Spitting cobras shoot venom from their fangs eight feet away into the eyes of anyone who gets too close. If they do reach it, the poison is so painful that it immediately deters the victim and may even blind them. What appears to be a ruthless attack is actually a defensive strategy that, as revealed by the magazine “Science” this Thursday, has evolved in three different places and practically at the same time, when the first human ancestors began to roam the Earth ago millions of years. A clear example, say the authors of the study, of how natural selection can give the same solution several times to similar problems.
The work, led by the University of Liverpool (United Kingdom), is based on analyzes carried out by the Institute of Biomedicine of Valencia (IBV), the center of the Higher Council for Scientific Research (CSIC). Using complementary mass spectrometry strategies, the researchers participated in determining the venom composition of three lineages of cobras that independently developed their ability to spit it. They are snakes of the genus Naja that live in Africa and Asia.
The analyzes showed that the three different groups of spitting cobras had independently increased the production of PLA2 toxins or phospholipases, a type of enzyme present in the venom of snakes, spiders, and other insects, and which has toxic effects in mammals. They result in an instantly painful poison. Previous studies suggested that differences in snake venom are largely due to variations in diet. However, the new analysis points to a different mechanism, the need to defend oneself, as the cause of these unique pain-causing poisons. For the vast majority of snakes, venom is used primarily for predation, to defuse or dispatch a possible meal. In this case, the venom targets specific sensory tissues and is the only harmful long-distance defensive adaptation among nearly 4,000 species of snake.
The evolution of the more painful venom allows these types of cobras to defend themselves more effectively against predators or aggressors by spitting venom into their eyes, causing pain, inflammation, and even blindness. “This reveals that the composition of the venom of spitting cobras has been modified to favor the defensive function,” says Jose Calvete, director of the IBV’s Evolutionary and Translational Venomics Laboratory, the only one in Spain. That each independent lineage has developed the same solution to defend itself represents an exemplary case of ‘convergent evolution’ in the natural world, the researchers note.
The international team studied the evolutionary family tree of the three groups of cobras in the Elapidaehan family. They related the ability to spit venom with two important pre-adaptations: the elevation of the front third of the cobras’ body, which provides them with an ideal posture to defensively spit venom into the eyes with great precision and, second, the existence of cytotoxins into the poison before the ability to spit evolved.
But there is something even more interesting. Molecular dating suggests that African cobras began spitting about 6.7 million years ago, shortly after the lineage diverged that led to humans on the one hand and bonobos and chimpanzees on the other. In Asians, this behavior evolved later, about 2.5 million years ago, approximately at the same time as the arrival in Asia of our ancestor Homo erectus. In the third species, it cannot be dated beyond having occurred less than 17 million years ago.
“Many primates attack snakes with sticks and stones. The arrival of bipedal hominids, with both hands-free for mischief, may have been the kind of selection pressure that favored long-range defense, with spit and specially adapted defensive poison, “explains Wolfgang Wüster, from the Faculty of Bangor University Natural Sciences and project co-investigator. In his view, this idea “emphasizes how our origins were closely intertwined with the broader ecosystems of Africa and Asia at that time.
The authors hope that the study will serve to develop better remedies against this type of poisons, whose potential victims are farmers and children in rural settings in Africa and Asia. According to the World Health Organization, between 1.8 and 2.7 million cases of poisoning caused by snakes occur each year, mainly in Africa, Asia, and Latin America. This amounts to nearly 100,000 deaths and triple the number of amputations and other permanent disabilities, condemning victims to marginalization and poverty.