The origin of green

A billion years ago or longer, a photosynthesising bacterium found its way into a proto-plant cell. The bacteria and the cell became symbiotic, each helping the other to survive and thrive. All land plants today are descended from that chance meeting.

Chloroplasts inside leaf cells
Des_Callaghan, CC BY-SA 4.0, via Wikimedia Commons

Simplistic narratives of evolution paint it as an adversarial process – survival of the fittest, nature raw in tooth and claw. The reality is much more complicated. Some of the most fundamental and influential evolutionary events were not competitive, but cooperative. And plants are a great example.

Plants photosynthesise sunlight and turn it into energy. They do this using chloroplasts, little organelles inside plant cells. The chloroplasts are packed full of chlorophyll, which powers the photosynthesis process. Some plant cells have a hundred such chloroplasts; most have fewer. Those green globes in the close-up of moss above are chloroplasts.

Last century, some botanists took a closer look at these extremely productive organelles and noticed a very suspicious similarity. Chloroplasts bear a strong resemblance to free and independent cyanobacteria. They’re structured the same, and they both photosynthesise. And, what is more surprising, chloroplasts have their own DNA, mostly independent of the plant around them.

The inevitable conclusion: some time in the distant past, chloroplasts were independent organisms, a variety of cyanobacteria. A very long time ago (maybe a billion years?) cyanobacteria got into a cell’s walls… and have stayed there ever since. This was a single chance event, a single proto-plant, but all green land plants are descended from that chance event.

(Side note: there were other cyanobacteria symbiosis events, such as the one that created red algae. But green plants on land? That’s all down to a single encounter.)

This state – foreign bacteria being absorbed into a cell and becoming part of its operation – is called symbiogenesis. It’s not just plants, though. We have good evidence that the energy organelle in our own cells (mitochondria) got in there the same way. We are all, at our cellular level, products of inter-species cooperation.

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