Though I very much enjoy the taste of food I have always thought that the actual act of eating is very primitive and mildly repulsive. Described abstractly eating involves the mastication of organic substances which are then broken down in digestive acids to produce sugars that are then used to fuel metabolic activity. The mastication process involves mechanically breaking down the organic substances and mixing them with saliva and in the process the organic substance is rubbed over the taste buds in our tongues and released gasses interact with the olfactory receptors via the nasal passages.
Now compare this process with the process of photosynthesis. In photosynthesis light energy is converted to sugar with oxygen as a waste product. This process is more elegant and much cleaner than eating (eating/digestion has excrement as its waste product versus oxygen for photosynthesis). However, the naturally evolved photosynthesis we find here on Earth is not very efficient (it captures somewhere in the area of 3-6% of the energy available in sunlight) and as a result we do not find vertebrates that use photosynthesis, though there is recent evidence that salamanders have photosynthetic cells and we might have an invertebrate or two that uses it.
So is it possible that humans might be able to someday use photosynthesis? Some have recently argued that we have a moral obligation to get rid of meat eating animals and replace them with herbivores. But herbivores are carnivores as well in the strict sense. While I don’t think that eating plants is as morally problematic as eating animals I still think it would be nice to free ourselves from eating all together. At least I would like to be able to do so for myself. So might I ever be able to? As it stands it looks like it would be difficult to do because extant photosynthetic processes are relatively inefficient and so even if we did successfully integrate photosynthetic cells over the entire area of our skin we would need to be exposed to dangerous amounts of radiation to meet our energy demands. Still it is certainly possible to improve photosynthesis and in fact scientists are working on it now and so while it doesn’t seem likely to happen anytime soon it certainly seems possible that there could be photosynthetic humans at some point in the future. We might not be able to quit eating all together, we might still do it for pleasurable experiences of taste etc or we might have to eat some limited amount to supplement the energy from photosynthetic process, but still it is not physically impossible that this could happen and it does seem morally and aesthetically preferable to what we have now.
Aside from this bioethical issue I think this raises issues for those who think that there is evidence for design in the human body and it also puts new light on the problem of evil. Certainly if it is conceivable that we could produce a photosynthetic animal, human or not, then it must be possible for God to have created such a creature. But if so then why aren’t we photosynthetic? I mean God could have made it so that we run on solar energy, and even had the pleasurable taste and ‘mouthfeel’ experiences that makes eating enjoyable (perhaps different wavelengths and/or frequencies of light would produce different gustatory experiences). Doing this 1.) seems like a much better design. It is simpler and more elegant than eating is and 2.) seems much more humane. The sheer amount of suffering produced by eating meat over the course of evolution is nearly unimaginable. It seems to me that this argument from photosynthesis is as decisive as one can get in this area and I wonder why it has not received more attention…or maybe it has and I just haven’t found it yet?