Might I point out that I wasn't the one making the statement that called for proof?
All the same, one could point to the existence of antibiotic resistant bacteria, which have evolved due to our overuse of antibiotics (evolution in bacteria happens more quickly than in higher life forms, because their life span is so short).
As far as humans go, have you ever seen suits of armor from the middle ages? A child might be able to fit in them, but most grown men could not today because humans are generally taller than they were 400 years ago.
Their diets where atrocious 400 years ago, among other factors.