Evolution of Venom

  How did venom first appear in the evolutionary path?  While the principles of natural selection easily explain away questions about alterations of structural size and purpose, it fails to adequately describe how such an odd adaptation could appear in so many organisms.  The idea of variations allowing organisms to survive and thus reproduce more effectively than their brethren explains why venom continued to exist after its initial creation, yet can not say how it first appeared.  The most likely explanation would be a mutation, but current scientific knowledge, or at least what the writer has seen of it, cannot say for certain.  In fact, it is likely that there is no single origin, but various events that caused its formation in different species at different times.  This would explain why the platypus and the snake have different toxins making up their respective venoms, or why two different spiders will affect completely different sets of insects.  Snakes seem to have evolved at one central point and then diverged, as do platypus, while spider venom is not understood well enough to reach such a conclusion.  What is probably the least understood is the venom in now-extinct organisms.
         Snakes, reptiles organized into the suborder Serpentes, of the order Squamata, are famous for their venomous capabilities.  Most scientists into three groups organize venomous snakes:  Colubrids, Viperids, and Elapids.  The largest distinguishing feature between Colubrids and the other two groups is the lack of a front-fanged delivery system.  Because of this, Colubrids are often considered non-venomous, although they do possess primitive venom sequences.  Many scientists believe that snake venom evolved among Colubrids, which then branched out into Elapids and possibly Viperids.  Elapids and Colubrids are thought to be of closer relation due to the fact that they share 3FTx (3 fingered toxins).  On the other hand, both Viperids and Elapids possess PLA2 (Phospholipase A2), which Colubrids lack.  Venom-secreting glands of Colubrids are distributed along all lineages, and the morphology and homology of the venom-secreting glands provide evidence for Colubrids being the basis of snake venom’s evolution, though some state that snakes of different groups developed venom independently of one another.  This probably happened through protein recruitment as snakes lost muscle mass.  Through time, these toxins underwent significant changes, yet maintained their molecular scaffold.  The oldest venom found in snakes thus far works by stopping the nicotinic acetylcholine receptor, or stops nerve signals from affecting muscles.  The Colubrid origin theory maintains that venom preceded fangs, although several within the scientific community claim that different lineages probably developed venom independently.  Different evidence suggests, however, that snakes did not develop venom on their own, but that it originally developed in the common ancestor of Squamates.  This is evidenced by the fact that lizards and snakes share approx. nine toxins within their venoms’ molecular make-up.  This development process is quite different from the one found in another venomous animal, the platypus.
         Platypus (which have no recognized plural name), are known for many things.  One such thing is that they are one of the only mammals to make use of venom glands.  The lack of venom in mammals is quite intriguing, as they were one of the last groups to form, and in most cases the most advanced.  Even among the platypus, venom is procured from the hind legs instead of the mouth, is only present in males, and is used to subdue mates rather than catch prey.  Another interesting aspect of platypus venom is that, while similar in chemical structure to snake venom, it seems to have a traceable origin.  The similarities between toxin proteins and defensin proteins have led scientists to conclude that platypus venom is actually a variation of genes in the animal’s immune system.  This is much more than can be concluded about spider venom.
         Spider venom is of particular interest to the writer because of its evolutionary mystery.  The venom in spiders can have completely different chemical compositions, even among closely related species.  In some cases, spider venom can be closer to a completely different organism than it is to other spiders.  For example, brown recluse venom has venom that is almost identical to the toxins released by Corynebacteria.  Even the secretive structures are identical in shape.  This fact may be evidence of a common ancestry among different kingdoms, or a possible lateral gene transfer.  Another problem with the evolution of spider venom is the apparent uselessness of the substance found in funnel web spiders.  Their venom is also effective against humans, monkeys, rats, and fruit flies, none of which make up the spider’s diet.  Animals it actually eats are unaffected.  Through all of these confusing and incomplete relationships, haw did venom appear in the first place?
         The oldest fossil remains found, which provide evidence for having used venom, are that of the conodonts.  These were a group of marine animals that are now extinct.  Their teeth were long and conical, and possessed longitudinal grooves, all of which are similar to snake fangs.  Their mouth structures were similar to the grasping apparatus of an arrow worm, which use tetrodotoxin-based venom, a very common marine toxin.
         The evolution of venom is not a complete picture.  There are different theories and speculations about it, but it still cannot be adequately explained.  Snakes probably started with Colubrids, but that is still debated.  The platypus is an oddity among mammals, which in itself is odd.  Spiders are different on all fronts.  This leads the writer to conclude that, as the first sentence says, venom’s path is not fully understood.  The differences may lead one to conclude that venom evolved independently in different organisms.