Photosynthesis is why plants are green. But did you know there is a theory that plant-life on early Earth was once purple? Let’s have a look at why.
Plants need sunlight. We all know that. But under the hood, there’s a lot going on. Plants convert light energy (a.k.a. photons) from the sun into chemical energy (a.k.a carbon-based molecules).
So, the plant takes water, light, and CO2, and out comes oxygen and carbohydrate. Carbohydrate is then converted to other molecules. This doesn’t just happen by itself, though. To make this possible, a plant needs chlorophyll. The chlorophyll absorbs light, and it facilitates the reaction described above. It’s slightly more complicated than that, but my chemistry is rusty, and I’m not Wikipedia.
The reason a plant is green, is because chlorophyll absorbs red and blue light. And something absorbs red and blue, reflects green, and is thus colored green.
Like I wrote above, the mechanism with chlorophyll is pretty complicated. Nature has a simpler way to harvest light energy. It’s based on retinal. Retinal works slightly differently from chlorophyll. We see it mostly in bacteria.
The thing is: it is still unclear how chlorophyll plants evolved. Evidence suggest that billions of years ago, chlorophyll- and retinal-based bacteria existed side by side. And then… stuff happened.
So, enter the purple earth hypothesis. The idea is that retinal-based plants evolved alongside chlorophyll-based plants. However, since the retinal-based reaction is simpler, they evolved first. In other words, the idea is that the world was first covered by retinal plants, and then by the more efficient chlorophyll ones.
Retinal is complementary to chlorophyll in that it absorbs green light. That means it looks purple. And there your have it. The theory is that before green chlorophyll plants evolved, purple retinal plants ruled the world.
There is not a whole lot of proof of this theory. It is interesting though, and one reason this could be interesting, is to use it to research exoplanets. We look for planets with green plant-life, but maybe we should also be looking for purple planets.