Eons

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 didnt the many other reptile species that did make it through - become the planets dominant megafauna instead, keeping us mammals confined to the ecological margins, where we had been for so long?

Why didnt 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 wasnt 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 offungi!

Sometimes, its 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 becausewell, 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.

Its 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 dont 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, weve known about the fungal spore spike at the KPg boundary for over twenty years.

But it usually isnt 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 wont 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 youd 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 its cold theyre 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 dont 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.