I think daily usage really is the answer for most prehistoric peoples.
However, it’s also worth pointing out that you can get obsidian (and other rock implements) extremely sharp. I once used a pocket knife to trim my nails, and actually felt like it was superior to my clippers. (I also recognized the risk of taking off more than just a nail, though, so I stick to the safer methods). Stone tools take us way, way back into prehistory.
I have never used a tool of any kind to trim my toenails in my 35 years of toenail growing. Like some others above, I just tear away the excess with my fingers every once in a long while. Not surprisingly, I’ve always “known” how prehistoric people trimmed their toenails.