What better day to celebrate the green than St. Patrick's day. And what green is more important than chlorophyll?
Chlorophyll is only found in two places those being green bacteria and chloroplasts, and as it turns out those are one and the same. Diverse evidences indicate that chloroplasts had their origin as a cyanobacterium (blue-green algae) that became a cellular slave rather than lunch. This particular symbiosis has been rather successful, in fact that bacterium that became a chloroplast is one of the most successful organisms in all of life history. And this success has had its impact on life.
The particular type of photosynthesis found only in cyanobacteria and chloroplasts liberates oxygen as a by-product because if obtains hydrogen from water. One of the more remarkable insights of biology is that all of the free oxygen in our atmosphere, the oxygen that makes aerobic respiration possible, the oxygen we think of as a life-giving gas, accumulated as a by-product of photosynthesis. And of course oxygen was toxic because life was anaerobic.
That much oxygen doesn't accumulate overnight, and diverse evidence suggests that oxygen-liberating photosynthesis has been around for 3.2 to 3.5 billion years.
So let's hear if for the green of chlorophyll and the most successful organism of all time, a cyanobacterial cellular symbiont! I'll drink to that.