What’s in it for me? Learn how plants have influenced major changes in the environment.
In his 1985 book Science Made Stupid, writer and artist Tom Weller said: “The evolution of plants is an important chapter in the history of life.
However, it’s a pretty dull chapter, so we’ll skip it.
” These blinks will show you that nothing could be further from the truth.
Snapshots from 400 million years ago up to the present day will give you insight into the fascinating, interconnected history between plants, animal life, and the climate.
By better understanding how things like mass extinction events and global warming happened in the past, we’ll be more prepared to tackle our current environmental situation.
In these blinks, you’ll learn why leaves got bigger 375 million years ago; how giant insects came to be; and what might have caused a large hole in the ozone layer.
Plants developed leaves to cope with a decrease in carbon dioxide, thereby kickstarting the evolution of animals and insects.
From the hottest deserts to the coldest Arctic tundras, plants are everywhere.
Their diversity is staggering – just compare the smallest dandelion to the tallest fir – but so, too, are their similarities.
You probably know that nearly all plants convert carbon dioxide and sunlight into energy through a process called photosynthesis.
And there’s one key evolutionary development that makes photosynthesis far more efficient: leaves.
The key message here is: Plants developed leaves to cope with a decrease in carbon dioxide, thereby kickstarting the evolution of animals and insects.
The first plants were completely leafless – and for the first 40 million years of their history, they did just fine.
For decades, this remained a mystery to scientists; even in evolutionary terms, this is a very long time.
But the latest research reveals a missing piece of the puzzle: the levels of CO2 in the atmosphere.
Plants absorb CO2 through tiny pores called stomata, located on the surface of their leaves.
The number of these stomata varies with CO2 levels.
When there’s plenty of it, plants don’t need quite so many pores.
And when CO2 becomes more scarce, the number of stomata increases.
So plants are constantly adjusting to the changing environment.
Older leaves can even communicate with younger ones, telling them how many stomata to make.
And understanding stomata can explain how leaves evolved.
Approximately 375 million years ago, CO2 levels in the atmosphere plummeted.
This meant that plants needed more stomata in their leaves to capture the same amount of CO2.
As a result, their leaves got bigger.
Plants adjusted to the new environment and became abundant.
That, in turn, paved the way for an evolutionary boom of animal and insect species.
But what brought about this dip in CO2 levels to begin with?
Well, it’s plausible that plants themselves were responsible.
Plants can disrupt what’s called the long-term carbon cycle – a process that manages the exchange of CO2 between rocks, oceans, and the atmosphere.
Fungi and the roots of plants actively removed CO2 from the atmosphere.
As carbon dioxide levels continued to drop, plants needed ever bigger leaves.
This set off a feedback loop, which spurred on the triumphant march of leafy plants across the planet.
A spike in oxygen levels supersized all living organisms.
About 100 million years after the dip in CO2 levels helped plants develop leaves, another substantial atmospheric change occurred.
It kick-started a period of supergrowth.
Many organisms – plants, insects, even early animals – grew in size.
Swamplands became home to centipedes a meter or more in length, and ancient relatives of today’s foot-long clubmosses grew up to 40 meters tall.
To understand this growth spurt, we need to look into the environmental conditions that made it possible.
The key message here is: A spike in oxygen levels supersized all living organisms.
Between 1877 and 1894, paleontologist Charles Brongniart was working in central France.
He unearthed lots of fossils of huge plants, animals, and even insects – including a dragonfly whose wingspan was wider than most modern computer monitors.
And they all came from the same era, the Carboniferous Period, which began about 300 million years ago and lasted for another 50 million years.
Brongniart and his contemporaries struggled to explain why exactly Carboniferous organisms became so big.
Hypotheses mushroomed, and even today we’re not quite sure.
One compelling theory points to atmospheric pressure as the culprit.
In the past, it may have been much higher, which would have resulted in increased air density.
Wings work more efficiently in dense air – so for insects, this would have meant spending less energy.
And with plentiful resources, they grew bigger.
But why was the pressure higher in the past?
Scientists think the only possible answer is that, at some point, either nitrogen, oxygen, or both gases became more concentrated.
