Did snakes evolve to spit venom in the eyes of humans?
Some snakes can spit venom.
Now, this is the sort of thing that I assume is common knowledge, because I assume that everyone went through a phase at age 8 where they read obsessively about snakes and other reptiles after discovering that libraries exist.
What’s that? This isn’t a universal experience? Wild :0
Well, if you weren’t a deeply weird 8-year-old child who likes snakes a lot, then you get TWO fun facts this week!
First, there are snakes that can spit venom. This is kind of crazy, because generally, venom is all about close-contact. if the toxin is delivered via a bite, it’s venom. otherwise, it’s poison. Right? Right?!
well... things can get complicated when the venom is being shot out of the tooth.
(A slightly more correct way to think of venomous-vs-poisonous is whether the toxin delivery is active or passive. If the animal as actively trying to put the toxin in you, it’s venomous. If you get the toxin in you without the animal needing to do anything, then it’s poisonous. Probably there are exceptions to this, because nature LOVES an exception, but that’s the basic gist of it)
There’s only a handful of species that can spit venom, all of which are cobras. That said, it appears that this trait evolved at least 3 different times: in Asian spitting cobras (Naja, subgenus Naja), African spitting cobras (Naja, subgenus Afronaja), and Rinkhals (Hemachatus; yes I’m counting rinkhals as cobras, because i think they deserve it).
How do snakes spit venom? Basically, they flex their head and neck really hard and shoot the venom out of a little hole in the front of their fangs, which are modified from the standard cobra fang venom-delivery-system:
the small round hole (& the angle leading to the hole) lets them spew their venom up to 2.5 meters. And to be clear: they DO aim for the eyes. Which: rude. But also: fair.
To make it even more interesting, the venom of spitting cobras is an altered version of the standard cobra venom recipe (which was already deadly) which is EVEN MORE PAINFUL. Ow.
Okay, so that’s the basics about snakes that spit venom.
Fun fact number 2: these snakes (maybe) evolved this venom spitting ability specifically to spit in the eyes of humans. Or, hominins, at least.
Why do we think this? Well, first of all, there’s the fact that it seems like this ability evolved 3 different times (at least). Rather than one species developing a venom-spitting ability, and then evolved into the different species we see today, it appears that there were 3 different times where the ability arose:
this is what we call convergent evolution. (This diagram is kind of a lot, so I over-simplified it into tree on ms paint. This diagram is NOT to-scale with the time line)
The stars are where we think the ability to spit venom arose (again: not to scale). There is actually a good amount of evidence to support the 3 different origins of this ability (which can be read about here or here).
Generally, convergent evolution happens when species are exposed to the same sorts of environments/ predators/ threats. Sometimes, a problem has one solution that just works. Dolphins and sharks look similar because they found a really good design for cruising in the ocean.
So, what problem does spitting venom solve?
Well. Around the time these adaptations appeared, there were these weird apes that started walking around on two legs and, importantly, using tools that let them attack snakes from a distance. They could extend their reach with altered sticks, or throw rocks, or tie a rock to a stick and throw that, and when the snakes bit these weapons, nothing happened, because the predator was over there, just out of reach.
Problem: the weird ape predator is too far away to bite & envenomate
Solution: throw the venom in their eyes.
(to be clear, the weird apes are hominins)
Now, this isn’t 100% certain, because palaeontology and anthropology are notoriously difficult fields in which to prove things, on account of the lack of a time machine. (seriously, the academic fights in palaeontology can get spicy).
BUT scientists noted that a) spitting venom is particularly effective against humans, and b) the ability to spit venom seems to have appeared not long after bipedal hominins arrived in their areas.
African spitting cobras: evolved ~6.7 million years ago.
Hominins (genus Pan): appeared in the area ~7 million years ago.
Asian spitting cobras: evolved ~2.5 million years ago.
Hominins (Homo erectus): appeared in the area ~2.5 million years ago.
(since rinkhals branched off so long ago that we don’t really have a good idea of how long ago they evolved the ability to spit venom; they split off from other cobras ~17 MYA, but their ability to spit venom could have arisen at any point since then. Monotypic genera are like that sometimes (ಥ_ಥ). Genetic studies pending while we Figure Shit Out)
Is it possible that the spitting venom evolved to deal with something else? Yes, it is possible. But, with the evidence we have currently, this is a hypothesis with a reasonable amount of support.
Also, I just think it’s delightful to think of snakes evolving to spit in my eye.