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?
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.
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.
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.