Cobras.

Taipans.

Sea snakes.

Mambas.

In an evolutionary sense,   much of the animal kingdom has been  at war with them for a very long time.

They’re all part of a family  of snakes called elapids that   evolved around 37 million years  ago and spread around the globe.

And as they did, they brought  with them some adaptations that   suddenly made the world a much more  dangerous place for the rest of us.

Specifically, a type of venom that  targets an ancient Achilles heel   found in the nervous systems  of basically all vertebrates.

Faced with these snakes, species all  over the vertebrate family tree found   themselves locked in a desperate  evolutionary arms race to cope.

For some, the rise and spread of venomous  elapids was just another challenge to adapt to.

For others, it was a catastrophe  of almost apocalyptic proportions.

And we humans are no exception, because  it seems that, when elapids slithered   onto the ecological scene, not  even our ancestors were safe… Despite having so many well-known  species, the family Elapidae is   only a relatively recent addition to the snakes.

While snakes as a whole have been  around for well over 100 million years,   molecular analyses date the earliest  elapids to around 37 million years ago,   with the oldest fossils dating  to just 25 million years ago.

But once they arrived on the scene,  elapids diversified at some of the   highest rates ever seen among reptiles  and rapidly spread across the world.

Of the 600 or so species of  venomous snakes that exist,   about half of them are elapids -  including many of the most deadly species… From coral snakes in the Americas, to black  mambas in Africa, to king cobras in Asia,   to, like a dozen different kinds of snakes  in Australia, because obviously, Australia.

And a big part of their success probably came  down to the sophistication of their weapons.

See, elapids didn’t invent venom –  that’s an ancient trait in snakes   that may have even been present in  their four-legged lizard ancestors.

But they did turn venom into an artform.

Many other snakes have solid  fangs at the back of their   mouths that can deliver only small  amounts of venom at low pressure.

Elapids, on the other hand, no, no...

They have hollow, syringe-like fangs at the front   of their mouths that can deliver  much more venom at high pressure.

And while some of the other families of  snakes with front fangs, like vipers,   usually have relatively slow-acting  venom that ravages the victim’s cells   and tissues, the venom of elapids  is generally much more insidious….

It’s mostly neurotoxic, causing rapid  paralysis and death by interfering with   the body’s ability to communicate with itself.

The venom’s neurotoxins do this  by binding to specific nervous   system receptors that are deeply  rooted in vertebrate evolution.

These receptors are shared by species across  the family tree, like a molecular achilles heel.

When the neurotoxins bind to these receptors, they  can no longer send nerve signals around the body.

Suddenly, this disrupts basic  functions like movement and breathing,   and death can occur in hours or even minutes.

Armed with this especially  deadly combination of traits,   the rise of elapids triggered the  unfolding of an evolutionary arms race… One that occurred in parallel around  the world, as different vertebrate   groups that were unlucky enough to run into  elapids suddenly had to find a way to cope.

And in 2023, researchers found perhaps  the strongest signal of this ancient   struggle in one especially  hard-hit group: Caecilians!

And despite the fact that my name ends in a  vowel, I'm not talking about the kind that   talks with their hands [Sicilians],  like my dad's side of the family.

I'm talking about the way it's  spelled this way [Caecilians]...

This order of legless amphibians may  look snake-like, but they’re actually   a much older group than snakes, so it’s more  accurate to say that snakes are caecilian-like.

Either way, despite the fact that these two  groups have some superficial similarities,   the researchers’ analysis showed that they  are definitely not on good evolutionary terms.

Because, by analyzing the nervous system receptors   of caecilian lineages from around the world,  the researchers found something incredible… Around the time that elapids began to  diversify and spread, different caecilian   lineages convergently evolved resistance to  their neurotoxic venom at least 15 times!

This happened in parallel all  over the planet - from ancient   caecilian populations in the Americas,  to those in Africa, to those in Asia.

But they didn’t all converge  on the same kind of resistance.

Some lineages have a mutation  that basically blocked the   toxins from reaching the receptors at all.

Others changed the physical  shape of the receptors,   which made the toxin no longer fit – like  trying to open a lock with the wrong key.

A third set of lineages switched  the electromagnetic charge of   their receptors, essentially repelling the toxin.

