I'm not sure if what you're trying to say here is that our remote primate ancestors were herbivores...if so, you do realize that's irrelevant? (As well as unsupported). Dietary adaptation can change in just one step on the family tree--from one species to the next.

Homo neanderthalus was a CARNIVORE. Analysis of fossil dung found in neanderthal areas shows that they had the digestive enzymes of a carnivore. They were physiologically adapted to a diet of almost exclusively meat. These guys are our close cousins (kissing cousins, even, according to some theories).

They came down the same evolutionary branch we did, they're VERY close to us. They had sophisticated tools, they had religion, fire, and culture. They were exceptionally intelligent...just not very creative, alas.

Now, it is understood that Homonids evolved from a lemur-like ancestor. Lemurs eat a variety of fruits, shoots, roots, insects, eggs, and small animals (including lizards, frogs, small rodents, and birds). So, right at the base of our tree, we have a primate omnivore.

Most primates are omnivores, you know. I hate to break it to you, but insects are animals. They are not plants. They do count. Eggs are also animal matter. The difference between eating a mouse or lizard, and eating a deer, is one of degree, not type.

Rabbits will play and tussle with each other. They do not spend 4 hours stalking butterflies by themselves. Human children do this. I know this, because I was one once, and I have 2 of them.

Homo sapiens is unquestionably an omnivore--are you disagreeing with that? If you do not disagree with that, then what relevance is there in saying that SOME of the ancestors of this species were herbivores? It's surely true, but some may have been carnivores like neanderthal, and many (probably most) were clearly omnivores.

You mistake the enlarged canines of our relatives, the great apes, for weapons used to procure or eat meat. They are for display and inter-personal combat, not for hunting. Hippos are herbivores...have you seen the teeth on those things?

Gorillas eat scant little animal matter, and their canines are very pronounced. Chimps eat plenty, including monkeys--they kill them by beating them to death with their hands, they don't kill them with their teeth.
Bears are physiologically more like carnivores than anything else, yet they are omnivores in practice. It's now been discovered that some species of fanged deer (the Muntjac is a commonly known species) actually hunt down and eat small animals, lol.

Chimpanzee molars are more impressive than ours, by the way--they're better suited for grinding shoots and leaves and plant matter. Yet they are omnivores.
Chimps are Homonids. I know, it hasn't made the front page of the Times just yet, but it will. Genetic studies and fossil records finally made it clear that our closest relatives are on the SAME BRANCH of the family tree that we're on...they diverged from the Homonid line, not from the line the split off prior to the Homonids. They're really another type of 'man', not another type of 'ape'....so to speak. Or, we are all the same type of ape...

Pigs are are good example of an omnivore, and they're useful as a comparison with humans because they are actually physiologically very similar to humans. Bears are not very similar to humans.

As for dentition...it tells us a lot less than we would like to think it does.

Neanderthal skull: http://museumstorecompany.com/images/285.jpg
Remember. Neanderthals were carnivores.

It appears than when diet changes, the intestinal tract changes first--the acids it produces, and how it functions, are the first to adapt to new food sources via evolution. More impressive morphological changes in bone change later...and they only change if they HAVE TO. Selective pressure is responsible for these changes, and for tool using animals, the change becomes unnecessary, so it will never happen. Humans don't have large canines because we have no use for them. We don't display using our teeth that way, we don't defend ourselves or fight with them, and we don't hunt with them. No matter what we eat, we will never need large canines. We can digest meat, and plant matter...and we don't need to do better than we do now, because we can process and cook our food. And our primitive homonid ancestors could, too.

We will never need shearing teeth, because we have knives. We will never need grinding molars--we can chop our food, and cook it. Our ancestors didn't need them, either...for those exact reasons. Their tools may have been primitive, but they eliminated the need for radical changes in dentition to follow changes in diet. We're learning that teeth are adapted to deal with worst-case scenarios. An ancestor with huge grinding choppers may have eaten primarily soft foods...but if it had to, it could handle the rough stuff, to get it through lean times. We see examples of this alive in the world today in other species.

Woodpecker finches will never develop the long, brush-like tongue of a woodpecker, because they have solved their problem through tool use...and they no longer need a long tongue.

Humans, weak jaws? We can exert 120 to 160 PSI of pressure!
We can bite just as hard, proportionally, as a chimpanzee-or the huge-jawed 'nutcracker man' in the homonid family tree.
Why? Because we are more efficient biters than either of them. Our jaw strength is deceptive. Our skulls can be lighter, as a result of higher efficiency.

http://www.sciencealert.com.au/news/20102206-21081.html

In conclusion, our jaws and teeth, as they are today, are great for cracking open nuts or small bones, but they're not good for grinding up coarse plant matter (such as what gorillas eat). Sustained chewing is not our forte.

We have STRONG chompers, though.

Tell me, where is the evidence that any homonid was ever truly an herbivore? We already know that lemurs are omnivores.