The Argument from Photosynthesis

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?

9 thoughts on “The Argument from Photosynthesis

  1. Evolution does not necessarily achieve the optimum solution out of all possible solutions. It just achieves the easiest one as compared to the advantages and disadvantages an organism already has. It doesn’t need to be the best, it just needs to be better than what came before. Similarly an intelligent designer just needs to choose a solution that works for his purposes. The designer might like unnecessarily complex solutions. Why? Well you’d have to ask it. This is why intelligent design as an argument is a non-starter. To understand how it might have resulted in the way things are now, you have to refer to information we have no access to. We could assume it away, that the designer likes “elegant” solutions, but then you have to explain what does “elegant” mean. Then once you have defined that you have to say something about why you want to assume that the designer would care about elegance.

    But it does make some sense to assume that God dislikes suffering and wishes to minimize or eliminate it. That is what religion tells us, so that’s where this argument is of some use because we can sidestep the whole “Why would the designer have certain goals or desires and how do you define them” question. If suffering is something God wants to prevent, then he should have made all animals photosynthetic, or at least if they are to eat, only eat non organic materials like minerals or silt or something. Any matter can be converted into energy. As a matter of fact, why wouldn’t we get all the nutrients and energy we need from just breathing, if the designer can design anything he wants? Why are there more than two supply lines from the environment?

    • Good points David. I agree with you about the arguments for intelligent design, but I think that this actually works for my argument. Traditional theists have appealed to notions like simplicity, beauty, and elegance as traits of God’s creation. I would guess it is because they identify God as a rationally (and morally) perfect. So, for those that do this counts as evidence against their view. But I agree that many give up this conception of god and that this wouldn’t rule out any kind of design.

  2. There’s an engineering issue at work here – what is the limit in terms of efficient energy conversion/hording through photosynthesis. I can’t say this as a fact, but it does not seem feasible to be able to move to the extent we do on solar energy alone.

    But IF it were feasible, I would say that that would be a crushing argument against any design notions.

  3. Hi Tomer, I agree the engineering question is really interesting. I am not an expert on this stuff at all but the post I linked to in my original post had some calculations that are relevant so I’ll reproduce it below:

    The average human being has 1.8m2 of skin, approximately half of which would be exposed to the sun (if naked and lying as you would if tanning). The Earth is bathed in much energy from the sun, but of that solar radiation only the wavelengths from 400-700nm are usable by plants (termed photosynthetically active radiation, or PAR). Even at midday on a very sunny day, the PAR energy flux density (or, the amount of plant-usable light energy per unit area of ground per second) is only 400W/m2 (Warrington, 1978). Photosynthetic efficiency (amount of light energy converted into usable chemical potential energy) typically is about 3-6%, so let’s assume 5. So the energy produced by a human being lying in the sun for an hour (3600 seconds) at midday would be:

    400 J/s/m2 x (0.5 x 1.8m2) x 0.05 x 3600s = 64800J = 64.8kJ (or 15.43 kcal)

    By comparison, an apple has about 400kJ of usable food energy. So an hour in the sun is about the same as a sixth of an apple. The daily energy requirements for a human being sit around 10,000 kJ per day, so that’s going to require 150 hours per day of sitting in the sun. Needless to say, that’s impossible.

    But notice that the author assumes that we can only capture 5% of the 400 w/m2…but suppose we could improve that to 20% (somewhere around the amount that solar panels currently get) then we would net 259.2 kJ…which would require just 37 hours of sunlight to get the 10,000 kJ per day we need. This still isn’t good but is better. If we could boost it up to 40% then we would net 518.4 kJ per hour and that would mean only 19 hours of sunlight. If we could get near to 100% then we would need only 8-9 hours of direct sunlight. Much better!

    And of course that assumes that we can only harvest the energy in the PAR but with the recent discovery of the so-called radiotrophic fungi we now know that it is possible to use other parts of the spectrum, and they use melanin rather than chlorophyl! So I think it is feasible that we could move as much as we do now. Also notice that the author assumes that we would need natural daylight but we could probably design much more efficient ‘charging houses’ (ok, ok, so they would be tanning salons 🙂

  4. To my surprise, I find the issues raised in this thread damned interesting. The “argument from photosynthesis”, as I’m sure you’re aware, fits a pattern common to many problem-of-evil type arguments against intelligent design: X is the case, but Y would involve less suffering; Y is (metaphysically?) possible, therefore no benevolent Designer. Pardon me for diverting the discussion from the possibility of human photosynthesis to the problem of evil in general, but what interests me is the last premise: substantive assumptions about which alternative universes are (actually) possible must be made to get the argument off the ground. In short, the problem of evil is fueled by contingencies: there must be some genuinely possible state of affairs Y preferable to actual state of affairs X. But not only this: it must also be the case that the universes in which Y is the case are better overall than the universes in which X is the case. Until we have a total theory of the universe, and know precisely what is contingent upon what, I don’t think we have a solid problem-of-evil argument. Pushed further, this line of thought yields an interesting conclusion: if it’s possible to have a total theory of the universe (that is, a theory that successfully explains everything), then there will be no contingencies (since any apparently contingent state of affairs is a consequence of the theory), and thus nothing for the problem of evil to operate on: if it’s possible to understand the world completely, then we live in the best (and worst) of all possible worlds, i.e. the only world that any Designer could have created.

    Or am I missing something??

    PS, I do hope that we live in one of those possible worlds in which human photosynthesis is eventually real

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