Around 420 million years ago, at the start of the Devonian Period,
plants were juuuust starting to cover the land.
They were small and kind of moss-like, and rarely grew larger than a few inches.
But by the end of the Devonian, 60 million years later, giant trees,
like Archaeopteris, had spread around the world - and changed it, forever.
And figuring out how that happened started in the 1920s at a quarry in Gilboa, New York,
where workers uncovered something surprising.
Buried among the rocks and dirt were sandstone casts of what looked like
ancient trees dating back 390 million years.
This “fossil forest” influenced how scientists viewed the evolution of plants for decades.
You see, previously, they thought that the evolution of wood was a key
adaptation that was linked to being able to reach sunlight. As species diversified,
having wood helped plants grow taller and break through the canopy created by their competitors.
But in 2010, a new excavation at Gilboa threw a wrench in this old hypothesis.
Those so-called ancient trees weren’t actually trees at all - because they
weren’t made of wood, which is one of the things that makes a tree a tree.
But the forest floor was covered with roots from a different plant - one that didn’t look
like a tree at all...except its roots were made of wood.
So, instead of looking up to learn about the evolution of trees, it turns
out paleobotanists should’ve been looking down all along.
The discoveries made in that ancient forest have sent scientists down a twisting path
to understand how our planet went from the reign of algae to the rule of trees.
Plants first began colonizing the land during the Ordovician period, about 470 million years ago.
And these earliest plants had a lot to adapt to in this new environment.
The land was harsh, dry, and poor in nutrients.
This left the early plants with two options: live somewhere that was wet all the time, like a swamp,
or let their water content vary with their surroundings and develop the
ability to recover from being almost entirely dried out.
And while there are plants today that use this second option,
sometimes called “resurrection plants,” it’s really hard on a plant’s metabolism. So,
for a long time, most plants remained very small and pretty flimsy.
But things started to change about a quarter of the way into the Devonian Period.
We know this because in 2011, researchers reported a new species of plant dating back to 407 million years ago,
almost 20 million years older than the woody plants of the Gilboa forest.
By this time, many plants had developed a kind of vascular tissue called “xylem.”
This helped move water from the bottom to the top of the plant.
And pro-tip, xylem is one of my favorite scrabble words.
If you play it right you're looking at 30 points at least.
But that new species of plants had more than one layer of xylem - it had
what’s called secondary xylem, also known as wood.
Wood consists of cellulose fibers and a compound called lignin, an organic polymer.
Lignin and cellulose are major components of the cell walls of land plants,
giving them structure and support, and together, they’re what makes wood so tough.
Although these plants would probably have grown just a little higher than
your ankles - they still spent energy on creating wood-like structures.
So if these plants were so small then why bother with wood at all?
Well, probably to keep from getting thirsty.
The earliest wood seems to have been a mechanism for transporting water more effectively
than one layer of xylem could on its own.
Because, at this point in the Devonian, holding onto water was a real challenge.
Stop me if you've heard this before but
plants need water, sunlight, and carbon dioxide to photosynthesize,
and they absorb that carbon dioxide through pores called stomata.
But those pores also allow for the evaporation of water.
And! In the early Devonian, the concentration of carbon dioxide in the atmosphere was going down,
so plants needed to keep their stomata open for longer to absorb all the CO2 they needed.
And, with those pores open longer, more water could escape.
The development of wood was becoming an excellent strategy for keeping the whole plant hydrated,
because it’s really good at conducting water.
So woody tissues seems to have first appeared as an adaptation for keeping plants hydrated.
But plants didn’t stop there.
It’s now thought that wood evolved at least five
different times across different families by the Late Devonian.
It’s a pretty clear case of convergent evolution,
and as more plants developed this structure, they started exploiting new niches.
Woody roots helped plants anchor themselves more
solidly into the earth and allowed them to grow taller.
And at the same time as plants were developing woody tissues,
they were also making a few more modifications.
When early plants first came onto land, most of them reproduced by releasing spores.
This meant that they were really dependent on a wet environment,
otherwise the spores would fail to grow - the different reproductive cells they contained
needed a thin layer of water on the plant’s surface to move around and come together.
But once wood helped plants stay hydrated, some plants didn’t need to rely quite so
much on wet landscapes - and they began developing different ways to reproduce.
Take, for example, that plant I mentioned earlier: Archaeopteris from the late Devonian.
Its name means “early fern,” but that isn’t entirely accurate.
It was actually something between a fern and a group of woody plants
that exists today called gymnosperms, which includes conifers and gingkos.
And it reproduced like ferns do, with spores - but it had both male and female spores,
making it unique among its peers at the time.
The female spores were bigger and stored a food supply for the embryo,
so they’re thought to be a precursor to seeds.
And this helped Archaeopteris spread around the planet,
by making its reproduction even less
dependent on perfect environmental conditions.
Fossils of Archaeopteris have been found on every continent, including Antarctica.
It's considered one of the earliest modern trees, appearing around 370 million years ago,
and it could grow to be 30 meters tall, which made it a giant of the Devonian forests.
And unlike those plants found at Gilboa that were originally mistaken for “trees,”
even though they didn’t have true woody tissue, Archaeopteris was full of the stuff.
Their roots spread deep and wide, allowing for more effective collection of water and nutrients.
And Archaeopteris and other Devonian plants also created more habitats for animals, and more food
for them, too. And their roots broke down rocks, releasing new minerals into bodies of water.
At the same time, the root systems of all this vegetation created more
stability in the sediment, which allowed meandering rivers to form.
The cycle of decay and renewal of these plants increased the amount of nutritious soil available.
And the wood in these plants was directly responsible for a lot
of the soil generation that took place in the Devonian.
The organic component of soil, called humus, is mostly made up of broken down lignin.
And between the weathering of rocks and the amount of carbon dioxide that the trees pulled out of the
atmosphere, CO2 levels fell drastically from the beginning of the Devonian to the end.
This, admittedly, was not great for a lot of the species living in the oceans at the time.
It was probably actually a major cause of the mass extinction that happened at the
end of the Devonian...which really deserves its own episode.
But the thing is, these trees directly created the world that we live in today,
by changing both the atmosphere and the land.
So, while they helped end the world of the Devonian,
if it wasn’t for them, our world might not have gotten started.
By the end of the Devonian period,
plants had developed almost all of the traits they would need to thrive.
It would take another 200 million years for the next big plant innovation to appear: flowers.
So from the false trees of the Gilboa forest,
to the true trees like Archaeopteris, it’s
clear how big a difference the evolution of wood made in helping trees take over.
And once plants were using seeds to reproduce -
- they had the handful of adaptations that made them mostly modern: roots, wood, leaves and seeds.
So while we might think that it’s humans, or at least mammals, that have transformed the world
the most, there’s a chance we might not even be here without the trees had not put in the work first.
So thanks trees for giving us a planet we can actually live on
Guess what?! Eons is now on TikTok! It's one of my favorite things.
We’ll be posting short stories from deep time,
casual geology in the outdoors, and more!
You can find us at @PBSEons.
And thanks to this month’s treemendous Eontologists:
Sean Dennis, Jake Hart, Annie & Eric Higgins, John Davison Ng, and Patrick Seifert!
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a joke for us to read, like this one from Steph....ok brace yourselves: Why did
the archaeopteryx catch the worm? - Because it was an early bird (I’m so sorry). You should be sorry,
Steph! Early bird gets the worm...I get it.
And as always thanks for joining me in
the Konstantin Haase studio. Subscribe at youtube.com/eons for more evolutionary escapades.