Whether Homo sapiens as a species have always eaten meat is uncertain. It is undeniably the case that meat and other products derived from non-human animals have a long history as a part of the human diet, but this fact says nothing at all about the moral or ethical propriety of breeding, enslaving, killing, and eating animals. Humans also have a long history of killing, raping, and butchering one another and until somewhat recently, it was perfectly legal in the United States to buy and sell human beings. Those slave-owners in the antebellum South who argued against the Abolitionists were fond of using the Argument From Antiquity to justify slavery, but it was not valid then and quite unlike a fine wine, it has not improved with age.
The brilliant poet and naturalist George Crabbe (1754-1832) articulated this fallacy beautifully in the following verse: “Habit with him was all the test of truth, / It must be right: I've done it from my youth.”
And it mustn't be forgotten that throughout the greater part of modern history, animal by-products constituted only a tiny part of the human diet. It wasn't until the advent of modern factory farming techniques in the mid- to late-twentieth century that meat, eggs, and dairy became such ubiquitous items on the plates of anyone not in the Ruling Class. This is thanks in very large part to the ability of factory farms to externalize the costs of their operations, an issue we'll address elsewhere. And it must also be remembered that abstinence from meat has a long and storied history within our species as well. From Pythagoras, to DaVinci, to Einstein, to Edison, and to Ellen Degeneres, there have always been those who recognize the indecency of killing and eating animals and who follow the dictates of their own consciences and not the example of the majority, be it past or present.
Your argument is invalid.