When these get too long, they are prone to breaking and causing hangnails etc., and in extreme cases you see people with these really unweildy hands due to nails that are too long. So I assume that primitive people, i.e. those who didn’t have scissors or nailclippers or the equivalent, had some way of cutting them. But how? (biting them?)
FTM, what about the fingernails of apes and monkeys? Are they stronger than our or what?
People who do a lot of physical work or lots of hand manipulating will naturally have short finger nails. This is especially true for those who work outside and with nature (sand, wood, dirt). The process of handling rough material will keep the nails trimmed. Monkeys do lots of climbing and this keeps their nails trimmed.
I would also bet that nails grow slower with a poor diet.
I don’t really buy this though; I’ve never noticed any wear on my fingernails from doing work of the sort you describe; true, I don’t do it all the time but I would think there would be some wear. That said, the main way I keep my nails short is to peel them off (not the whole nail of course, just the end).
Biting would be the simplest way, and is an instinctive behaviour. It’s also easy enough to cut them or file them using stone tools. I don’t quite see what the mystery is here, nails aren’t that hard to cut.
Chimps will chew their nails if they get too long.
Having worked in several jobs that involved strenuous physical labour with wood, metals, soil and concrete, and having spent most of my childhood barefoot, I can categorically say that this is bunkum. People who work with their hands keep their nails trimmed short because it hurts like hell when they break or get bent back. Work does nothing whatsoever to prevent nail growth, and I have never heard anyone else suggest that they don’t need to trim their nails because of their work.
I worked for a year, in college, in the school library, reshelving books. Not exactly harsh manual labor, but lots of moving books into smallish spaces. While school was in session, my nails never got long. It’s not that they wore down, it’s that they broke. That’s with a decent modern diet. Finger nails break. If you get a jagged edge, you bite it off.
When I was a kid, I trimmed my nails by “clipping” them using other nails.
It’s hardly primitive, but northern Cameroon doesn’t have a lot of manufactured goods, and things like nailclippers would be relatively rare.
You could buy volcanic rocks at the market, and people would use these to keep their feet and elbows smooth. Presumably, they can also be used to file down nails if, for whatever reason, you didn’t feel like biting them.
As a poster mentioned before, cutting or filing with stone tools. And once we entered the chalcolithic and beyond, metal implements to cut them. Some can be just a simple knife like a straight razor, others can be more specialized, and look sort of like weed pullers you just push across the fingertip.
Been there, still there.
It took me years to teach myself not to chew my (finger)nails. I always shortened my toenails by peeling them until about age 40, when I started to buy nailclippers for my fingernails.
If a nail is long enough, it can be peeled and torn across to make it shorter. If it is too long it chips with some types of activity and you have a starting point to peel - if you don’t, regular activity will make it tear until yo can grab the “flap” and tear across the nail to finish the job.
If you’ve never done anything like that, well la-di-da aren’t we the self-preening, nail-clipping-and-filing type…! 
This, I assume, was the origin of fancy nails denoting rich ladies - it says “I don’t have to do housework and such that would damage my nails”. Painting them emphasized the point.
