Wednesday, January 11, 2017

Evolution: Using the Hand You Are Dealt

A mistake common among both “creationists” and “evolutionists” is that their preferred mode of origin produces perfection: an adaptation, whether created or evolved, is the best possible solution to the challenges of existence. The exceptions to this view are so numerous that I believe no one could list all of them. I just wanted to tell you about a recently-published example.

Those of us who are enchanted by the beauty of photosynthesis, all the way from the deep emerald color of chlorophyll a to the utter transformation of Earth that photosynthesis has wrought in the last three billion years, are tempted to think that it is a perfect process. I can hardly contain my enthusiasm for photosynthesis. But, for all of its elegance and global importance, photosynthesis has several flaws. One is that the light-absorption reactions consist, for no good reason other than evolutionary history, of two cycles rather than just one. Somewhere back in time two different bacterial systems merged together into the chloroplast photosynthetic system that covers the Earth today so much that large parts of the planet appear green from outer space. I suspect that, had a Designer made photosynthesis, this Designer might have made a single, efficient cycle rather than smooshing to previous cycles together. But perhaps the most noteworthy limitation of photosynthesis is rubisco.

Meet rubisco. You gotta love it. About half of the water-soluble protein in a leaf is rubisco. It is an enzyme that removes carbon dioxide from the air and fixes (attaches) it to other molecules, which will ultimately become sugar. This is the major short-term process that removes carbon dioxide from the air and almost the only process that creates food upon which all the food chains on Earth depend. That is, rubisco is a carboxylase. But it is also an oxygenase. Oxygen molecules can get into rubisco and crowd out the carbon dioxide molecules. This starts a whole cascade of reactions called photorespiration. Rubisco does not react very much with oxygen, but oxygen is over 5000 times as common in the air as carbon dioxide, so it turns out that photorespiration significantly inhibits photosynthesis—by as much as one-quarter. If only rubisco were not such an inefficient carboxylase, the world would be a lot greener—probably over 30 percent greener. Forests would probably grow 30 percent more biomass, although deserts and tundra, limited by water and temperature, might not look very different. Most physiologists consider rubisco to be the rate limiting step in photosynthesis, the slow guy that holds everything else up. Come to think of it, this is probably why there is so much rubisco. Each molecule is so slow that chloroplasts have to make a whole lot of them just to get the job done.

But rubisco is not the only game in town. There are apparently at least five other carboxylases that are found in cells. That is, the genes for them already exist, but are not used in the most common form of photosynthesis. And they are all more efficient than rubisco.

Thomas Schwander and his colleagues in Germany have devised an artificial pathway of carbon fixation that they call the CETCH pathway (read about it here and here). While it would be difficult to insert the enzymes of this pathway into living plant cells (in vivo), they are working on a commercially viable industrial system that removes excess carbon dioxide from the air and makes them into organic molecules. This system is not just a little bit more efficient than a system based on rubisco; it is thirty-seven times more efficient!

A Designer would have built photosynthesis on something like the CETCH pathway; or, who knows, maybe something even more efficient that the Designer would be able to think of. But evolution uses whatever hand of cards it is dealt. At the time and place when the prevalent modern form of photosynthesis evolved, rubisco was ready and available to be conscripted for that job. And today the natural world is pretty much stuck with it.

Photosynthesis is no different from any other biological process in being the result of an evolutionary pathway that consists of lucky adaptations. Your DNA is not as efficient as it could be. It is filled with dead genes and dead viruses and repeated elements from nucleotide duplication that went a little crazy. Your DNA is not like an orderly house or office. It is like an attic, or the offices of some of my professorly colleagues, in which piles of papers totter in corners and occasionally fall over but in which they can eventually find the papers they need. And almost all the food in the world (unless you live at volcanoes at the bottom of the sea) comes from photosynthesis, which could be a lot more efficient if only it had not been designed by the brainless process of evolution.

1 comment:

  1. And why? Rubisco evolved in an anaerobic world and the production of oxygen as a PS byproduct was its own undoing.