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?

>>9
yeah but really, whats the chance that all humans would evolve out of the ability to digest dietary fiber? It just seems that at least some people would have kept the ability unless it was harmful.