When you think of plants you probably think of lush green trees swaying in the breeze, producing oxygen that keeps us alive and making life on the surface of the planet more pleasant in a hundred different ways. Plants are the foundation of the terrestrial food web, but the pathway they took to reach that position was filled with several brushes with disaster on a planetary scale.
The primary power of plants comes from their ability to do photosynthesis, a neat trick that allows a massive increase in available energy from sunlight. Previous forms of photosynthesis in a few groups of bacteria were limited by the availability of the specific chemicals they required such as sulfur compounds. Green photosynthesis that was first developed by cyanobacteria (blue green algae) made the key advancement of using common old water as one of its ingredients, converting it to oxygen in the process. Today we know oxygen is vital for our survival, but when blue green algae first started producing this highly reactive molecule it was unknown on the planet, not that different to how humans recently started pumping out ozone destroying CFC gases. The simple organisms that had evolved up to this point had no way to deal with this new toxic oxygen that was polluting their pristine atmosphere. As oxygen accumulated in the atmosphere over time these original oxygen sensitive organisms were forced to retreat into low oxygen habitats such as deep in the mud.
The accumulating oxygen also dramatically changed the chemistry of the oceans. Previously a soluble form of iron was dissolved in the water, but reacting with oxygen turned it into an insoluble form that sank to the bottom of the ocean, forming the great rusty red iron ore deposits that are now a cornerstone of our industrial society. The oxygen also reacted with the methane in the atmosphere, that had previously provided a sufficient greenhouse effect to keep the early Earth warm enough to be hospitable for life at a time when the sun’s output was significantly lower than today. Once this insulating blanket was stripped away the planet was plunged into a severe ice age known as snowball Earth. The entire planet froze solid almost to the equator and life nearly ground to a halt.
Once they figured out how to exploit the new toxin in the atmosphere the highly reactive oxygen seems to have allowed the evolution of larger and more complex cells than the tiny bacteria that came before, with new internal compartments like the nucleus. Some of these cells engulfed tiny blue green algae, forming a symbiotic relationship and transforming those bacteria into the chloroplasts we find in plants today. This new photosynthetic partnership became multicellular plants that then marched onto the land in their drive for conquest. Previously the land was only inhabited by lichens, a symbiont themselves weaving together fungi and single celled algae. While lichens slowly ate away at the rocks by secreting acids the first land plants (in symbiosis with their own root fungi) dramatically stepped things up a level by pushing roots ever deeper into the soil. These acid secreting roots remind me of the terrifying xenomorphs with acid for blood from the Alien movies. By pumping acid into the soil the plants could mobilise the minerals they needed to grow, but in the process also destabilised a massive amount of minerals to leech and run off into the oceans. Once dissolved in the ocean these minerals combined with carbon dioxide from the atmosphere, forming limestone and dolomite that sank to the bottom. The carbon dioxide had previously helped warm the earth through a greenhouse effect, and once again plants stripped this protective layer off the atmosphere and plunged the planet into another severe and prolonged ice age. When will you learn, plants?
Photosynthesis is powerful stuff indeed. You would think the caring and supportive plants of the world would have learned their lesson after nearly destroying the planet twice in a row, but they weren’t done yet. In their quest to selfishly shove other plants out of the sunlight plants started developing ways of growing ever taller. One key development was the evolution of a chemical called lignin, a tough kind of bioplastic found in the woody tissue of modern plants. This gave the more ancient cellulose found in plants more mechanical toughness, allowing plants to grow as tall as modern trees do today. There was one tiny problem however. While plants quickly became experts at making lignin rich wood there were no organisms around that could use it in any way, so it simply accumulated and lay around in great heaps, not that different to how the plastic rubbish created by industrial humans now piles up around the planet. That adds a whole new meaning to the term “leaf litter”. It took fungi and bacteria tens of millions of years to catch up and become effective decomposers of this new woody waste material. In the meantime the great rubbish layers were buried by geological activity, eventually forming the coal beds that form another cornerstone of modern industrial civilisation.
In some ways decomposing microbes haven’t completely caught up with the capacity for plants to produce waste materials. Another fate for excess plant material is fire, an issue on everyone’s mind around the world today. Debates about “natural” fire regimes aside, is this really the best end use for all that solar energy and biomass in the form of a conflagration? To paraphrase an old saying, you don’t have a surplus plant problem but a herbivore deficiency. Pre-humans learned to tap into this underutilised resource through the harnessing of fire, a key step that laid the foundation for becoming who we are today. We more recently tapped into coal and iron and transformed our society and planet through the industrial revolution, though those ancient resources won’t last forever. Will our society transform again to harness the power of the sun directly or will it go back to relying on burning biomass?
So next time you are laying in the grass and looking up at the trees, remember how their great green power was responsible for transforming the planet both for the better and the worst, depending on the timescale of the story. Perhaps humans, with their newly awakened powers of transformation, will also settle down over time to make the planet a more interesting place. Just don’t be surprised if we mess it up first a few times along the way.