No one can know for certain. In any case, an activity does not become morally justified simply because it has been practised for a long time. Slavery, genocide, rape and torture are all practices that have existed since prehistoric times.
The original answer also states that although people have eaten meat throughout history, it made up a relatively small proportion of our diet until the twentieth century.
It further argues that a vegetarian diet is closer to what people ate through most of history than a typical modern diet.
More answers: Vegan Ethics FAQ.