Why is there a belief that our ancestors had a singularly common diet? Humans have lived in all areas of the earth with vastly different diets. Some were high in fat, some were high in protein, some relied on plants/fruit, others were more balanced, etc.

Yes we probably on average have a higher carb intake than most humans in the past. But that doesn't mean there was some standardized diet that all pre-historic humans ate. They ate whatever was around to survive.

And sure, the evolutionary pressures were huge. This seems to be the source of things like lactose tolerance (the default is intolerance)

... after childhood.

The modern western diet is mostly made of vegetable oils, caffeine containing leaves and beans, huge amounts of refined sugar, ridiculous quantities of grains, antibiotic infused meats, fruit juices, milks, fermented products, huge amounts of corn syrup. Its no great challenge to go to the store for groceries and come home with nothing not on the list above, yet almost none of it existed more than a couple centuries ago. I realize this is considered an incredibly controversial opinion; I assume "most people" strongly believe our ancestors in 8000 BC mostly ate McDonalds and drank MtDew for every meal.

Primarily, although massive grain consumption is evolutionarily very recent, so that counts too.

"Also, what was the diet of our ancestors, and how was this information about our ancestral diet ascertained?"

Talk to the archeologists. However it doesn't require much research to determine very few people ate twinkies or drank coffee a thousand years ago. I would not recommend talking to public relations firms, or people collecting a paycheck from the food industry in general.