Eons

On a warm summer night around 130 million years ago, in an early Cretaceous conifer forest, a bolt of lightning struck, and the smell of smoke began to fill the air.

But the fire didn't start among the towering seed plants - or gymnosperms - that dominated the plant life of the time.

It broke out below and between the trees instead, fueled by the newest additions to the forest understory: flowering plants, or angiosperms.

Helped by high oxygen levels, the fire quickly grew among the dried leaves and weedy stems of these strange, new, and very flammable plants.

Soon, the entire forest was ablaze.

Flowering plants had only just arrived on the scene, but already, they had sparked a revolution literally.

By the end of the Cretaceous Period, they would de-throne the gymnosperms to become the new dominant form of plant life on Earth.

And today, they represent up t ~90% of all land plant species.

But it seems that, for flowering plants to take over the world, first they may have had to help burn the old one away and then put those fires out.

The sudden rise of flowering plants is one of the most profound ecological shifts in the history of complex life - and one of the most puzzling, too.

Charles Darwin himself described it as an abominable mystery Flowering plants underpin most terrestrial ecosystems today, yet, according to the fossil record, they didnt even exist until relatively recently!

Over the course of a single period of geologic time - the Cretaceous - they seem to have emerged, diversified, and achieved world domination, pushing all other land-plants into the background.

So howd they do it?

Well, it was almost certainly a mix of factors - including the symbiotic relationships they developed with insects, their faster rates of growth, and their more efficient photosynthesis, to name a few.

And its also possible that angiosperms are actually older than the Cretaceous, but the fossils from earlier periods are controversial.

My point is, something big clearly changed during the Cretaceous that catapulted them into the ecological spotlight across the world.

And in 2010, two researchers proposed that a physical force may have played a major role One that could have literally cleared the way for their rise: fire.

See, weve known for decades that the Cretaceous Period was a pretty hot and fiery chapter in the history of life.

The world was in a greenhouse phase, with evidence of forests and dinosaurs thriving as far north and south as the poles, which would have been ice-free at the time.

And abundant charcoal found in Cretaceous-aged layers around the world shows that ancient wildfires were common and intense.

So it was clear that early angiosperms must have experienced a baptism by fire - but what if it had actually been a warm welcome?

And more than that, what if early flowering plants themselves had been partly responsible for fueling the flames?

This hypothesis is known as The Early Angiosperm Fire-Cycle, and it suggests that a key part of the dramatic success of angiosperms was their unusual ability to play with fire But how could delicate, early flowering plants have harnessed a force of nature to reshape the world in their favor?

Well, some of their descendants still exploit fires creative destruction today.

Modern grasslands are an inherently flammable type of angiosperm ecosystem, and that seems to often help them spread into new areas and become dominant there.

By increasing the frequency and intensity of local fires - simply by existing and being an ideal kind of fuel - invasive grasses set in motion a grass-fire cycle.

The fire burns away the areas native plants, and the grass is adapted to grow back faster than the competition - turning forests of big woody trees into plains of open grasslands.

More grasslands cause more fires, and more fires cause more grasslands its a positive feedback loop that can rapidly reshape whole ecosystems.

So maybe early angiosperms also influenced the fire cycle in a similar way, helping them invade, and eventually displace, the gymnosperm forests of the Cretaceous.

In 2021, researchers published a study combining plant fossils, charcoal abundance, and biogeochemical modeling to trace how this process may have played out.

Starting around 135 million years ago, the earliest known flowering plants appear in the forest understories for the first time.

Fossil evidence suggests that they were small, weedy, and fast growing, with short-lived leaves that dried out easily.

In theory, this would have caused a new kind of fuel to build up in the forest understory that, during the dry season, would have been highly flammable.

Coupled with rising oxygen in the atmosphere at the time, this could have begun to significantly influence the fire cycle of those ancient ecosystems.

And this is supported by the charcoal data, which starts to skyrocket a little under 130 million years ago, indicating a change towards more frequent and intense wildfires.

