Where you get this information? Vegetables was seasonal, fruits was not even remotely that tasty like nowadays (do you ever tried wild apples? bitter as fuck and small as peanuts) and in some regions (take inuits) was nothing else but meat. Ratio was more like 95% animal, 5% - vegeterian.
Agriculture allowed all this boom of population, because it's so easy to settle and feed on crops instead of running down mammots.
Firstly our ancestors were unlikely to have been eating wild apples because of where we are from and not all wild fruit is bad. Besides that I wonder why our ancestors would care more about bitter fruit than other primates?
What you say about the inuit and other northern peoples is true but only because of the limited availability of plant foods. More equatorial peoples tend to get far more of their food from plants (up to 50%). The average hunter gatherer diet today consists of 64–68% animal and 32–36% plant based foods.
Our nearest relative the chimpanzee and ourselves have a functionally very similar digestive tract. They are largely fruit eaters with 95% of their calories coming from plants supplemented by animal based foods. Don't get me wrong, when it comes to diet I see an omnivorous one to be the healthier option most of the time and most clearly what we evolved for. I just disagree that we should only eat "some" vegetables.
As for tying the population boom to agriculture: This is a chicken and egg situation. Did we start farming because an increased population left us no choice or did population increase because we had more food from farming? It certainly appears that early farming peoples were less of stature than neighbouring hunters but there is debate about if it was diet that caused this also. The stature of modern farming peoples is certainly greater than that of modern hunters. If you haven't read it before I strongly suggest
Gun Germs and Steel which talks about this in some detail.
Humans and their ancestors will have been eating fruit and other plants long before we started hunting.
Pandas' and pigs' ancestors were purely carnivorous, ancestors of our fruit eating ancestors were eating insects, what's your point?
That line was meant to contrast gogis point about us shifting from hunters into farmers. Mostly it was to highlight the flaw in choosing to mainly eat meat based on our having been mostly hunters for longer than we've been mostly farmers by pointing out that we've gone through long times of mostly eating fruit also.
Our fruit eating ancestors would have likely been eating insects too.