The story of the rise of mammals  is one you may have heard before… 66 million years ago, an asteroid impact  triggered the K-Pg mass extinction,   wiping out the non-avian dinosaurs and  ending the so-called Age of Reptiles.

In the post-apocalyptic wasteland that  followed, our small, furry mammalian ancestors   and relatives emerged from the shadows,  and, well, the rest is natural history.

As the planet began to recover, they  rapidly diversified and replaced the   dinosaurs as the dominant large  land animals, ushering in the   Age of Mammals that continues to  this day – and led to you and I.

But there's a mystery at the heart of  this famous evolutionary turning point… Why was it the mammals who triumphed?

How did they manage to pull off such   a stunning ecological coup in  the wreckage of planet Earth?

Why didn’t the many other reptile species   that did make it through - become the  planet’s dominant megafauna instead,   keeping us mammals confined to the ecological  margins, where we had been for so long?

Why didn’t this simply mark the  start of a second age of reptiles,   after the first had been cut tragically short?

Well, perhaps it might have -  and even should have - if it   wasn’t for an unlikely ally that  some scientists believe may have   given ancient mammals a competitive edge  at this pivotal, once-in-an-era moment.

A global explosion of…fungi!

Sometimes, it’s tempting to see the story  of life on our planet as an inevitable,   linear chain of events that was just  ‘destined’ to play out the way it did.

Mammals rose to become the dominant  megafauna of planet earth after the   extinction event because…well, it was simply  our turn - and our reward for being smarter,   scrappier, and more adaptable  than our reptilian cousins.

But as regular viewers of this show will  know, evolution doesn't work like that at all.

Sometimes, things happened the way  they did for reasons that were far   from inevitable and even downright bizarre.

To uncover the factors at play, scientists  often need to get a little creative when   generating hypotheses - connecting dots that  no one had previously thought to connect.

And in the case of asking why mammals rose to  ecological prominence after the K-Pg instead   of the surviving reptiles, an especially  strange idea has emerged in recent years.

It’s called the ‘Fungal Infection  Mammalian Selection’ hypothesis, or FIMS.

And, according to its proponents, it could be the  solution to the mystery behind the Age of Mammals.

So, how exactly does the FIMS hypothesis  work?

How could fungi of all things have   been the secret ingredient behind the last  66 million years of mammalian success?

Well, our story begins at the end of the world.

When the asteroid hit, clouds of ash, dust,  and soot were kicked into the atmosphere,   blotting out the sun and causing an impact  winter that lasted for years or even decades.

Photosynthesis basically shut down  worldwide, plants withered away,   and the food chains that relied on  them collapsed like a house of cards.

Any big megafauna from the age of  reptiles that had been lucky enough   to survive the immediate effects of the  impact - like the shockwaves, firestorms,   and tsunamis - were then finished off by  the ecological collapse that followed.

Those medium and large sized stragglers eventually  died hungry, cold, confused, and in darkness.

All in all, more than 75% of species went extinct,   most likely including every land  animal bigger than 25 kilograms.

But a wasteland ain't so bad if you  actually like your land full of waste… And one kingdom of life in particular  has a notorious fondness for darkness,   death, and decay: fungi!

Seeing as they don’t photosynthesise and  generally prefer dark and cool environments,   an impact winter would have been  a pretty fun time to be a fungi.

Plus, being decomposers that feed on dead stuff,  the aftermath of the mass extinction event was   basically an all-you-can-eat buffet of dead  plant and animal matter for fungi to feast on.

While the impact winter was one  of the worst times in history   to be pretty much any other form of  complex life, if you were a fungus,   this was maybe the closest to  paradise that the world had ever been.

So, unsurprisingly perhaps, a rich layer of fungal  spores abruptly appears in the fossil record of   this time - literal hard evidence of this fungal  bloom in the wake of the extinction event.

Now, we’ve known about the fungal spore spike  at the KPg boundary for over twenty years.

But it usually isn’t seen as connected  to one of the other notable changes   in the fossil record that began  soon after the extinction event… Like the great radiation of  mammals as they took over the   medium and large-sized animal niches that  had previously been dominated by reptiles.

