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Is it possible for humans to evolve to eat something like wood or stone or something similar? Any hard material you would find or any abundant resource. So is it possible that humans in the past had a chance of evolving to eat this type of thing?

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    $\begingroup$ Eat wood or stone, or live off of wood or stone? There are records of starving cultures eating dirt to subsist on the small amount of organic matter it contains. $\endgroup$
    – Cort Ammon
    Dec 1, 2014 at 15:21
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    $\begingroup$ For open-ended "is it possible.."-type questions, the answere is almost always yes, unless it violates some law of physics or logic. Since termites evolved to eat wood, and lichen evolved to eat rocks, its a safe bet that it is possible for humans to evolve similarly. Of course it would take millions of year, very contrived conditions and they may look more like termites or lichen than humans when completed. $\endgroup$ Dec 1, 2014 at 16:45
  • $\begingroup$ If you include genetic manipulation under the umbrella of "evolve", then the possibility of ingesting wood and minerals from soil (not so much stone), becomes much more reasonable. Nancy Kress addresses this possibility beautifully in her Beggars In Spain series of novels, including some unexpected social and economic aspects to such a food-chain modification. $\endgroup$ Dec 1, 2014 at 18:52
  • $\begingroup$ Wood and stone are chemically very different so "like wood or stone" is near meaningless. $\endgroup$
    – smithkm
    Dec 1, 2014 at 23:19

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Important rule of thumb is: If it burns, it is in principle possible to eat it. Remember that food is primarily a source of energy. Stones are not, they are oxides of silicon and some other elements and they are very low-energetic. So nothing can really eat stones. (If we do not consider possibility that it would perform nuclear fusion in its stomach.)

It is possible to eat wood and some animals evolved to do so. Humans eating wood are possible, but not very likely, since there would have to be really strong evolutionary pressure to do so and we would have to be lucky to find the right adaptation before we go extinct. So far, it seems there are many, many more easier ways how to find food than such a big adaptation as eating wood. I can easily imagine, that in future, we would be able to develop bacteria by a genetic manipulation that would digest cellulose and help us to digest wood. Chances for this occurring naturally seem very small to me.

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    $\begingroup$ Fun fact: Humans already do eat wood. Very finely-grained cedar wood is sometimes used as "natural flavoring" to substitute raspberries in processed food. But it doesn't provide any nutrition value. $\endgroup$
    – Philipp
    Dec 1, 2014 at 13:00
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    $\begingroup$ Wood products (IIRC sawdust derived) are also used to keep the shredded cheese you can buy in bags at the store from clumping. IIRC in the ingredients list it says something like natural cellulose. But, IMO, there's a very large difference between simply ingesting something and being able to obtain nutrition from it. The wood fiber added to food products passed through your digestive system intact. $\endgroup$ Dec 1, 2014 at 14:50
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    $\begingroup$ Your "rule of thumb" is dangerous. "If A then B" does not mean "If NOT B then NOT A". Stones could certainly have energy and nutrients we could access through acids that would not burn. $\endgroup$
    – corsiKa
    Dec 1, 2014 at 16:31
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    $\begingroup$ To extract energy from something, you must perform a chemical reaction that releases heat. This means that the product has to have lower energy than the reactant. Oxides are usually energetically very low, so it is not very easy to extract any additional energy from them. And vice versa - if the material can be oxidized, it can be used as an energy source. This is foundation of my rule of thumb. What chemical reaction do you propose for releasing energy from stone? $\endgroup$
    – Irigi
    Dec 1, 2014 at 17:02
  • $\begingroup$ But I admit that it is is not general and can be tricky. For example calcium oxide does not burn, but could be used as an excellent energy source if mixed with water. $\endgroup$
    – Irigi
    Dec 1, 2014 at 17:07
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Termites 'eat wood' by passing the raw cellulose to symbiotic bacteria in their gut to turn this into sugars. A human could in theory be changed to do something similar with these bacteria. The body could then digest the raw sugars as normal.

Rocks are harder, because there is no lifeform that metabolizes rock for fuel, and the relative hardness would make it difficult to work with even if you found some energy releasing reaction. Plus, biologically just releasing heat is not enough - it has to be a reaction that can be controlled step by step in the context of the body's cells to release energy as needed. Our bodies aren't the kind that can work off explosions like an internal combustion engine.

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Humans do, in a manner of speaking, eat wood. We use it to augment the heat produced within our bodies by eating food and oxidizing it by instead burning it outside of our bodies. The end products of the process are the same/similar but the mechanism is different. We also use the heat of fire to pre-digest food by cooking it.

In very cold climates humans without fire would die of cold before dying of starvation.

Since humans already can 'effectively metabolize' wood by burning it to keep warm, process food, scare off animals, smelt metals etc there is little evolutionary pressure to do so more directly by eating it.

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    $\begingroup$ This isn't really answering the question, just comparing current eating to burning wood. $\endgroup$
    – bowlturner
    Dec 1, 2014 at 17:59
  • $\begingroup$ It lowers the evolutionary pressure pushing for the direct consumption of wood since we can already, effectively metabolize it. So I would argue that it is at least tangentially relevant. $\endgroup$
    – Ymareth
    Dec 1, 2014 at 18:02

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