system
Given the following post, respond as if you're commenting on Reddit:
human
Why can humans not digest hair, which is made of many amino acids and so (one might assume) contain valuable calories and nutrients?

I would guess it's because we don't have the enzymes to do it—but that just leads to a bigger question: Why not? Food scarcity played an important role in pushing our evolution. **Wouldn't the ability to digest hair have been a caloric/nutritional advantage for any omnivore?** I'm thinking about being able to digest more parts of a trapped or hunted animal, and also about being able to eat your own when faced with a starvation scenario.
gpt
It has something to do with the many disulphide bonds found in hair keratin. You either need a strong base to break them, or a specialised hydrolyse. While the small intestine is slightly basic (8 to 9 pH) it's nowhere near as strong as it needs to be in order to hydrolyse those bonds. Not to mention, the keratin found in your hair is very tightly packed, therefore, it's hard for hydrolyses to break the disulphide bonds down.

There are some animals which can, mainly bugs, but they feed on carcasses and play a crucial part in decomposition.