It's a green, green world outside my window, a patchwork of green as far as the eye can see. No wonder they call this the Emerald Isle. No wonder the Irish everywhere wear green on Saint Paddy's Day.
Billions of years ago single-celled organism were living a hardscrabble life in the sea, scrounging for food, for bits of carbohydrate brewed up catch-as-catch-can by random chemical reactions. Hunting and gathering. Hanging on for dear life.
Then some lucky line of cells pieced together or appropriated what surely turned out to be the most important chemical process ever contrived in the history of life -- a way of absorbing sunlight and using the energy to cobble together carbon dioxide and water to make -- Ta-da! -- energy-packed sugar. Food! Inexhaustible food. The invention of agricultural by one-celled PLANTS.
All the microbes in the sea benefited, even the ones that couldn't do photosynthesis themselves. Even the ones without that necessary agent chlorophyll.
And here's the thing: the new chemical reaction absorbed sunlight in the red and violet ends of the solar spectrum. Leaving green behind. I look out my window and I'm seeing leftover sunlight. Table scraps.
There is a certain purple bacterium that lives in salt lakes, called Halobacterium halobium, that accomplishes a primitive sort of photosynthesis using a pigment other than chlorophyll. The pigment absorbs light in the middle (green) part of the solar spectrum. This leaves reflected red and blue light at opposite ends of the spectrum to give the bacterium its purple color. H. halobium may have appeared earlier in the history of life than organisms that use chlorophyl. It is not unreasonable to imagine that they might have remained the template for all successive plant life, in which case even land plants might be purple. Would I like that? Living on the Amethyst Isle? Wearing purple on Saint Paddy's Day?
Well, it happened otherwise. Green is the color of the planet's livelihood. Green is nine-to-five, nose-to-the- grindstone, earning one's keep. "In summer, greenness is cheap," said Thoreau. He was simply applying the law of supply and demand. Green is the ubiquitous badge of a planet plugged into a star.