The fungal spike and the dawn of the age  of mammals have generally been thought of   simply as two unrelated Things That  Happened in the early Paleocene.

But in 2005, an expert in fungal immunology  proposed that the fungal bloom could have   been exactly what gave mammals the edge  over reptiles at this critical time,   changing the trajectory of life on land forever.

Now, to be clear, the FIMS hypothesis is   not yet widely accepted or even widely  known within the scientific community.

You probably won’t find a  mention of FIMS in any textbooks,   but it is based on a few key facts  and plausible, logical arguments.

First off, there are reasons to  think that reptiles should have   coped better with and bounced back faster from  the devastation of the K-Pg compared to mammals.

Because their energy requirements are  much lower than mammals, meaning they   need way less food to stay alive - a pretty  handy trait when food chains have collapsed.

They also have more offspring  on average than mammals do,   so, in theory, they can repopulate  faster following an extinction event.

And their offspring require less parental  care than mammals’, who produce generally   pretty helpless babies, for a while at least  - which is not ideal when times are tough.

The mammalian lifestyle is just so much  more expensive and complicated than the   reptilian one, so you’d think that  they would have fared better during   the apocalypse and seized ecological  opportunities much faster in its wake.

The fact that the total opposite actually happened  is the first big hint that some other factor must   have been at play - one that acted as a powerful  filter that selected for mammals over reptiles.

Enter the fungal bloom.

See, we know from modern evidence that reptiles  and amphibians, who are cold-blooded, are much   much more vulnerable to fungal infections than us  mammals, as well as birds, who are warm blooded.

Our body temperatures are simply too hot  for most pathogenic fungi to cope with… It's the reason why there are actually  relatively few fungal diseases that affect   mammals compared with the many bacterial,  viral, or parasitic diseases that threaten   us… Far fewer in fact than the many fungal  diseases that infect our coldblooded cousins.

And in most cases, the few fungal pathogens   that do infect mammals are minor  annoyances rather than life-threatening.

Mammals were pretty much just as hot 66 million  years ago, and reptiles just as cool, meaning the   fungal bloom right after the K-Pg would have been  a much worse time for them than it was for us.

The air was suddenly filled with fungal spores,  at least some of which would have been pathogenic,   infecting reptiles and their eggs en masse  while leaving us mammals relatively unbothered.

In fact, all those mushrooms sprouting  up all over the place would have been a   great new food source for mammals  when little else was available.

And while reptiles could have eaten mushrooms too,   we know that when it’s cold they’re less able  to move around, reducing their foraging ability.

They also struggle to digest at  lower temperatures - neither of   which are issues that mammals  would have had to deal with.

Put all these facts together and suddenly, the  K-Pg aftermath - with its mushroom meadows and   dense mists of spores - suddenly begins to look  like a much better time for mammals than reptiles.

If the fungal bloom offered even a modest survival  advantage for us over them, then it becomes easier   to imagine how those mammals were able to  outcompete reptiles at this critical juncture.

Mammals would have repopulated the world  in greater numbers and had a head start   over reptiles as both groups scrambled  to fill all the medium-and-large sized   animal niches that had been left open by  the demise of the non-avian dinosaurs.

And so, a second Age of Reptiles was  averted, and the Age of Mammals began.

Now, the FIMS hypothesis is hard to prove for  sure - and scientists may never be able to.

But it points to something that we  do know for sure - disease must have   played a massive role in the story of life on  earth, just as it plays a massive role today.

Seeing as it rarely leaves tangible  evidence behind in the fossil record,   though, we often overlook it when  reconstructing events from deep time.

While we struggle to know exactly what  role disease played, the only definitely   wrong answer is that it played no role, even  though that tends to be the default assumption.

And the FIMS hypothesis shows that  big radical ideas about the history   of our world don’t just come from  people digging through the dirt   and discovering new fossils and new  data that no-one had found before… Sometimes, they come from digging  through old papers and old discoveries,   but making new connections  that no-one had made before.

And if true, the FIMS hypothesis means that  we don't just owe the evolutionary success of   mammals to the innate resilience and ecological  street-smarts of our underdog ancient ancestors… Instead, we might have had a helping hand  from a brief period of fungal paradise   that kept our competition down, without  which you and I would never have evolved.