Friday, 23 October 2009

The Evolution of snakes' poison apparatus


Alan Hayward said that : Professor G A Kerkut of Southampton University, was an evolutionist. His standing was such that he was appointed General Editor of an important series of more than 50 textbooks of Zoology "The International Series of Monographs in Pure and Applied Biology - Zoology Division" published by Pergamon Press, Oxford. His own book in the series was entitled Implications of Evolution.

Kerkut took a very hard look at the theory in his book, and one point that stands out was his recognition that gradual evolution could not explain the origin of the poison apparatus of snakes. If we look at the requirements of the poison apparatus, we can soon see why he took the point of view that he did.

All or nothing

AS we’ve shown on several occasions this blog, there is a very serious number of all or nothing mechanisms. The yucca moth is extraordinary; the eumenes wasp is nearly so; the Bell spider is amazing; the life history of the butterfly is inexplicable – and so we could go on.

The poison apparatus of snakes is no less amazing than these.

As we all know, venomous snakes bite and poison their prey by injecting venom through their fangs. Each one of that short list has serious physical, and biochemical implications which are inexplicable on any but the creationist model.

Whatever else we may think of the mechanics of poisoning its prey, a snake has to possess the necessary instincts required BEFORE any such apparatus is of any use. It somehow has to know that it must identify, kill or stun its prey; it must know how to get near; it must know how to hide, how to camouflage itself etc etc.

Without all these things, the best apparatus would be useless. So since poisonous snakes represent a successful group, the inevitable question arises: where did it get these instincts from, and how did the information enter the genes? And just as important, WHY did they arise when there was no poison apparatus for the animal to use?

The Poison and the Poison Glands

Let’s begin with the poison itself.

Somehow, the poison which is going to be used, is extracted by the poison glands themselves from the blood. This alone represents an enormous biochemical feat. To become a poisoner, one needs to know how to obtain the poison. It is impossible for a snake to learn this deadly skill from its non-poisonous parents or from anywhere else.

It has to take the constituent substances from its own blood, and biochemically manufacture them safely in the poison glands which must be expressly designed for this purpose. The glands are sealed, so that the venom cannot re-enter the blood of the snake once it has been manufactured, which would be really too bad. This is an all or nothing, life or death mechanism – literally.

Not only that, but the glands are surrounded by a double capsule whose outermost and strongest layer is muscular, and is used to contract, and squeeze the gland, and express the venom once the fangs are sunk in the prey’s tissue. Without this layer, the venom would remain in the gland, and useless to the snake. Some snakes eat toads, and amazingly enough take the poisonous substances from the toads, and store them for their own use if this article is to be believed:

So this process is absolutely fraught with danger to the reptile, and it is a wonder they survive – but survive they do, and successfully. To postulate that such a remarkably ingenious mechanism could possibly arise by chance mutations etc etc is to be absurd.

The Fangs

Not only are the poison glands remarkable – but the fangs are too. There are two sorts, the erectile fangs, which only become bared and upright when the snake is ready to bite, and the permanently erect type which is always upright.

The fangs are hollow, and connected to the poison glands by a tube. Wouldn’t it have been pointless (ho ho!) if the fangs and the glands were not connected? The snake would have starved. And if the fangs were not pointed and capable of entering the prey’s tissues, all the snake would manage to do would be to give it a good lick. But they are pointed AND hollow AND connected to the poison factory. Three mighty statistical impossibilities.

But this isn’t the only problem the original poisonous snake had to solve. Once it had killed the prey, it had to avoid poisoning itself with its own poison that was IN the prey. How did it manage this by chance mutations, one wonders.

The eyes and musculature

The spitting cobra is a most remarkable animal. We have seen on TV series just how far it can spit its venom. The presenters wore sunglasses to prevent the venom getting into their eyes, and the snakes rarely missed.

How did that ability evolve – and from what? Then, of course, there is the question of the snake’s eyesight. It must be able to measure distances to the prey accurately, by sight. A single eyed animal couldn’t do this – so they have 2 eyes. Perfectly functioning, as people who have been bitten will testify only too sadly.

There is a most unusual feature of their eyes, which alone separates them from any supposed ancestor. Here's wikipedia on the point:

"Most snakes focus by moving the lens back and forth in relation to the retina, while in the other amniote groups, the lens is stretched."

In our eyes, when we focus on a near object, the muscles surrounding it (the ciliary muscles) contract, and the lens becomes thicker. When we look at a distant object, the lens becomes thinner.

In the snake's eye, instead of becoming thicker or thinner, the lens is moved forward or backward to adjust the focus on the object.

It is so totally different that Gordon Walls wrote:

"It has long seemed strange that the eyes of snakes should differ so much from those of lizards considering the closeness of the relationship of the two groups. Indeed, the ophidian (meaning snake) eye exhibits not one solitary structural feature which would enable a comparative ophtalmologist, handed a microscopic preparation of it, to place its owner in the Sauropsidia at all."

Here is a heavy blow in the eye for common descent. Many of the optimistic lineages or alleged connections between the snakes and other groups founder on this single extraordinary fact, which doesn't come from some molecular biochemist's test tube, but from field and anatomical observations. There is no satisfactory evolutionary explanation of this fact available.

The nervous system

There is the question too, of the nervous system of the snake: it has to synchronise, co-ordinate and oversee the whole process: from poison manufacture to getting the snake’s body into the correct position to strike, to injecting the venom, to swallowing the prey whole, and so on.

An enormously complex, and perfectly synchronised operation.

The snake’s jaw

Let me remind readers that the jaw of a snake is doubly hinged, so that the gape is much larger than would be expected from the head size of the animal. It is larger so the serpent can swallow the prey whole. Think of a python swallowing a calf, for example.

It is tempting to try and imagine how many snakes with perfectly functioning poison apparatus starved to death before their jaws opened wide enough!

As we all know, the snake uncoils itself amazingly fast – ‘as fast as a striking snake’ has come into the language. The musculature required to do this must be present. The snake erects itself, the extreme of this being seen in the cobras, most notably the spitting cobras. How could such musculature evolve, and from what?

As usual, nobody knows. “The discovery of a new fossil snake may shed light on the poorly known evolution and ancestry of snakes, paleontologists announced April 16, 1997.”

If this finding is confirmed, it means that we have to completely re-think our view of how snakes evolved. It appears that the snakes underwent a rapid radiation in their initial burst of evolution, with a number of different lifestyles appearing at once and then developing independently and in parallel afterwards. Much work remains to be done on the evolution of snakes.

They don’t seem to learn, do they.




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1 comment:

  1. Snakes sound like pretty amazing animals. Do many people own poisonous snakes for pet or just ball pythons?