A common account is that once upon a time, "ne" was the word that actually denoted negation.
Originally, the idea is that essentially, ne was the negative marker, and a sentence such as je ne marche pas would have meant something like "I do not walk a single step", i.e. pas carried it's literal interpretation of meaning "a step" (similarly je ne parle mot, je ne bois goutte etc). Over time, pas, mot, goutte, point, personne etc-- became interpreted as the words that actually carried negativity. Nowadays, this process has happened to the extent that ne is usually omitted altogether in non-emphatic speech, and ne is practically never used on its own as a negative marker (though you'll find occasional examples such as je n'en ai que faire = "I have no use for it", si ce n'est... = "notwithstanding...", "if not...", n'empêche que... = "the fact remains that...", je n'ose/ne puis te le dire... = "I dare not/cannot tell you" etc-- obviously some of these are quite formal/literary-sounding).
I'm not an expert specifically on language history, but my understanding is that there is a complication in this account, in that from the very first texts that are ostensibly "French" (such as the infamous Chanson de Roland), the range of nouns used with ne is arguably not as great as you'd expect-- so making the examples up out of thin air, right "from the beginning" (whenever you say that really was) it was far more common to say e.g. je ne vois goutte, je ne vois point, than e.g. je ne vois chaise, je ne vois chien, je ne vois arbre.
I insist-- historical linguistics isn't my speciality, but what I've briefly studied, that's my understanding; if anyone has more knowledge, I would be glad to be corrected!