Why would you think humans lost this ability? Haven’t you ever heard of a craving?
Plus, any such instinct would be developed over time by direct experience of the organism, plus shared knowledge of others (like a parent teaching it’s young). It would also be developed within the context of what sort of plants and such are available. Most humans aren’t going around eating grass or tree bark that’s natively growing in their backyard, but we do have access to a variety of plants and such through stores, and even gardening. This suggests that this instinct is developed differently in modern humans. If you don’t have willow bark trees growing in your backyard to gnaw on the bark, you can buy aspirin instead. This could also be viewed as a more refined version of the instinct, as humans have isolated the compound that seems most beneficial, and you can get it pretty easily and cheaply, and don’t have to go through the work of processing all that willow bark on your own.
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