How do (or did) native peoples keep their finger and toenails trimmed?

I would say the same way other animals such as our selves do. Through use. If you notice no one ever clips a wild dogs nails, but we do clip domesticated dogs nails. WHy? Because the D-dogs just sit in your house, they are not grinding there nails agains the ground. We did the same thing, we climbed trees we ran long distances, we did things that caused the nail to wear down. Also they probable pick at them too.