The Remarkable Mystery of Land Plants

This episode is sponsored by Wren, a website where you calculate your carbon emissions. You can also sign up to make a monthly contribution to offset your emissions or support rainforest protection projects by clicking our link in the description. 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 that 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, you know, they're just about everything else. Trees, flowers, that 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 just a little closer to land compared to marine habitats. It could also be that 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 relative to land plants, a group of streptophyte algae called Zygnomatales that include 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 Aegiria 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 cook stoves to refugees in Uganda, or a project that uses satellite monitoring and drones to detect deforestation in the Amazon rainforest. We'll need a lot of different approaches to stop the climate crisis, and this is one way that you can learn more about your carbon contribution and take some action. You'll answer a few questions about your lifestyle, and they'll also show you ways you can start reducing your carbon emissions. No one can reduce their footprint to zero, but using Wren you can help offset what you have left. Once you sign up, you'll receive updates from the tree planting, rainforest protection, and other projects you support. And we've partnered with Wren to plant 10 additional trees for the first 100 people who sign up using our link in the description. Before we go, we'd also like to say thank you to each and every one of our patrons. Some of their names are on the screen right now, and these are the people that make this channel and videos like this possible, and we are so grateful. If you'd like to become one of them, you can go to patreon.com slash journey to micro. If you'd like to see more from our master of microscopes, James Weiss, you can check out Jam & Germs on Instagram. And if you'd like to see more from us, there's probably a subscribe button somewhere nearby.