When It Was Too Hot for Leaves

In the 1980s, researchers excavating plant fossils near a village in Yunnan, China unearthed a tiny specimen that would have big implications for our understanding of early land plants.

The fossil had leaves less than a centimeter in length along its stems and branches.

These were a type of leaf called a megaphyll, the kind produced in over 99% of modern leafy plants.

And it was estimated to be 390 million years old, making these the oldest leaves of their kind ever discovered in the fossil record.

But although this tiny leafy plant lived a long time ago, it wasn’t one of the first plants on land...not by a long shot.

Plants first made their way onto land by at least 470 million years ago, during the Ordovician period.

That means that for those first 80 million years, leaves as we know them today didn’t exist.

But even after fossil leaves showed up in the middle Devonian Period, they seemed to have kinda stalled - evolutionarily speaking - for roughly 5 million years.

After that, suddenly, leaves evolved in a bunch of different groups of plants, including ferns, horsetails, and seed plants.

Scientists aren’t sure exactly how many times leaves originated in plants - with numbers ranging from 2 to 9 separate origins - but they all seem to have appeared right around the same time.

So, if leaves are so essential to modern plants, what held them back?

And then what allowed them to break through, emerging in so many different groups all at once?

And what happened when they did?

Well, looks like it all comes back to how plants themselves interact with the planet’s climate - for better or for worse.

Now, we’ve talked about the first plants before, but it’s worth repeating how totally strange they were, compared to a lot of the plants we’re familiar with today.

They didn’t have roots, leaves, or tissue that could carry water and nutrients throughout the plant, which limited where they could grow.

So, the first plants were confined to moist lowland areas where water was always available.

By around 430 million years ago, in the Silurian Period, the first traces of vascular tissue show up in the fossil record in the form of a tiny plant named Cooksonia.

With the ability to transport water and nutrients, plants could lift themselves above the ground for the first time and increase in size.

But if the first plants didn’t have leaves, then how did they photosynthesize?

Well, instead of having leaves to capture sunlight, they had green stems full of chlorophyll where they converted light into sugar.

These early vascular plants were mostly made up of simple, photosynthetic stems with forked branches capped by spore-bearing structures for reproduction.

And that worked - for a while.

But a single, flattened leaf can capture 200% more sunlight than a photosynthetic stem, which means that any plant that evolved leaves during this time would’ve been able to quickly outgrow its competition.

And a simple type of miniature leaf called a microphyll did show up in a group of plants called Lycophytes somewhere around 420 million years ago.

Microphylls are small - most are no more than a few millimeters to centimeters in length.

But the biggest difference between microphylls and the leaves we’re more familiar with today is their architecture.

Microphylls have only a single vein of vascular tissue that pulls in water and pumps out sugar, which limits how long and wide they can grow.

In contrast, the leaves of all other modern plants - the megaphylls - have a dense network of veins that supply water to even the most distant parts of the leaf.

This allows them to get bigger.

For example, the modern coccoloba tree in the Amazon basin has leaves that can grow up to two and a half meters long.

Lycophytes are still around today and they still photosynthesize with the same version of miniature leaves they developed in the Devonian, they’re just much less diverse than leafier plants.

Now the first megaphylls show up 390 million years ago with that little fossil plant from China.

And, even though megaphylls can grow large, they didn’t start out that way.

For the first 80 million years of life on land, plants were basically leafless, and when they finally did evolve megaphyll leaves, they were tiny, no larger than the microphylls of lycophytes.

It was almost as if something was keeping them small.

And in 2001, a team of researchers set out to find what that ‘something’ might be, by searching for clues in the ancient Devonian fossil record.

They started by comparing the large, fossilized leaves of the Carboniferous period, when much of the world was covered in swamps and forests, to the photosynthetic stems of early Devonian land plants.

And one of the first clues they found was in the number of tiny, bean-shaped pores called stomata that they observed.

Plants take in carbon dioxide through their stomata and expel oxygen, which allows them to create sugars through photosynthesis.

