The skulls of ancient NW native Americans show worn down teeth from the sand in their shellfish diets. The teeth of these people did not not harden through the generations in response to their food. This author draws some bizarre conclusions from the data.
If faces change to a softer shape--I think it might have to do with the male attraction to childlike faces. Hardwired for the fecund and the nubile--and might go a long way towards explaining the intractability of the seduction of the adolescent. Yet again, in this article, the muddling-up of selection for type and "evolution."
Since all teeth are the same fundamental chemical composition, it is impossible for evolution to make them harder. Have to switch over to a different chemical subsrate, and since the calcium system has been selected for over millions of years, that just ain't gonna happen.