the dna that decided to group with other dna that eventually became plants, fruits, vegetables, doesn't have anything to do with the dna that grouped together to form us, which is why we're seperate organisms
once organisms were formed, organisms began to co-evolve, and this did happen...if you look at animals like you say, they have canines or flat teeth....the ones with sharp teeth eat meat, the ones with flat teeth eat plants....but if you take time to look in your mirror and open your mouth you see a vareity of teeth
why?
well evolution says (and you should agree with this...) that humans are different from every different species on earth
we can collaborate, communicate, think of new things, want new things, we have the most advanced brain EVER
so what is to prevent a primitve human from getting sick of eating wild Mastadon and deciding to eat a plant? Nothing! What is to prevent that human from realizing plants can be grown and provide a source of food that doesn't have to be hunted...Nothing!
if the tribe of this primitive human had a glut of vegetables in their environment and NO meat for the longest period of time, and they had to eat vegetables, the humans that had flatter teeth would survive better htan those with mostly sharp teeth.....this is the kind of thing evolution says
such is the beauty of humans