Around 48 million years ago, in a warm  subtropical forest in what's now Germany,   zombies were on the march.

These zombies were ants, and their bodies and  behavior were no longer under their control.

They had become infected with the spores of  a fungus that was now growing inside them.

This fungus was actually  manipulating their movements,   forcing them to do something they’d never  ordinarily do, something strange, and specific… The ants abandoned their colony, climbed  up a nearby plant onto the underside of   one of its leaves, and bit down hard on the  leaf’s veins, locking themselves in place.

This behavior is known as the  ‘death-grip,’ and as the name suggests,   it's the last thing these ants would ever do.

Soon, the fungus sprouted – probably   from their heads – releasing spores to  infect new hosts and continue the cycle.

All that was left behind were the marks  on that ancient leaf.

It fossilized and   was published in 2011, complete with  the distinctive death-grip scars.

This single fossil leaf is the oldest known   evidence of zombified ants  – which still exist today.

Modern zombie ants even perform the   exact same death-grip behavior –  something healthy ants never do.

So we know that this creepy capability  to manipulate behavior is ancient.

But how and why did it come  to exist in the first place?

Well, it turns out the fungus’ ability to turn  ants into the walking dead may have evolved as   a response to the development of an equally  complex behavior of the ants themselves:   a thing called social immunity.

Over hundreds of millions of years,   many fungi have become specialized for  infecting insects and other arthropods.

Generally, it happens like this: a  fungal spore will attach to a host,   penetrate its exoskeleton, and  start growing within its body.

Eventually, when the fungus is ready,   it erupts from the host and releases  spores to find and infect new victims.

And if you’re an old-school sci-fi fan like I  am, then I know exactly what you’re thinking   and you are right.

These fungi are basically  xenomorphs.

And the insects that they infect are   like human space colonists whose last moments will  be watching parasites explode out of their bodies.

The insects have it a little better,   I guess, because they’re dead  before the xenomorph pops out.

Anyway, only some fungi have worked   behavioral manipulation - aka  zombification - into this cycle.

And none of them infect people, so don’t  worry – probably because they’re adapted   for infecting arthropods specifically, which makes  it really hard for them to jump to other animals.

In any case, the most well known  of these fungal puppet masters   are probably the species of Ophiocordyceps  that infect and manipulate ants.

And this manipulation has become incredibly  fine-tuned over evolutionary time.

When the ants are forced to death-grip  on the undersides of leaves, they do   it at a specific height - around  25 centimeters above the ground,   and at a specific time of day  - when the sun is highest.

This is probably to ensure optimal temperature and  humidity for the reproductive part of the fungi   to erupt and spread its spores, which fungi  often need very particular conditions to do.

Positioning the ants on the underside of a leaf   might also give the growing fungi  protection from UV damage and rain.

And directing the ants to bite  down around noon might ensure that,   by the time they die hours later, the fungi  can begin to emerge at night when it’s cooler.

Researchers have even found evidence that  these fungi have tweaked their strategy   over millions of years to keep up  with their changing environments.

See, leaf-biting is the typical  behavior of zombified ants in   tropical forests dominated by evergreen plants.

But in temperate forests filled with  deciduous plants, which shed their leaves,   zombified ants almost always perform their  death grip on twigs and bark instead.

To dig into this difference, researchers  sampled DNA from Ophiocordyceps fungi that   infect ants from as many locations as they could  - including both temperate and tropical species.

Then, they used that data to create a  family tree for these fungi, mapping out the   evolutionary relationships between the different  species based on their genetic similarity.

And this family tree suggested that,  while leaf-biting is the ancestral trait,   the shift to biting twigs  and bark evolved convergently   at least four times between  20 and 40 million years ago.

The researchers hypothesized that this  was a result of changes in climate and   environment that occurred over that period.

Because, around 47 million years ago, in the  mid-Eocene epoch - when that fossil leaf from   Germany dates to - the world was a much  warmer place and had less seasonal change.

Back then, evergreen forests went from  the equator to the far north and south,   even approaching the poles.

So it makes sense that, as the study suggested,   manipulating ants to lock onto leaves  would have been the original strategy.

But as the climate cooled during the late Eocene,  tropical and subtropical evergreen forests were   gradually replaced by temperate, deciduous  forests at northern and southern latitudes.

Zombified ants that only gripped onto leaves in   these regions would have fallen to the  forest floor when the leaves were shed.

And because the fungus seems to have  pretty picky requirements when it comes   to temperature and humidity, this  would have been a major problem… One that provided a pretty strong  selective pressure for ensuring   the ants bit onto something a little more secure.

Ok but why did such a complex ability  evolve in the fungi in the first place?

I mean, this seems like a really complicated way  for a little fungus to reproduce.

Why not just   release your spores into the air and  call it a day?

That's what I would do.

Well, the fossil record doesn't  really give us a clear answer.

The oldest known example of a fungus parasitizing   ants is an ant trapped in amber dated  to between 45 and 55 million years old.

And it clearly shows a fungus  erupting from the ant's body.

Specifically, from its butt.

Which is  probably the most embarrassing way you   could be immortalized for eternity.

That's  high on the list of ways I do not want to go.

As amazing as this fossil is though,  it can’t really tell us how or even if   the fungus was controlling  the ant’s behavior back then.

It might have just infected and   killed the ant without manipulating its  behavior, like many species do today.

But we can use DNA again to give us insight into   the history of ant-zombification -  this time, right back to its origins… In a paper published in 2019, a team of  researchers analyzed the DNA of over 600   species within the order of fungi  to which Ophiocordyceps belongs.

This more extensive family tree suggested  that the modern group of Ophiocordyceps   that zombifies ants can trace its origins  to an ancestor that infected beetle larvae.

Now, Ophiocordyceps infect  many species of beetle today,   but those fungi don’t manipulate  the behavior of their hosts at all.

So the researchers hypothesize that what  led to the emergence of zombification   was the fact that the fungi switched hosts, from  solitary beetles to insects like ants and wasps,   because – and this is the important  part – those insects are more social.

Highly social insects like  ants have some impressive group   strategies to defend themselves against pathogens.

And one of those defenses is what’s  known as social immunity, the collective   defenses against disease that have developed  within social groups – like ant colonies.

See, ants live in very close quarters and don’t  have much genetic diversity within a colony,   and those are perfect conditions  for the spread of pathogens.

So a pretty strictly enforced public health  system has evolved to keep the colony safe.

Ants groom each other to clean off spores,  quickly remove or destroy dead bodies, and if   an individual shows signs of infection, they’re  often kicked out of the colony or even killed.

And this system of social immunity  is pretty effective at stopping the   growth of fungus from hosts  that die inside the colony.

It takes hours to days after the host dies for  the fungus to emerge and disperse its spores,   and by then the ants have usually  destroyed or discarded the corpse.

But by manipulating the host to leave the colony  and die at a spot nearby instead - usually   close to a busy foraging trail - the fungi  could evade the colony’s social immunity.

And this would ensure that there’d be enough  time and the right conditions to emerge.

Now there are still many mysteries to  be solved surrounding zombie ant fungi,   like how exactly they take control of their hosts   to manipulate their behavior.

We  just don’t know how they do that.

But, this ability clearly has ancient roots,  stretching back tens of millions of years.

Over that time, it has changed to  suit the fungi’s changing environment,   as well as their changing hosts.

And it’s this ongoing arms race between pathogen  and host that seems to have led to the emergence   of zombification in the first place, as the  perfect counter to a complex behavioral defense.

Which is pretty messed up, when  you think about it.

I mean,