[ajross]

Interestingly, though (just to digress -- your point is quite valid) it's not an essential amino acid for humans. Obviously it's required to make proteins, but if we don't get enough in our diet our bodies will synthesize it.

It's just interesting that the molecule we evolved to recognize as a "protein!" marker in our food turns out not to be one of the protein submolecules we actually need to surive.

[tschwimmer]

I agree with you, but it makes sense that we may have evolved to be able to synthesize it. Since it's such a crucial amino acid, it would be an advantageous adaptation to do so.

[jannotti]

I suppose that makes sense if it's one of the easier ones to produce, so many animals can produce it. So it would be a better indicator of general protein than any of the rarer ones would be.