How did venom evolve and why are some animals so startlingly venomous?
The second question is easier to answer. Heightened toxicity is a result of evolution compensating for shortcomings in size and habitat. A David needs toxicity that will fell a Goliath. And in desert conditions where food is in short supply, a hungry killer needs to make absolutely sure of a kill.
Moreover, as Josh Gabbatiss notes in “Why some animals have venoms so lethal, they cannot use them” (BBC Earth, 5 April 2016):
“Most venomous animals target a specific and narrow array of prey species, and it is these species that shape the evolution of their venom. What results is a co-evolutionary arms race. The prey species evolves resistance to venom, only to then be faced with a more potent venom further down the line.”
Originating over 100 million years ago, by the time the venom arms race had peaked, plenty of creatures were going around with cocktails of chemicals that appeared to be vastly more potent than they needed to be. Not only that, but the venoms differed in their effect. Snake venom might contain one component to prevent blood from clotting and another to break down the walls of blood vessels. The production of multiple toxins favours the hunter, since victims are unlikely to be immune to all of them. Complex venoms reinforce natural selection. As Gabbatiss makes clear:
“Snakes, jellyfish and cone snails did not evolve powerfully potent venoms just for the sake of it. Their venoms are specialised, and capable of doing exactly the job they are meant for – even if that job is not immediately obvious to us.”
None of which answers the most interesting questions: At what point in the development of life on earth and under what conditions did venom and its accoutrements (stings and fangs) evolve? When did saliva acquire an element of toxicity that began to give its bearer an advantage? How did that manifest itself before a means of delivery evolved? Or was it the other way around – fangs acquired the value-added of poison?
Scientists are getting closer to figuring out what happened. A recent study supports the theory that highly toxic venom proteins were “born” from non-toxic genes that had other ordinary jobs around the body, such as regulating cellular functions or digesting food. This also suggests that instead of a single ancient origin, venom and venom-delivery systems most likely evolved independently in several distinct lineages of reptiles.
In “New model for snake venom evolution proposed” (ScienceDaily, 8 December 2014), the lead author of the study states:
“First, these potentially venomous genes end up in the oral gland by default, because they are expressed in low but consistent ways throughout the body. Then, because of natural selection on this expression in the oral gland being beneficial, tissues in the mouth begin expressing those genes in higher levels than in other parts of the body. Finally, as the venom evolves to become more toxic, the expression of those genes in other organs is decreased to limit potentially harmful effects of secreting such toxins in other body tissues.”
In other words, the evolution of venom was pure chance. Where it served a purpose (in a snake’s mouth) it gradually got stronger and in doing so its prevalence in other parts of the body diminished. Fascinating stuff. But what about scorpions? And bees and wasps? And stinging nettles? The jury is still out.