Evolution isnt always reactive though. Sometimes organisms mutate for no specific reason(radiation, limited gene pool) and if that mutation is beneficial or leads the organism to excel in a different niche it will propagate. This is not probably what happened with humans though since it would be a pretty big mutation to suddenly have sharp front teeth.
I think what the previous post was saying was that 2 different groups of human ancestors had different jaw/tooth structures that suggested they ate different diets. The one that was not like us died while the other lived so it concludes that being able to eat meat and plants is better for survival.