Now, small and fast-growing angiosperms are often the first plants to recover after major ecosystem disturbances today.

So ancient angiosperms mightve had a similar advantage in this new fire regime that helped them initially spread, start to diversify, and keep the cycle going.

Each new fire came with opportunities in the aftermath, allowing them to outcompete other plant groups and quickly establish new niches as the ecosystem re-formed from scratch.

The dominant seed plant species wouldnt go down without a fight, though.

Fossil evidence shows that around 126 million years ago, the pine family of conifers evolved thick fire-protective bark as they adapted to endure the new, more flammable world.

And by around 100 million years ago, angiosperms had diversified into larger and more complex forms like shrubs and small trees.

This brought flammability up yet another notch.

Models of fire behavior with different fuel types suggest the larger and shrubbier angiosperms would have led to more intense and destructive fires in the understory.

They could spread through the canopy more easily, igniting the treetops and jumping between them in what are known as crown fires. Frequent and intense crown fires would have been even more deadly for the older, taller seed plants, like conifer trees.

Its also right around this point, 100 million years ago, that charcoal abundance reaches its peak in the geologic record of the Cretaceous.

At this time, ancient forests were being regularly destroyed by the planets new fire regime.

And potentially, thanks to the early angiosperm fire cycle, more and more flowery ones were growing back in their place.

Plus, the fossils of these flowering plants show some important changes around this time: the density of veins in their leaves jumps way above the level of non-angiosperms.

This boosted their rates of gas exchange - allowing them to take in more CO2 during photosynthesis and release more water vapor during transpiration.

These harder-working leaves would have given them an even greater growth advantage in the fires aftermath, as they raced to out compete the slower-growing competition.

And in the late Cretaceous, angiosperms were no longer just ecological oddities - their global takeover was well underway.

In many areas, they had already become the dominant form of terrestrial vegetation, and formed entirely new kinds of plant ecosystems - some of which had fire built into their very DNA Like highly flammable open shrublands in Gondwana that emerged around 88 million years ago.

Or the earliest Eucalypt woodlands in Australia from at least 62 million years ago, a little after the end of the Cretaceous.

These kinds of flowering plant ecosystems are known as pyrophytic - they encourage fires and even rely on them to release and germinate their seeds.

Its a legacy of the tens of millions of years their ancestors spent harnessing the fire cycle to conquer the Cretaceous world.

But if the early angiosperm fire cycle idea is correct why isn't everything on fire right now?

Angiosperms have only become more widespread and diverse since the Cretaceous, yet the world is generally a lot less fiery than it was back then.

Well, according to the researchers model, thats partly because, as the Cretaceous gave way to the Paleogene Period, another new angiosperm ecosystem emerged One that began to quench the flames rather than fuel them: damp tropical rainforests.

Theyre the single most diverse flowering plant ecosystems in the world today, but they only appear in the fossil record after the end of the Cretaceous, around 58 million years ago.

And, as you might imagine, tropical rainforests are not pyrophytic, like, at all.

So for them to spread and take on their modern form, they had to put the angiosperm fire cycle into reverse.

See, the higher rates of photosynthesis and transpiration of their leaves that we mentioned earlier increased local rainfall and trapped moisture beneath the closed canopy.

In those dense, humid forests, the fuel was a lot damper and much harder to ignite, which created a new twist on the angiosperm fire-cycle.

As initial pockets of moist rainforest emerged, they set a new feedback loop in motion that began to make the world a lot less flammable Which allowed those tropical rainforests to expand further and eventually become the richest flowering plant ecosystems of all.

So it seems like part of the explosive success of flowering plants may have come down to the ways that fire worked to their competitive advantage.

And by playing with fire, flowering plants remind us that life doesnt just respond to Earths conditions, it actively shapes them.

The world we see today, from fields, to forests and rainforests, is one that rose from the ashes of ancient fires millions of years ago.