I agree with belle and vecta
In terms of evolution, the real breakthrough in diet, wasnt eating meat in itself, or wansnt necassarily the diversity in the diet, its when we COOKED food.
Cooking half digests the food for us, therfore our digestive systems never really needed to adapt. There was a great experiment done, where 10 "average" people ate like our closest relative who was as strict raw veggie eater (I think it was homo habilis). His jaw is larger and so are his teeth, as well as them being flatter then ours.
Anyways, the people basically sat down all day eating the 10 pounds of raw food we need to eat on a raw food diet. After a week, all people were going to the bathroom 5 times a day, and not having "regular" bowel movements. They felt bloated, gassy and uncomfertable. This is because our bodies cannot break down only raw food, we need to cook it to release the startches and break down the cellulose which surrounds the plant cells.
Of course, I am not by any means suggesting that we CANT or SHOULDNT eat raw food. Nothing beats a fresh fridge cold apple or nectarine!