An article in last month's Science, "Cooked starchy rhizomes in Africa 170 thousand years ago", provides some archaeological evidence of early humans cooking and eating tuber-like vegetables.
As others have pointed out with earlier finds, there are abundant artifacts (weapons, tools and bones) suggesting early humans ate meat. However, this doesn't mean they frequently ate meat, or it was a regular part of their diet - it just means that the artifacts survived.
If they pulled up a carrot and ate it there wouldn't be any archaeological artifacts to provide evidence for that thousands of years later. Occasionally we find something which has been unusually preserved, as in the recent example, but the frequency of each find says nothing about diet - just that buried knives last longer than buried potato peelings.
On the other hand, there is excellent biochemical evidence that our brains evolved with a carb-heavy diet to fuel it, and metabolism to match. A snarkier blog in SA puts it more simply:
"So what do other living primates eat, the ones with guts mostly like ours, eat? The diets of nearly all monkeys and apes (except the leaf-eaters) are composed of fruits, nuts, leaves, insects, and sometimes the odd snack of a bird or a lizard. ... The majority of the food consumed by primates today--and every indication is for the last thirty million years--is vegetable, not animal. Plants are what our apey and even earlier ancestors ate; they were our paleo diet for most of the last thirty million years during which our bodies, and our guts in particular, were evolving. In other words, there is very little evidence that our guts are terribly special and the job of a generalist primate gut is primarily to eat pieces of plants."