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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. 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.

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SOURCES:
https://nph.onlinelibrary.wiley.com/doi/10.1111/nph.14975
https://www.cambridge.org/core/books/abs/climate-change-ecology-and-systematics/terrestrial-green-algae-systematics-biogeography-and-expected-responses-to-climate-change/438694D1D2435BAE81DDF807F0375E7F
https://www.sciencedaily.com/releases/2011/04/110417214202.htm
https://bmcecolevol.biomedcentral.com/articles/10.1186/1471-2148-11-104
https://academic.oup.com/plphys/article/172/1/533/6115716
https://www.sciencedirect.com/science/article/pii/S0960982220304188
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5462063/

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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. Wren is a website where you can  calculate your carbon emissions,   then offset it by funding projects like providing  clean burning fuel and cookstoves to refugees   in Uganda or a project that uses satellite  monitoring and drones to detect deforestation   in the Amazon Rainforest.

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