I once heard that this used to be a way to emphasize, by adding the thing you do not do. Not very clear, bit here are some examples:

- je ne marche _pas_ (I not not walk _any step_)

- je ne me bat _guerre_ (I do not fight _any war_)

- je ne coud _point_ (I do not sew _any point_)

And then slowly disconnected. All those words are still used as negations, and I cannot think of any word of negation that cannot be interpreted this way, so that might be true (but I cannot really think of more than those 3 either). What makes me doubt is what words would have persisted (why _ step_ as the one most common?)