Human insulin and rat insulin are nearly identical. After 100 million years of separate evolution, the differences between them are minimal. The same precision holds for ghrelin and leptin, a counterbalanced pair that regulates hunger and satiety.
These signals were running before the first animal grew an eye. That is not a metaphor. It is a timeline. Insulin-like peptides appear in organisms that diverged from humans over 500 million years ago. Across that entire span, the basic design remained virtually unchanged.
Evolution doesn’t change what’s already optimal. When a peptide signal is so precisely tuned to its biological task that small variations reduce effectiveness, mutation pressure stops. Natural selection locks the design in place.
The design was already optimal.