By relying on the "humans were meant to eat meat" logic, Niman fails to examine the assumption upon which it rests. At its foundation, the claim implies that any adaptive quality that humans might have evolved to survive is, to quote Niman, "so fundamental to the functioning of nature" that it "cannot be regarded as morally problematic."
The problem here is that evolutionary adaptation - the essence of the "functioning of nature"- includes untold morally disgusting behaviors that, while perfectly natural in the same way that eating animals is considered natural, are rightly deemed abhorrent by decent people living in a civil society.
Take infanticide. The adaptive advantage of infanticide for many vertebrates is well-supported. This is true for humans as well as primates. Among the !Kung hunter gatherers of Kalahari, about one in a hundred births end in infanticide. In regions of New Guinea, according to anthropologist Sarah Hrdy, infanticide is "off the charts," as mothers who wanted sons (or whose partners wanted sons) will often kill their daughters.