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Somewhere around 470 million years ago, something happened that shouldn’t have
been particularly striking. An algae found its way onto land.
On its own, that’s not an unusual event. Over billions of years, many different types of algae
have been able to find their way from their freshwater or marine homes,
and carve out new lives in niches they found on land.
But something was different about this algae,
something special. It would take the algae on an evolutionary journey
through millions and millions of years, changing our whole world in the process.
This algae turned the lands of this earth green,
altered the chemistry of our atmosphere, and created homes for future life.
This algae would give rise to all of the land plants we know of today.
Land plants are incredibly diverse. They’ve had all those hundreds of millions of years
to get that way. Some have trunks, some have flowers, some are low to the ground.
Even in a small patch of forest, the variations can seem endless.
But all of those plants belong to a single clade called the Embryophyta.
They’re divided into two broad categories. There are the bryophytes,
which lack the vasculature to grow tall. These include mosses and liverworts.
And then there are the tracheophytes, which do have vascular tissues that carry water
and support their growth. These are, well, just about everything else.
Trees, flowers, the plant climbing up the corner of your room.
All land plants are descended from an ancient member of a group
that’s still around called the Streptophyte algae. But what was so special about this algae?
What enabled it to cultivate lands and take them over to an extent that no other algae could?
We don’t know. It’s a fundamental mystery of plants, because their existence
is just that remarkable.
Land is a stressful place to settle down on, especially for an algae that is making its way
out of water. It would have had to contend with so much more sun than before, unfiltered and
unrefracted by water. And that would put the algae at risk for damage to its cell and its DNA.
On top of that, land is dry. It’s easy to take water for granted when
you live in a pond or an ocean. But land makes you even more dependent on the weather to provide
moisture, and on the morsels of water you can gather from your environment.
So whatever algae made its way onto land, it had to have contained incredible potential,
an enormous capacity to contend with the stresses
of an environment that was fundamentally different from the home it was leaving.
While we don’t know exactly what that potential looked like, scientists do have some theories.
One idea is that this mysterious ancestral algae may have been able to cheat a little…
that it was somewhat predisposed to life on land because maybe it originally lived in freshwater,
where conditions are a little closer to land compared to marine habitats.
It could also be that the streptophyte algae just got here first. That maybe it beat other algae to
the punch, and that because it was first, it had very little competition as it adapted to
the new challenges it was facing. It could thrive without having to struggle for space or resources,
taking over the land and developing new structures
that would give it an advantage over any other algae that tried to come onto land later.
Those are a few general theories about what might have enabled the streptophyte algae to
adapt readily to land. But scientists also wonder about the specifics—the details of the algae’s
appearance that would have morphed into the traits that allowed embryophytes to dominate land.
And with the genetic tools available today, scientists have made progress on understanding
more of this mysterious past. They’ve identified the closest living relatives to land plants,
a group of streptophyte algae called Zygnematales that includes this spirogyra. And
scientists have used this relationship to form new hypotheses around ancient family traits.
Plus, there are many, many, many plants in the world whose present-day forms
reveal the messy web that evolution has spun.
Take, for example, the stomata. You can see them here, the microscopic openings that help the plant
take in carbon dioxide and release water. The stomata is the kind of adaptation
that is key to plant survival on land, creating many tiny valves that tune the plant’s interior
to best match whatever the changing world around it demands. Scientists have even
found stomata-like openings on plant fossils dating back more than 400 million years ago.
Not all modern day plants have stomata though, including some bryophytes like liverworts.
So scientists had to dive into the phylogenetic history woven into plant DNA to figure out that
the first land plants likely did have stomata, and that bryophytes may have reduced their stomata
over time to the point that some even lost their stomata entirely.
It’s all a bit like looking at your family members today to try and figure out whether an ancestor
millions of years ago had dimples. The results are a clue into one small feature of an ancient
algae that made its way onto land, a sign that it may have had its own ancestral form of a stomata.
And if that’s the case, then perhaps an ancient bryophyte inherited that stomata
only to eventually lose it through many cycles of change and selection. It would hardly be the first
plant to take its inheritance from that remarkable algae and then decide to do away with it.
These cells belong to Egeria densa. You may have seen it before. It’s a plant
commonly found in aquariums. And yet this is an embryophyte, a member of the land plants.
It just happened to have found its way back into the water,
leaving behind the land that its unknown ancestor once conquered.
It can be difficult to piece together why anything in evolution
happens the way it happens. It’s easy, with hundreds of millions of years of hindsight,
to describe these events with some sort of inevitability.
After all, we live in a world covered in plants. We live because of a world covered in plants.
To conceive of our world without plants would be to conceive of a world without us as we are today.
But maybe the point of all this is that hindsight is a convenient story we tell ourselves,
and that none of this was inevitable.
An algae came onto land, and it changed the world. The rest, as they say, is history.
Thank you for coming on this journey with us as we explore the unseen world that surrounds us.
And thank you again to Wren for sponsoring this episode of Journey to the Microcosmos.
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