From Science ABC:
Before humans developed blades or social expectations of hygiene, how did we handle the inexorably growing nails at the ends of our fingers?
The answer to this question is quite simple… the fingernails probably took care of themselves. Fingernails are largely made up of keratin, a hardened protein that is also found in the skin and hair. While keratin is hardy and durable, it is far from unbreakable, as any woman with a chipped nail will attest. Similarly, when you clip your nails with any of the clippers explained above, there is some resistance, but they are relatively easy to snip off.
Now, think back 100,000 years, when early humans behaved as hunter-gatherers, engaging in physically demanding activities to survive. Over the course of their normal days, they may have been digging tubers out of the ground, sharpening a rudimentary spear, carrying temporary shelters or trying to start a fire. With all of this manual labor, it is believed that the fingernails would have naturally been worn down and chipped away. The daily demands of survival would have kept the fingernails from growing to unruly or unmanageable lengths. As mentioned above, we see this passive maintenance in other species as well, such as dogs that are often walked on pavement, which gradually wears down their nails, thus requiring fewer nail trimmings at the vet.If the fingernails of these early humans did break or chip, they likely solved the problem as we do today—giving them a nibble and maybe tugging off the occasional irritating hangnail. Again, we see this same behavior in other species who lick at, soften, and bite their nails when they grow too long.
The tribal elites probably didn’t to as much physical labor as the proletarians. so I imaging they and others could have used flat rocks as nail files.
The linked article covers nail trimming over the last 10,000 years, too.
Steve Parker, M.D.