I hate to be the one to bring some reality to this discussion but I used to be a bit of an anthropology buff.

All my research, lectures, presentations I went to detailed that paleo man was an omnivore and an opportunistic diner.

All the analysis that I've seen of paleo human coprolites (dung) included both vegetable and offal matter. In the case of vegetable matter, more or less vegetable matter in the dung was used to place the season. A lot of this dung was found preserved in caves. Analysis has gotten so good, via DNA, they were able to identify the type of matter and even sometimes the species. The coprolites contained protein (both muscle and offal), insects, berries, grasses and other vegetation.

Further for paleo, depending on your timeline definition--mine is before settlement--the area of habitation was quite widespread and the resulting diet diverse. If you want to look at it as a time when there were still mastadons, they found vegetable matter and offal in those coprolites as well.

Another idea to think of...before there could be agriculture, as primitive as the first agriculture must have been, there must have been a selection from the wild of what to grow. That means that certain grasses, bulbs, fruits, and yes vegetables were growing and available in the wild.

And looking to todays woods and forests for sustenance is quite useless, the forests of today are domesticated and barren. And further, why limit yourself to this area/climate/continent. But you don't even have to look far past what was available in the forests and fields of North America when Native Americans were living in this country to see what might have been availble in the wild--onlions, garlic, cabbage, gourds, fruits and berries, greens, and yes even grasses/grains.

Oh and another thing...the liver is a great source of vitamin A, and offal are concentrated sources of other nutrients. I hardly think there would be so many primitive peoples who had culteral traditions and ceremonies for the eating of brains, hearts, and livers if these things weren't part of a more ancient diet.