Evidence for this comes from a technique that allows scientists to measure oxygen levels in ancient rocks.
This analysis shows that oxygen levels peaked at 35 percent around 300 million years ago.
They then fell to a suffocating 15 percent 200 million years ago, before creeping up to our current level of 21 percent some 25 million years ago.
All these changes match neatly with paleontological data.
When oxygen levels spiked, all living organisms became bigger.
And when they dropped, giants went extinct – like they did at the end of the Carboniferous Period.
But what caused this oxygen spike?
Well, oxygen is closely linked to plant evolution.
Photosynthesis releases plenty of oxygen.
Normally, when organisms die and decompose, this oxygen gets used up.
But not always; some oxygen can’t be reclaimed.
Over the course of millions of years, it all added up.
Oxygen levels rose, and so, too, did the atmospheric pressure.
It wasn’t oxygen alone that created these Carboniferous giants, but its increased concentration did contribute to the world in which they thrived.
Destruction of the ozone layer caused mass extinction for some and genetic mutations for others.
So rising oxygen levels can create giants.
But what happens when oxygen levels go down?
Well, at the end of the Permian Period, approximately 250 million years ago, oxygen levels dipped to their lowest level ever – to just 15 percent.
That event wiped out 95 percent of all species on Earth.
How exactly did this happen?
For years, researchers who studied animal fossils lacked key details; the puzzle seemed incomplete.
But in the early 2000s, Dutch paleontologists shifted the focus from animals to plants.
And then it clicked: the largest mass extinction event in history could have been associated with changes in the ozone layer.
The key message here is: Destruction of the ozone layer caused mass extinction for some and genetic mutations for others.
The Dutch team analyzed fossils of conifers preserved in rocks in East Greenland.
The trees were surrounded by lycopsids – a type of short, green plant with spiky leaves.
As the forests died out, lycopsids quickly colonized new ground.
The unlikely success of these plants lies in a strange mutation which, counterintuitively, sterilized them.
They could no longer germinate like most plants.
Instead, they developed a survival mechanism that gave them the upper hand over struggling contemporaries: asexual reproduction.
Without the need for a sexual partner, lycopsids could capture new ground even as other plants became ever rarer and, eventually, died out.
Similar mutations were later found in other fossils from the same period.
And, increasingly, one thing became clear: while many species were being destroyed, others were living through a period of rapid genetic change.
But what brought about all these mutations?
One possible answer is the destruction of the stratospheric ozone layer, which protects life on Earth from ultraviolet radiation.
This, in turn, may have been brought about by volcanic eruptions.
A lot of volcanic eruptions.
Half a million years’ worth of them, to be exact!
These volcanoes ejected material which, in effect, pressure-cooked Siberian sediments of coal and salt, setting off a chemical reaction that produced organohalogens – substances that can obliterate the ozone layer.
Without the protective ozone layer, ultraviolet radiation could now reach Earth.
And that’s precisely what may have precipitated the extinctions and mutations discovered in Permian fossils.
Increasing CO2 levels made temperatures go up, which caused ancient life to go extinct and dinosaurs to flourish.
The Permian extinction wasn’t the only one to hit our planet.
Between the Triassic and Jurassic Periods, 200 million years ago, the third-worst mass extinction took place.
It eradicated a fifth of all marine animal families and a quarter of all land animal families.
What caused this event is disputed; theories range from a meteorite impact to volcanic eruptions.
But the latest research points to global warming.
The key clues come from the study of plant fossils, and they suggest that the late-Triassic extinction began with an increase in carbon dioxide.
The key message here is: Increasing CO2 levels made temperatures go up, which caused ancient life to go extinct and dinosaurs to flourish.
Evidence for this theory came from an unlikely place – plant fossils, which were collected almost 100 years ago.
In 1925, a British botanist called Thomas Harris traveled to Greenland to dig for samples of ancient Arctic flora.
Harris’s samples lay nearly forgotten, until, 70 years later, they again went under the microscope.
The leaves he’d gathered revealed something very important: reduced numbers of stomata.
As we’ve learned, stomata shrink when CO2 levels rise.
At the end of the Triassic Period, atmospheric CO2 levels appear to have tripled in just a few hundred thousand years.
