Teeth and knives.

You can just tear the excess off when they get too long

I imagine our ancestors didn't have to worry about that too much since if they weren't working at something every waking moment, they'd starve to death. The nails would have been kept in check just through constant contact with abrasive surfaces of some type.

When I was younger I just used my teeth. Not particularly good for your teeth AFAIK, but it's very effective.

If mine get too long they start chipping and I don’t even work with my hands too much. I imagine it was just a steady state of broken nails.

A rough porous rock to sand 'em down might work too?

😬

I imagine our ancestors didn't have to worry about that too much since if they weren't working at something every waking moment for a few hours per day , they'd starve to death.

FTFY

Sandpaper has been around for a while

You can peel your nails. I haven't used a clipper in decades