The Best Color

Happy Saint Patrick's Day! Yes, the best color is indeed green. It is the color of this text, it is the color of this blog, it is the color in the middle of the visible spectrum, it is the color of plants, it is the color of spring (which is of course the best season), it is the color of nature, it is the color of life! 

The etymology of green involves the proto-indoeuropean root "ghre-" for "to grow", and the OED notes that "the associations with verdure, freshness, newness, health, and vitality are widespread among the Germanic languages." GREEN IS THE BEST!

But really, why are plants green anyway? Because plants use chlorophyll for photosynthesis, and chlorophyll absorbs and uses red and purple but reflects green. (Incidentally, if it weren't for chlorophyll and photosynthesis, there wouldn't be so much oxygen around, and we wouldn't be around, so green really is the color of life!)

Ok, but why is chlorophyll green? Why doesn't it use the entire visible spectrum instead of just the edges of it? Scientists don't know for sure, but there are three major hypotheses that seem the most reasonable (compared to others that just sound like excuses):

1. In the same way that we can't take in 100% of the oxygen in the air we breathe (it would be toxic) and don't release all the energy in glucose when we undergo cellular respiration (it would be explosive), using all the visible spectrum might involve dangerous energy levels that would damage cells.
2. Evolution is not engineering. Having evolved chlorophyll and photosynthesis (which, let's be honest, work pretty well as is), it might not be trivial to make the switch without passing through suboptimal levels. In general, this evolutionary constraint explains many differences between man-made and biological machines.
3. Certain archaea use retinal to utilize solar energy, and retinal does absorb the middle of the visible spectrum (these appear purple in color). If these evolved first in the ocean, the first cyanobacteria/algae might have lived under this purple film and thus been restricted to only using the tail ends of the visual spectrum, resulting in the chlorophyll we know today. 

Anyway, I wonder if it would make a difference to us--if plants were black or purple instead of green, would we love green as much as we do now? Would black and purple seem less dark? Or would we just like trees less? These answers lie in the realm of neuroscience and psychology (and perhaps philosophy), but until they're answered, I'll just say that I'm very glad our world is green. :D