Before this spike, large-leaved trees needed a lot of stomatal pores to keep cool.
As carbon dioxide became more abundant, trees got rid of some of them.
But the greenhouse effect of CO2 meant that global temperatures rose.
With fewer stomata, plants could no longer stay cool.
Eventually, ancient large-leaved trees overheated and died out.
Meanwhile, other plants with smaller leaves pulled through and took over.
But what caused the upsurge in carbon dioxide?
One theory suggests that volcanic eruptions helped to release greenhouse gases that had remained trapped on the floor of the Arctic ocean.
These gases are known as methane hydrates.
So, here’s a scenario which scientists now consider very plausible.
Giant volcanoes threw rocks and noxious gases into the atmosphere.
This process destabilized the methane hydrates.
Methane turned into CO2, which acidified the oceans.
Coral reefs died, along with many other organisms.
The oceans became warmer – and this meant that water could now carry less oxygen.
Marine life literally suffocated, and many land animals struggled to adapt; those who couldn’t, perished.
This extinction freed up resources for the animals that could survive.
Dinosaurs were about to take over.
Deciduous trees became dominant in northern polar forests by growing fast and dying young.
When you think about the North or South Poles, you probably don’t imagine lush forests.
But, as we now know from the fossil record, these areas of our planet were once home to an abundance of plant life.
Something about those plants, though, always puzzled biologists.
Why did most of the trees near the North Pole evolve to shed their leaves, or, as scientists would say, become deciduous?
And why did the majority of trees growing near the Antarctic remain evergreen?
For decades, biologists believed that the deciduous trees had a superpower – they thought the trees could become dormant.
The idea was that a plant could sleep by “turning off” its leaves, so to speak.
In doing so, the plant would consume less energy, and, therefore, have a better chance of survival.
But there are two main problems with this theory.
First, computer simulations of virtual forests show that shedding leaves in the high northern latitudes is actually a very costly strategy.
Getting rid of the leaves costs about 20 times more energy than keeping them.
And second, evergreen trees also lose their leaves – just at a slower pace.
So the whole picture becomes a lot more complicated.
The key message here is: Deciduous trees became dominant in northern polar forests by growing fast and dying young.
So how did deciduous trees outpace the evergreens in the northern polar forests?
The answer may lie in how different types of trees grow.
Evergreens become larger slowly and steadily.
Bit by bit, their biomass increases throughout the year; the rate of their growth varies with temperature and available sunlight.
But in the ancient world, Arctic summers were much warmer.
Deciduous trees learned to make good use of the short summer months.
They evolved to photosynthesize quickly, and that ability sent such plants on a growth spurt.
However, over the course of a year, energy costs averaged out.
Deciduous and evergreen trees ended up on even footing.
There were other environmental factors that helped deciduous trees take the lead as well.
One of them was forest fires.
In the past, these natural fires were very frequent; even today, fires rage through the Arctic forests of North America every 50 to 60 years.
After each fire, deciduous trees can repopulate the territory right away.
Evergreens, on the other hand, struggle to keep up.
Over the last 30 years, winters in Alaska have warmed significantly.
As the snow melts, the soils and vegetation under it will be exposed.
And if polar trees and shrubs start growing again, this will accelerate climate change.
50 million years ago, greenhouse gases warmed up the Earth – resulting in a climate vastly different from today’s.
Plant fossils from 50 million years ago tell us that our planet was much hotter back then.
But why?
How did the Arctic and Antarctic reach such mild temperatures?
And why are things so different now – why are the tropics so warm and the poles so much colder?
In the past, the most popular explanations involved the oceans.
Some scientists suggested the culprit was a change in how water circulates around the Earth; others argued that planetary cooling all but stopped as the Pacific Ocean absorbed more and more atmospheric heat.
New research contradicts these theories.
Instead of blaming the oceans, it points to a rise in greenhouse gas levels.
The key message here is: 50 million years ago, greenhouse gases warmed up the Earth – resulting in a climate vastly different from today’s.
When we talk about the greenhouse effect and greenhouse gases, most people probably think of carbon dioxide.
