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! Become an Eonite at patreon.com/eons and
you can get fun perks like submitting 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.
Hey there, botanist here.
Technically, a "tree" is a type of growth habit, and we describe things as "woody" and as trees even if they don't have secondary xylem. Effectively, it just has to be big and not herbaceous. Lignin and other compounds contribute to the water proofing and rigidity in xylary cells, including in primary xylem.
Also, Archaeopteris sp. taxonomically is a fern and exhibits the tree growth habit. Furthermore, heterospory evolved multiple times.
Also, the breakdown of the first trees wouldn't occur until after the Carboniferous. The fact that nothing could breakdown lignin at the time resulted in the vast coal deposits from that time (along with the rich oxygen content in the air, frequent fires, etc). If you do an acetate peel of a ball of coal, you can actually still see cellular details of the plants that once made it up. A lot of organisms that specialize in breaking down wood are either specialized fungi or have commensal relationships with fungi in their gut flora.
A great video but just needed to air out those nitpicks.