I think we are opportunistic eaters. We can and will eat whatever is available. Our powerful brains, dexterous hands and the use of tools and planning have allowed us to acquire all sorts of foods in many different types of environments. The question here is what our ideal diet would be like, rather than what we can eat.
With any other non-domesticated animal, when determining what their ideal diet consists of, researchers would look at what that animal typically would be able to eat in the wild.
If one looks at what's available in the wild for humans to eat, while it does vary from one environment to another, in the raw, it's basically the same as what most bears eat. In the north pole and other northern very cold areas, polar bears will live on meat only, just like the humans who inhabit such areas. In other areas, while meat will still mostly play a crucial role, there are other foods available, such as fruits/berries, some mild herbs and bulbs/root vegetables and nuts and seeds and mushrooms that are not overly toxic, as well as insects, worms, honey and eggs.
Once humans developed cooking, however, more food sources became available, as there are many plants which are toxic or inedible raw, but become mild and edible when cooked. This is the case in particular with root vegetables and with nuts and seeds, notably with grains and legumes, which are generally a type of seed.
Then, with the development of agriculture, human diets were no longer bound by that which was available, but to an extent by that which they wanted to produce, or perhaps to another extent, by that which they could produce in enough quantities given scarce land, water and other resources.
In time, people learned to make intentionally fermented, refined or prepared foods and meals, such as wine, sugar, cheese, tea, and more recently hot dogs and icecream, then canned foods and potato chips, candy, and so on. Now much of our food is treated with artificial pesticides and is genetically modified, then treated in large factories and packed, often with additional artificial preservatives, then shipped over long distances, and it can sit in a shelf, often at room temperature, for extended periods of time before rotting.
The question then is, at what point do we make the cut, and say, this far is when we're gonna say we were no longer eating the foods that our wild ancestors would've been eating on a regular basis, and which were probably healthy for them (and thus, likely, for us too)?