The first thing that comes to mind when you need to separate types of food is to make a banal division into plant and animal food, but here a seemingly stupid question arises: how to distinguish an animal from a plant? Well, here it seems that we can simply sessile photosynthesizers are plants, and moving heterotrophs are animals. But there are both sessile heterotrophs (for example, mushrooms and sea anemones) and moving phototrophs (for example, some mollusks with plant chloroplasts), and how to understand which one is a plant and which one is an animal?
So I think we’ll need a different system for identifying food.
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I was sure we would already have a thread about food sources, but I can’t find one right now so if someone else remembers what that was called, feel free to post the link.
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I think better solution would be just having tissues made of cells, say, cellulose, and counter-tissue enzymes, like current lysosomes work. If your body digests cellulose, you can eat cellulose organism. Note I didn’t say plant as there is no guarantee that plantlife (or, more precisely sessily life. There is also no guarantee plants will ever evolve. Your world may be consisting entirely of fungus and animals, unless something prohibits it, like lack of oxygen) will evolve. And if your motile life decides to evolve cellulose cell wall tissues, then you won’t really need cellulase enzyme to eat plants, but you’ll need it to eat animals. It all depends on the environment game takes place in.
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The existence of an ecosystem and life without autotrophs is impossible, and since there is not a sufficient amount of hydrogen sulfide on the surface for chemosynthesis, life on the surface is impossible without photosynthetic organisms (these organisms do not have to be macroscopic, the ecosystem can also exist at the expense of phytoplankton (but then access to land is impossible ))
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Photosynthesis is not the only form of autotrophy. In some deep caves algae use redux reaction to thrive without sunlight. Insects eat the algae, other creatures eat the insects, vibrant ecosystem and food cycle without photosynthesis.
Also, some plants get their food from a symbiotic/parasitic relationship with fungi.
Edit: Also, Thermosynthesis
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These plants are not capable of being a full-fledged basis for an ecosystem because they are either heterotrophic or use redox reactions that require oxygen, which is produced only by photosynthetic organisms (that is, they are dependent on photosynthesis)
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There are true sunless ecosystems, the hydrothermal vents would work without marine snow. Sunlight is just a much more powerful source of energy that the heat of the earth or minerals or something, and redox reactions will quickly run out without the sun and other autotrophs to replenish them.
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I think that this entire food stuff will work similarly to how it does in the current microbe stage: you need to have the required elements in order to properly eat something. My best guess is that each cell wall type would require it’s own specific digestion element.
When it comes to stuff like “herbivores” eating “meat”, as far as I am aware the consumption of such foodsource is potentially dangerous due to certain parts of this type of food causing negative long-term consequences (for instance if a real-life herbivore is eating too much meat, cholesterol would start accumulating in it’s circulatory system, eventually leading to death via a heart attack).
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