Humans Driving Evolution: Did Cobras Evolve in Response to Us?

Sam Westreich, PhD

“Aargh, rock throwers! Time to turn my offense into defense!”

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Pictured: somebody concerned about these hairless apes wandering around.Photo byPhoto by sippakorn yamkasikorn on UnsplashonUnsplash

Snakes: they are scary to many, along with spiders, scorpions, and other creatures that don’t follow the typical four-limbed body plan. We see them in stories and cartoons, and perhaps the most recognizable snake is the cobra.

The cobra: flaring its hood to scare and intimidate, bearing its fangs and getting ready to spit venom. It’s certainly threatening to us!

And a paper published last year in Science suggests that this may be by design.

These scientists looked at the composition of cobra venom, compared to that of other snakes, as well as the behaviors of the spitting cobra. From these observations, they drew an interesting hypothesis about why cobras picked up these traits.

Let’s take a quick look at how a cobra operates, and then see what the scientists propose.

How can a cobra spit?

There are many venomous snakes in the world. (For anyone curious about the difference between poisonous and venomous, it comes down to who is eating whom. If you eat the snake and you die from its flesh, the snake is poisonous. If the snake bites you and you die from poison in its fangs, it is venomous.)

Cobras are a group of snakes with the ability to flare their neck ribs in order to form the appearance of a hood. They’re found in southern Africa and throughout Asia.

Most cobras cannot actually spit venom. Only about twenty species are able to “spit,” although all cobras are venomous. For the non-spitting species, they simply inject the poison through hollow tubes in their fangs. Bite the prey, hit the bloodstream with the fangs, inject venom. Once done, it’s just a matter of time before the prey drops dead, ready to become a meal.

Spitting cobras, unlike other non-spitting species, have smaller holes in their fangs as well. When threatened, they compress muscles behind their venom glands, increasing the pressure with which the venom is ejected and causing it to spray out of the fangs in a fine mist.

So technically, it’s not spitting, the same way we do it with our lungs and cheeks. It’s more like covering the end of a hose with your thumb in order to shoot out a thinner stream of water.

And cobras can propel that stream of water up to 2.5 meters (10 feet) away from themselves!

Why evolve the ability to spit?

What’s particularly interesting about the spitting ability is that it’s evolved independently, multiple times; at least three separate populations of cobras in Africa and Asia, different species, have all evolved the ability to spit their venom.

And when did this ability to spit evolve? In African cases, right around the time that our early ancestors were diverging from other primates, around 7 million years ago. In Asia, meanwhile, spitting didn’t appear until about 2.5 million years ago — which happens to roughly coincide with the time that Homo erectus, our ancestor, migrated to that continent.

Most primates show an instinctual hatred towards snakes. If a wild monkey or ape sees a snake, it will usually show immediate fear, and may attempt to kill the snake if possible. (Fascinatingly, this seems to be a learned behavior; monkeys raised in captivity that have never seen a snake before are not scared of it, unlike their wild relatives.)

The fear makes sense; that snake is dangerous. Best to kill it before it can hurt you.

And for our ancestors, walking on two legs and with the ability to use rudimentary tools (like thrown sticks and rocks), killing snakes became easier, since we had the range advantage. Keep away from the fangs, throw a rock at it.

Snakes needed a countermeasure. And spitting seems to be the trait that proved successful.

What about the venom?

In the Science paper, the researchers also took a look at the venom that spitting cobras produce, compared to the venom of non-spitting cobras. It turns out there are some significant differences.

Specifically, the spitting cobra venom had higher levels of a protein called phospholipase A2, or PLA2. This protein isn’t painful to humans on its own, but when combined with the natural toxins in the venom, it greatly amplified their effects in activating pain neurons.

In other words, spitting cobra venom isn’t any deadlier than that of non-spitting species — but it hurts a lot more. That’s a useful trait if you’re projecting the venom out as a defensive measure, rather than using it offensively to kill your prey.

This is also why cobras rear up and sway back and forth. They’re trying to improve depth perception, to better gauge the distance to their target.

After all, they need a direct hit, because…

What’s the danger of getting spat on?

Cobra venom is cytotoxic, meaning that it kills cells. Specifically, it targets neurons. By killing neurons, we are no longer able to breathe, and our heart stops receiving the signals to keep beating.

But this only works if the venom gets into the bloodstream. What happens if it’s just sprayed out through the fangs?

It’s not likely to kill you. It can’t be absorbed through the skin, and while it could cause issues if you swallow it, that will only happen if you have any exposed cuts, ulcers, or sores inside your upper digestive tract. (Still not a good idea to eat it, though.)

Snake venom will only really cause you any lasting irritation if it gets into your eyes. It won’t absorb through the eyes into your bloodstream and kill you, but it will kill the outer cells in the cornea of your eyes, which can cause scarring and may even lead to permanent blindness if not treated.

In most cases, when people received cobra venom spat into their eyes, they recovered after medical treatment with no lasting effects.

But they certainly stayed away from the snake, instead of sticking around to try and kill it; the venom still accomplished its goal.

In summary: cobras may have evolved spitting in response to human rock throwing

The most fascinating part of this hypothesis, to me? We always tend to think of the world as (largely) outside our influence, at least when it comes to evolution and the development of wild animals. Yes, we’ve bred captive dogs to take all shapes and sizes, and we have cows that simply do not exist in nature any longer. But we haven’t really changed wild animals…

(Did you know that the wild ancestor of the cattle we raise is extinct?)

But in spitting cobras, we see an example of how early humans did just that. They posed a significant threat to cobras, to the point where multiple species of cobra, independently, evolved the ability to use their venom as a defensive measure to keep themselves safe from angry, attacking early proto-humans.

So next time someone argues that humanity’s actions don’t affect wild animals, suggest that they consider the spitting cobra. They probably wouldn’t be around, if it weren’t for our ancestors.

And if you ever encounter a cobra, keep your eyes safe!

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A microbiome scientist working at a tech startup in Silicon Valley, Sam Westreich provides insights into science and technology, exploring the strangest areas of biology, science, and biotechnology.

Mountain View, CA
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