How is losing an ability part of evolution?

Alright, so it's believed the appendix and one point helped in our disgestion of dietary fibers, and we could consume grass/bark/plants like heribovores. So how is losing that ability evolutionary?

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Because the human race isn't stressed, and also because mutations happen over a long period of time. It generally takes AT LEAST 2 generations for a mutation to surface with any specific advantages.
Plus, humans don't need horns or a tail, and wings are too great of a leap without technology.