Haven't humans eaten meat ever since they walked
the Earth?
Yes. However, as discussed in the title page, an activity does not become morally justified simply because it has been going on for a long period of time. Slavery, genocide, rape, and torture are all activities which have been going on since long before the dawn of recorded human history.
Furthermore, it is interesting to note that although humans have eaten meat throughout their history, it was a relatively small portion of our diets up until the twentieth century. A vegetarian diet actually bears closer resemblance to what humans have eaten throughout most of our history than does a typical American diet.
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