But less prevalent gases, such as methane, ozone, nitrous oxide, and water vapor, are actually much more effective at trapping heat.
The Antarctic ice sheet has been accumulating signs of atmospheric change for more than 740 thousand years.
And its ice cores store all the evidence of that change.
By studying a core sample, scientists came to the conclusion that rising CO2 levels cause a warmer climate, which leads to higher methane concentrations and, eventually, global warming.
Plants play an important role in this process.
50 million years ago, wetlands and tropical forests were much larger than they are today.
Mild temperatures led to more plant growth, which meant more food for animals.
As these plants and animals died, they fed microbes, which put various greenhouse gases into the atmosphere.
On their own, each of these greenhouse gases didn’t have what it took to create such a big climate change.
But together, they magnified the warming and created a feedback loop.
So how did the climate leave this “hothouse?
” Why is our planet so much cooler today?
The change may have been due to another drop in CO2 levels.
As levels of this gas in the atmosphere dipped, plants became less abundant, and the feedback loop ran out of fuel.
Glaciers formed, and the sea level sunk by about 100 meters.
Between 30 and 40 million years ago, CO2 levels took one final dive, and the climate flipped.
The Arctic and Antarctic froze, while the tropics became, well, tropical.
A different method of photosynthesis helps some plants thrive when CO2 levels are low.
As we’ve already learned, most plants rely on photosynthesis to convert CO2 and sunlight into energy.
But, surprisingly, not all plants photosynthesize in the same way.
About 30 million years ago, tropical grasses started developing a better way of doing it.
Their method has since been taken up by at least 7,500 other plant species.
The key difference lies in the acids they manufacture during photosynthesis.
These acids are based on molecules consisting of four atoms of carbon – that’s one carbon atom more than in other plant acids.
These more efficient species are sometimes called C4 plants, and they make up a fifth of all vegetation on Earth.
So how did they evolve?
And how does their photosynthesis process work?
The key message here is: A different method of photosynthesis helps some plants thrive when CO2 levels are low.
The C4 upgrade happened after levels of CO2 dipped millions of years ago.
At the time, most plants relied on a CO2-rich atmosphere.
But when CO2 levels fell, C3 plants found themselves at a disadvantage; they were accidentally capturing oxygen molecules instead of CO2.
And this squandered energy.
C4 plants, on the other hand, could keep running at full capacity.
But there was a price to pay: they could only live in the subtropics.
So, we know that fluctuating CO2 levels may have played a hand in the evolution of C4 plants – but these levels had always fluctuated.
In fact, there was a CO2 dip three million years before C4 plants first appeared.
So something else must have happened to usher them onto the scene.
One theory is forest fire.
Remember that most C4 plants were grasses?
They were much more flammable than trees and other plants, but they also recovered more quickly.
As forest fires raged and trees burned down, grasses stepped in to fill the gaps.
The rising temperatures, the low CO2, the fires, and the grasses all became entangled in an elaborate feedback web that accelerated climate change.
So how does this affect the world in which we live today?
Well, some of the key plants driving our economies, like sugarcane or maize, are C4-based.
And such plants may also help us in the future.
By 2050, our planet will have nine billion people to feed.
Scientists have been trying to engineer better versions of C3 crops – like rice, potatoes, wheat, or soybeans.
What if we could give these plants the C4 advantage?
They could grow faster and feed more people, without negatively impacting the environment.
Final summary
The key message in these blinks: Plant fossils show us how small changes in the environment can make the entire world vastly different.
Spikes and drops in CO2 levels can set off transformations in the atmosphere, which are reflected in all plants, animals, and microorganisms on Earth.
The planet has seen giant floras and faunas created by high oxygen levels, and mass extinctions caused by climate change.
A lot about these transformations remains a mystery – but with new evidence, we’re beginning to understand much more about our planet’s past and how it affects us today.
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What to read next: The Incredible Journey of Plants, by Stefano Mancuso If you’re interested in further exploring the evolution of plant life, Stefano Mancuso has even more fascinating stories about plants and the environment.
In The Incredible Journey of Plants, you’ll dive deeper into how they adapt, invade new habitats, and survive – and even learn how certain plants ended up where they have today.