An article in a recent issue of Science discusses the origin of photosynthesis. "Try to picture the world without photosynthesis," the story begins. And it's difficult to do. Everything green and lush and live depends upon the ability of certain cells to capture and use the energy of sunlight. (Well, almost everything. At deep-sea volcanic vents, colonies of organisms tap into the heat energy of the Earth.)
Every high school student learns the basic equation of photosynthesis: Carbon dioxide plus water plus sunlight yields carbohydrates and oxygen (with all the C's, H's and O's appropriately balanced). The equation doesn't begin to convey an appreciation for the complex reactions that connect one side of the reaction to the other. That little arrow in the science book disguises a complexity that rivals a modern petrochemical factory. Crucial to the process, of course, is a boxy molecule -- chlorophyll -- with a magnesium heart and a long tail. Atomic electrons in the molecule are bumped up in energy by sunlight. As they return their bounty, they energize reactions that create intermediate products called ATP and NADPH, which then move along the assembly line. When all is said and done, it is sugar that appears at the factory door -- where non-photosynthesizing animals and fungi wait to appropriate their share.
The article in Science discusses current ideas for how all this chemical processing might have evolved -- mainly by looking at the simplest energy-trapping bacteria we find on the planet today. When did it happen? At least 2.4 billion years ago, and maybe much earlier. "Looking so far into the past is difficult," says Science. "The geological record for that time is skimpy and tricky to interpret. Eons of evolution have blurred the molecular vestiges of the early events that remain in living organisms."
I read somewhere that humans currently command between one-third and one-half of the products of all terrestrial photosynthesis, as food for ourselves and our domesticated animals, or for fuel, building material, and clothing -- the lion's share, we might say, except that the lion gets slim pickings. One twig on the tree of life finds itself sucking an inordinate proportion of the living sap that keeps the tree alive.
Will the tree survive our outsized depredations? Of course. But it may not be the tree we know and love today.