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| Naja nubiae slow motion spitting Credit: © The Trustees of the Natural History Museum, London and Callum Mair |
Researchers at Bangor University School of Natural Sciences, Bangor, Wales believe they have shown that the components of the venom ejected by spitting cobras has evolved on three separate occasions due to the same environmental pressures. The fact that there are components in these cobra's venom that exploit the mammalian pain receptors to give instantaneous pain, suggests that this evolved as a defensive, not an aggressive mechanism, aimed at deterring attacks by mammals.
Contrary to the theory that venoms are adapted primarily to enable snakes to kill prey, in spitting cobras, a venom which causes instant pain, and a delivery system which enables the snake to spray the venom to a distance of up to 2.5 metres towards the eyes of anything which comes too close, suggests a defence mechanism, rather than hunting weaponry.They have also suggested that early hominins may have played a part in this convergent evolution by attacking the snakes.
In a Bangor University News item, Dr Wolfgang Wüster explains:
Here’s a further strong piece of evidence to illustrate that similar evolutionary challenges often generate the same solutions. Even though we studied three different cobra groups, which evolved in different locations and at different evolutionary time periods, each evolved the same defensive mechanisms in the face of a threat.The researchers published their findings in Science yesterday.
All cobras have venom components that cause tissue destruction, called cytotoxins. But in spitting cobras, adding another group of toxins, phospholipases A2, has created a synergistic effect, resulting in an instantly painful venom, which can rapidly deter and even blind an aggressor.
Understanding to what extent evolution is unpredictable, almost random, or predictable, is a major question in biology. This is a remarkable example of the same problem leading to the evolution of the same solution several times – that is, predictable evolution.
Noting the coincidence of this evolving in three different locations at three different times, with the appearance of early hominins at those locations and time, the authors speculate that attacks by these archaic humans could have provided the environmental stimuli needed to drive the evolution of these components for defensive rather than aggressive purposes. Dr Wüster again:Abstract
Convergent evolution provides insights into the selective drivers underlying evolutionary change. Snake venoms, with a direct genetic basis and clearly defined functional phenotype, provide a model system for exploring the repeated evolution of adaptations. While snakes use venom primarily for predation, and venom composition often reflects diet specificity, three lineages of cobras have independently evolved the ability to spit venom at adversaries. Using gene, protein, and functional analyses, we show that the three spitting lineages possess venoms characterized by an up-regulation of phospholipase A2 (PLA2) toxins, which potentiate the action of preexisting venom cytotoxins to activate mammalian sensory neurons and cause enhanced pain. These repeated independent changes provide a fascinating example of convergent evolution across multiple phenotypic levels driven by selection for defense.
Kazandjian, T. D.; Petras, D.; Robinson, S. D.; van Thiel, J.; Greene, H. W.; Arbuckle, K.; Barlow, A.; Carter, D. A.; Wouters, R. M.; Whiteley, G.; Wagstaff, S. C.; Arias, A. S.; Albulescu, L.-O.; Plettenberg Laing, A.; Hall, C.; Heap, A.; Penrhyn-Lowe, S.; McCabe, C. V.; Ainsworth, S.; da Silva, R. R.; Dorrestein, P. C.; Richardson, M. K.; Gutiérrez, J. M.; Calvete, J. J.; Harrison, R. A.; Vetter, I.; Undheim, E. A. B.; Wüster, W.; Casewell, N. R.
Convergent evolution of pain-inducing defensive venom components in spitting cobras
Science 22 Jan 2021: Vol. 371, Issue 6527, pp. 386-390 DOI: 10.1126/science.abb9303
© 2021, American Association for the Advancement of Science
Reprinted with kind permission under licence #4993881327526
Many primates attack snakes with sticks and stones. The arrival of bipedal hominins, with both hands free for mischief, may have been just the kind of selection pressure that favoured long-distance defence through spitting and a specially adapted defensive venom.Creationists who have been fooled into beieving that the Theory of Evolution has been discredited and is a theory in crisis, about to be overthrown by the religious dogma of Intelligent [sic] Design, might like to note how the TOE is the underlying idea behind this interpretation of the evidence and how well-known changes in the environment at the right place and time fit perfectly into the theory. Nowhere do the authors need to invoke the intervention of a magic deity using magic to make things happen anywhere in their perfectly rational explanation. This is to be expected of a theory which is based on facts as science discovers them, of course.
The idea that early humans, millions of years ago, may have caused the evolution of spitting in cobras emphasises how our origins were very much entwined with the wider ecosystems of Africa and Asia at the time.
What would be more astonishing is if there were no environmental changes at those locations and times that could account for the appearance of these changes in cobra venom, since change of this nature is driven by environmental change, not magic.
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