Protein complexuty versus evolutionary steps

Hi /sci/
My father has a hypotheses which I agree, but have no real scientific data. Here it goes:
Protein is hard to digest(takes multiple steps in our body). Since animals at lower places in evolutionary steps(fish or prawns instead of beef and chicken) have simpler protein structure, they are also easier to digest.

This is based on simple intuition and high school biolgy. Can you confirm or deny?

I doubt that.
For one thing, fish and prawns aren't at a "lower" evolutionary "step", if anything they have lived longer in their current niche than we have.

Secondly, I don't know the specifics of how the body breaks down proteins, but for carbohydrates at least, it's less about the complexity and more about the size of the molecule. Even very primitive organism can manage to string together some huge-ass protein chains.

Creatures living in water don't have to be so rigid, though, so they tend to have less dense flesh and contain more soft, unsaturated fats, which could ease digestion.

Anyway, eat your fish. It's good for you (if it's not full of heavy metals, mind you), and typical diets in most developed countries contain far too little seafood.

what is this i don't even

Resistance to proteolysis will occur if the proteolytic enzymes cannot access their recognition sites present in the target protein.  This could happen if the protein does not carry these sites (unlikely) or if it is particularly acid resistant and does not denature, thereby hiding any potential digestion site.

Now I don't know what "simpler protein structure" is supposed to mean; proteins are chains of amino acids, if you mean "less complex," then that would imply fewer repeating elements and more sequence diversity.  For example  AAAAAAA is a less complex sequence than LAVLAV which is less complex than ACILVT.  But as far as I know, fish or prawns do not have less complex amino acid sequences than land vertebrates.

If you are basing this on the observation that eating fish gives you less indigestion than eating beef or chicken, well it may be that fish proteins are more easily denatured in your stomach than steak proteins, allowing the enzymes in your stomach easier access.