Most early leafless land plants had fewer than 5 pores per square millimeter in their photosynthetic stems.

But by the beginning of the Carboniferous, when leafy plants had become widespread, there were eight times as many stomata on those leaves, as if the plants were struggling to breathe.

So, researchers focused their attention on the most likely culprit: carbon dioxide.

The atmosphere in the late Silurian and early Devonian contained 7 times as much carbon dioxide as today, and temperatures were also high as a result.

Average temperatures for the entire planet hovered around 30 degrees C in the early Devonian, which is twice as high as the current average.

But starting around 410 million years ago, carbon dioxide levels started to drop throughout the middle Devonian.

And after a short warming period between 383 and 375 Ma, carbon dioxide took a nosedive and quickly declined in the atmosphere.

When the Devonian came crashing to an end about 360 million years ago, carbon dioxide had dropped by about 90%, which resulted in two major ice ages that wiped out more than three quarters of animal species in the world’s oceans.

With such a sudden dip in the molecule plants needed to photosynthesize, their leaves became packed with stomata to suck in as much of it as they could from the air.

But stomata do more than just regulate gas exchange.

When they’re open, these pores also allow water to escape, which cools leaves down the same way that sweating cools us down on a hot day.

This can be a trade off, though - one with important consequences.

Plants can get more carbon dioxide if they keep their stomata open longer, but they also lose more water.

When you forget to water your houseplants and they start to wilt, their stomata shut tight to avoid as much water loss as possible.

And experiments have shown that stomata are inextricably linked with the amount of carbon dioxide available in the air.

If you decrease the amount of carbon dioxide in a sealed chamber with plants inside, they’ll increase the number of stomata they produce on new leaves and vice versa.

Plants in the early Devonian only needed a few stomata to take in the abundant carbon dioxide, but they also wouldn’t have been nearly as efficient at keeping cool.

As an experiment, the team of researchers asked what would have happened if a large megaphyll leaf had evolved in the Devonian with a low density of stomata.

By running models, the answer they came up with was unmistakable: these leaves would’ve heated up to temperatures way above 50 degrees C. And over that temperature, the proteins responsible for just about everything that happens in a cell start to break down.

But why couldn’t these plants just produce a bunch of stomata to keep up with the heat?

Well, it comes back to that tradeoff.

The researchers showed that even if the density of stomata wasn’t linked to carbon dioxide levels, these leaves would’ve had to be packed with so many pores that the amount of water they lost would’ve been more than the roots of early plants could supply.

So it was physically impossible for plants to produce large leaves until carbon dioxide levels fell, which is why microphylls seemed to do fine in early Devonian environments and why the first megaphylls were also small.

And when the first forests appeared 387 million years ago, sunlight began to come at a premium for plants growing on the forest floor.

So when carbon dioxide levels rapidly fell in the late Devonian, several groups of unrelated plants evolved not only bigger megaphyll leaves but also tree-like forms, shooting up to compete for space in the newly-crowded skies.

And that wasn’t the only way they changed the world.

We know that the first land plants evolved under extremely harsh conditions.

Back 470 million years ago when algae took their ‘first steps’ onto land, they did so on a barren landscape under a blistering sun.

But land plants didn’t just change to adapt to these conditions; they also changed their environment.

They weathered bare rock, creating the first soils; they developed complex ecosystems that supported the animals that followed them onto land; and they also changed the climate.

And remember that 90% drop in atmospheric carbon dioxide that allowed plants to evolve leaves?

If you’re wondering where all that carbon went, plants turn out to be one of the primary suspects.

The development of the first soils led to nutrients from land being washed away into streams and oceans.

This process not only kept carbon out of the atmosphere, it also contributed to algal blooms that would have starved near-shore environments of oxygen.

So the story of leaves is really one of some not-so-subtle feedback loops between organisms and their environment.

Climate shaped how plants evolved, and plants, in turn, changed Earth’s climate.

But it also paints a vivid picture of what happens when that feedback loop leans too far one way or the other.

The drop in carbon dioxide allowed for the evolution of leaves and complex forest communities,