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 wasnât 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 didnât 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 arenât 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 planetâs climate - for better or for worse. Now, weâve talked about the first plants
before, but itâs worth repeating how totally strange they were, compared to a lot of the
plants weâre familiar with today. They didnât 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 didnât 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 wouldâve 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 weâre 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, theyâre 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 didnât 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 âsomethingâ 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 worldâs
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 theyâre 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, theyâll 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 wouldnât 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 wouldâve 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 couldnât 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 wasnât linked to carbon dioxide levels, these leaves wouldâve had to be
packed with so many pores that the amount of water they lost wouldâve 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 wasnât 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 âfirst stepsâ onto land, they did so on a barren landscape under a blistering
sun. But land plants didnât 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 youâre 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 Earthâs 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, which then helped trap even more carbonâŚ.ultimately
leading to a mass extinction. Before you go, we wanted to invite you to
participate in PBS Digital Studiosâ annual audience survey. Your feedback really helps us understand what
our audience is interested in, so we can give you more of it. You even get to vote on potential new shows! Thereâs a link in the description below. If you have a few minutes, weâd love your
input. Thanks! Thanks to this monthâs unbeleafably amazing
Eontologists : Mikail Afridi, Colton, Annie & Eric Higgins, John Davison Ng, Jake Hart
and Sean Dennis. By becoming an Eonite at patreon.com/eons you can get
fun perks like submitting a joke for us to read, like this one from Ed Borasky Where do kingfishers, cormorants and penguins
hang out? At a dive bar! A dive bar? Yeah, I like that one And as always thanks for joining me in the
Konstantin Haase studio. Subscribe at youtube.com/eons for more evolutionary
escapades.
I already watched it yesterday. It was quite interesting to learn about. An overall good episode from PBS Eons! đđđź