I recently caught an episode of the new BBC epic documentary series Earth. It tells the story of our planet - from the early solar system to the present day. That’s a lot of ground to cover. The episode I stumbled upon looked at the emergence of land plants - and the potentially planet-ending series of events they might yet lead to.
Today, plants appear in almost every niche, from steamy jungles to sparse mountain sides, deserts, grasslands and even cracks in the pavement. They are almost infinitely adaptable - and great at sticking around. But it wasn’t always that way. While photosynthesis emerged in the oceans 3 billion years ago, the first land plants took 2.5 billion years longer to become established.
Then, around 470 million years ago, protoplants came out of the seas during the Cambrian Explosion, a period of intense evolutionary change that led to increasingly complex lifeforms. For another multi-million year period, the exact duration of which is up for debate, plants could not venture beyond coastlines and estuaries. They were used to getting their nutrients from water and the land was just bare rock - soil hadn’t been formed yet.
However, the land was already home to fungi, which could absorb minerals from bare rock. To survive, early land plants adapted to absorb minerals from fungi, paying them back with carbohydrates made through photosynthesis. This sybiotic relationship was Earth’s first ecosystem. Both organisms benefitted, with fungi the early winner. Fossil records show that some could have grown up to 8m tall! Plants hadn’t even evolved roots yet.
But then they did... first as tiny hair-like appendages that wore away at the surface of the rocks beneath. Over another many thousands of years, this made land-based minerals more available than ever before. Generation upon generation of rotting plants provided organic matter on which their descendents would become larger and more complex. An evolutionary arms race led to the development of wood for taller and taller trees. The world had become a gigantic jungle.
And so to the plant Kingdom’s first near miss with global catastrophe. As they colonised Earth’s early continents, plants became a massive carbon sink, taking more CO2 from the atmosphere than was being produced. CO2 concentrations plummeted - in geological terms - from 6000 - 9000ppm down to ~300ppm in less than 100 million years. The average global temperature fell from 25 - 30°C to 10 - 12°C. Ice caps, absent for the best part of 250 million years, reemerged and began to advance across the continents, leaving smaller and smaller ice-free areas.
This was enough to eventually shut down the world’s main carbon sink - its swamp lands. With the ice receding at the ‘last minute’ Earth narrowly avoided a ‘snowball event.’ A new burst of plant and animal development then arrived, with the emergence of dinosaurs, early mammals and - eventually - humans. At the same time, plants bloomed like never before - literally - with the first flowers observable in the fossil record around 150 million years ago. Their symbiotic partners - flying insects - also take off at the same time.
But the story of the swamps is not finished. As part of their efforts towards maximum evolutionary fitness, many trees in these regions had developed an exceedingly robust outer protection system, similar to today’s cork. It was supremely effective at absorbing CO2 and almost entirely indestructable upon death. This led to piles upon piles of carbon-rich material that simply would not decompose. Over hundreds of millions of years of tectonic pressure it was transformed into an entirely new kind of rock - coal.
And so to the plant Kingdom’s second - and mankind’s first - brush with planet-wide catastrophe. Without coal, the fuel of the Industrial Revolution (and other fossil fuels), humans would have remained constrained within the carbon cycle, with a far lower impact on our environment. As it is, in as little as 200 years, we’ve released carbon sinks that took hundreds of millions of years to form, with CO2 levels now rising at least 10 times faster than ever before. We are simultaneously removing the remaining carbon sinks and eating away at biodiversity.
There is a lot of talk about ‘saving the Earth’ but, as these ~780 words hopefully convey, there have been several different ‘Earths’ over the years. We are now on the precipice of another. Science shows that reforestation would be a great place to start if humans want to be part of it. If we do cause our own demise through climate change, the geological record suggests that something could step up to restore Earth’s balance. My money’s on the plants...
