes. 
Traditionally, to understanding evolutionary relationships, we looked at various easily observed physical characteristics of animals, their skeleton, the size of their brain, and... and then classify them based on similarities and differences. The problem with this method is that characteristics that appear similar may actually have developed in quite different ways. For example, some venoms are chemical-based, and others are bacteria-based, so they clearly had to have developed along different routes and may not be as closely related as we thought. 
Now, and not everyone will agree about this. The classification based on DNA seems to be much more reliable. 
Ok, back to the research. The researchers found that venom evolved before snakes even existed, about a hundred million years before. Now, a couple of venomous lizards were included in this study. And the researchers found some of the same DNA in their venom as in the snakes’ venom. This suggested that the common ancestor of all snakes was actually venomous lizard, which means that actually, according to this research, anyway, in terms of the snakes’ ancestry, there is no such thing as a non-venomous snake, not even colubrids. What separates colubrids from other snakes we have been classifying is venomous, is not the lack of venom, but the lack of an effective way to deliver the venom into its prey. In most venomous snakes, like vipers and cobras, the venom is used to catch and inmoblize the prey; but in colubrids, venom drips onto the prey only after the prey is in the snake’s mouth. So for colubrids, the venom must serve some other purpose, maybe linked to digesting prey. As the different families of venomous snakes evolved, the teeth moved forward, becoming larger, and the venom becoming stronger, so the evolution of the obvious venomous snakes, like cobras and vipers, is about the evolution of an efficient delivery system, not so much the evolution of the venom itself. 
So, if there are no truly non-venomous snakes, were the so-called non-venomous snakes, like constrictors and pythons, were they venomous at some point in their evolution? Well, that’s not clear at this point. Constrictors have evolved to kill their prey by crushing, but perhaps they once were venomous, and then at some point their venom-producing apparatus wasn’t needed anymore, so it gradually disappeared. There’s one species of snake, the brown tree snake, that uses both constriction and venom, depending on its prey. So, well, it is possible. 
So, we have these new concepts of snakes’ evolution and a new DNA database, all these information on the genetic makeup of snake venom. And what we have learned from this has led researchers to believe that venom proteins may have some exciting applications in the field of medical research. You see, venom alters biological functions in the same way certain drugs do, and the big benefit of drugs made from snakes venom would be that they target only certain cells, so maybe that’ll create fewer side effects. Now, it sounds far-fetched, venom is the basis for human drugs. So far, only one protein has been targeted for study as a potential drug, but who knows, maybe someday.