Lecture 3 (biology)
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 colubers don’t neatly fit neatly into either category though.
Colubers, and you probably learnt this too, although there are often classified with poisonous snakes. They are actually generally non-venomous. There are classified with 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 relationships for reptiles. You know which reptiles descended from which ancestors. The researchers studied the proteins in the venom genes of various species of colubers. 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 revolution of snakes.
Traditionally, to understand the evolutionary relationships, we looked at various easily observed physical characteristics of animals, their skeleton, the size of their brain and 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 the study and the researchers found some of the same DNA in their venom as in the snake’s venom. This suggested that the common ancestors 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 colubers.
What separates colubers from other snakes we’ve been classifying as venomous is not the lack of venom, but a lack of an effective way to deliver the venom into its prey. In most venomous snakes like vipers and colubers, the venom is used to catch and mobilized the prey but in colubers, venom drips onto the prey only after the prey Is in the snake’s mouth. So for colubers, the venom must serve some other purpose, maybe linked to digesting prey. As the different families of venomous snakes evolve, the teeth moved forward, becoming larger and venom became stronger. So the evolution of the obvious venomous snakes like colubers 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 is 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 this new concept of snakes evolution and a new DNA database, all this information on the genetic makeup of snake venom. What we’ve 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 a big benefit of drugs made from snake venom would be that they target only certain cells, so maybe that will create a fewer side effects. Now it sounds far-fetched—venom as the bases for human drugs. So far, only one protein has been targeted for studies as a potential drug, but who knows? Maybe someday.