How the First Land Plants Forever Changed Earth's Atmosphere

Not many can say that they completely changed the world - that they built an empire from the ground up - and paved the way for the future of land-based life on Earth. Not many can say they did that and caused a major Extinction Event in one go. Yet, that's exactly what happened with the introduction of the first land plants, some 470 million years ago! The Earth we live on - and the atmosphere which enables us to exist - would not be as it is without the influence of the first land plants. The story you are about to hear will chill you to your roots... 

 Artistic Recreation of land plants celebrating their invasion (is this kind of cannibalism?)

The land plant invasion began... not on land, surprisingly. Before plants found their way onto land, the main photosynthetic producer of oxygen was Cyanobacteria - more commonly known as blue-green algae. You likely know that photosynthesis is an important process in which plants take in Carbon Dioxide (CO2), and release Oxygen into the atmosphere. Cyanobacteria, the sort of grandparent/parent of the first land plants, was confined to water, and was not as efficient an agent of oxygen production and carbon sequestration as land plants would be, as the water affected both the quality of sunlight and CO2 that Cyanobacteria could take in. 

An Accurate Depiction of Photosynthesis

With time, terrestrialisation began - plant life travelled from the water to the land - with land plants evolving from photosynthetic Cyanobacteria. The invasion found its genesis in wetlands, with troops starting off as small, leafless and seedless plants with shallow root systems. Since these early land plants did not have seeds, they used spores to reproduce and spread to new territory. Previously, plants could simply release spores into their aquatic environments, and have the spores transported by the currents - but outside of the water, conditions were harsh. Thankfully, land plant spores were protected by a highly resistant wall of sporopollenin, which allowed spores to brave the terrain, and further the invasion into new territories. A major threat to early land plants was the possibility of drying out, which is why the invasion began in wetlands and areas with an abundance of water. But life is an evolutionary arms race, and land plants grew more resistant to drier climates, and were thus able to colonise more of the earth's surface. Such advancement, meant that photosynthesis was occurring at a much greater scale than it had before, resulting in a significant decrease in atmospheric CO2 levels, and simultaneous increase in atmospheric oxygen levels. As you likely know, CO2 is what's known as a greenhouse gas, meaning it traps heat and increases global temperatures. Thus, it makes sense that a decrease in CO2 would result in a decrease in global temperatures - and so it did! Not only did temperatures drop, but the oxygenation of the atmosphere also lead to the diversification of life on earth!  

The plant army considered camouflage uniforms, but ultimately decided against it, as they would be camouflaged as themselves.

At this point, land plants were spreading - colonising the continents - and as they did so, they began diversifying and growing in both size and complexity. Where early land plants were small flora, tightly hugging the ground, evolution allowed land plants to grow stems, shoots, and eventually barks, climbing higher into the air to capture sunlight. Accompanying this growth, there was a need for plant organs/structures to evolve so that such heights could be supported. Root systems thus grew more complex - not only drawing nutrients from the soil, but also providing stabilisation for the growing flora. Major evolutions include the development of vascular tissue - which allowed for better absorption of water and minerals - and the first plants with seeds. During this period of colonisation, Earth's first forests developed, highlighting the extent to which the land plant invasion had grown. With land plant colonisation occurring at such a large scale, one can imagine the impact that it had on the global atmosphere. Carbon sequestration increased, and, as a result, global temperatures continued to plummet. And it is here, where an Extinction Event comes into play.  

Due to the land plant colonisation, CO2 levels plummeted, causing enough cooling to set off a major glaciation event, which caused the mass extinction of several oceanic species. The term "Snowball Earth" may give you an idea of the severity of this glaciation. All of this goes to show the incredible impact that plants can have on global climate - completely altering global atmospheres, paving the way for our very existence, and causing a major extinction event! Plants are certainly not to be messed with. 

Thank you for reading!