Fiendishly clever... And eight species had more  than one of these mechanisms.

The scale of this evolutionary resistance  to venom was completely unprecedented.

The selection pressure that caecilians faced  must have been pretty extreme for them to have   convergently evolved resistance so many times, in  so many places, and in multiple different ways.

They were probably succumbing  to elapid venom left, right,   and center in an absolute  caecilian apocalypse of sorts.

And it’s easy to see why.

Their worm-like anatomy would likely have  made them especially vulnerable, as would   their ecological niche of burrowing, as elapids  are the major snake predators in this niche.

The global carnage must have been so intense  that individuals with random mutations in their   receptors that decreased the effects of the venom  were consistently favored by natural selection.

These lucky survivors repopulated the world after  the elapids devastated caecilian populations.

And their modern descendants have receptors molded   by this ancient struggle that is  still playing out across the world… Apart from one specific corner of  the globe, that is: the Seychelles.

These islands separated from  India around 65 million years ago,   tens of millions of years before  the first elapids evolved.

And to this very day, no elapids  have ever set foot there…or belly?

So the lucky caecilians there have  never encountered elapids and their   venom in their entire evolutionary history.

And sure enough, the researchers found  that the caecilians of the Seychelles are   the only ones whose receptors show no signs  of having adapted against neurotoxic venoms.

They just completely sat out the arms race – if   you can call it that when no one  involved has, like, literal arms.

Now, while caecilians show the most  extreme signal of this arms race   with elapids found so far, they were not alone.

Their particular battle was just  part of a broader evolutionary   war that many other vertebrate  groups have also been fighting.

While no other group has been found to  have convergently evolved resistance   on so many independent occasions,  similar mutations have been found   in the receptors of animals like meerkats,  mongooses, and, of course, honey badgers.

Honey badgers don't care.

And in another recent paper, researchers  stumbled upon something pretty unexpected… It turns out that we too have some anti-elapid  tricks up our evolutionary sleeves… By studying the same nervous system receptors in  various primates and testing their susceptibility   to neurotoxic venoms, scientists were able  to identify some resistance adaptations.

And they found that resistance to elapid venom was   strongest in the last common ancestor  of chimpanzees, gorillas, and humans,   giving these species a greater ability to resist  neurotoxic venom than any other primate group.

This suggests that, of all the  primate lineages they looked at,   it was ours that had been  most vulnerable to elapids… Probably cobras specifically, which are  abundant in the same areas that we evolved in,   are active in the daytime like our ancestors were,  and readily bite bigger animals when threatened.

So it’s easy to imagine how this  evolutionary battle may have played out.

As our ancestors began spending less time  in the trees and more time on the ground,   they would have come into contact with  neurotoxic snakes like cobras much more often.

And this selection pressure favored  individuals with receptors that helped   them survive these encounters, and later  gifted us with at least partial resistance.

Now to be absolutely clear,   this does not mean that you’re immune to  the venom of cobras and other elapids.

This is not permission for you to  go out playing with them, okay?

They look like friends, but  they are not your friends.

You should still treat them with the  caution and respect they deserve.

It just means that we’re better able  to cope with neurotoxic venom than   other primate groups that have  had less exposure to this danger.

Like, for example, the lemurs of Madagascar  who have never coexisted with elapids and show   no signs of any mutations in their  receptors that confer resistance,   exactly like the caecilians in the Seychelles.

And, weirdly enough, our arms race with cobras may   have also driven them in a new  evolutionary direction, too… Some researchers have proposed that, by  learning to poke them with long sticks   and throw stuff at them from a distance,  we hominins may have inadvertently caused   some cobra lineages to evolve ranged  weapons, becoming…spitting cobras!

That's on us!

So yes, we might be responsible for  the evolution of spitting cobras,   a hypothesis we explored in another episode.

While there’s still a lot to learn about the  ancient, global arms race that the rise of elapids   triggered, what we’ve figured out so far shows  us just how interconnected all living things are.

A single group can cause an evolutionary  shock to reverberate across the tree of life,   impacting species from legless  amphibians to great apes,   changing them down to the  molecular level, forever.

And the more we explore the spread  of this successful group of snakes,   the more we see that our evolutionary  journeys are a complex, entangled hissssstory.