Mentioning previous answers is not intended to discourage more discussion -- follow-on questions, information, and debate is welcome.

Oddly, the FAQ under Hygiene And Healthy Lifestyles at https://www.reddit.com/r/AskHistorians/wiki/health#wiki_hygiene_and_healthy_lifestyles has only one discussion on nails:

• "What did people use before the invention of the toothbrush, floss and scissors?" at https://www.reddit.com/r/AskHistorians/comments/12m6x7/what_did_people_use_before_the_invention_of_the/ . But the parts about nails are scattered in the discussion. The main answer was by /u/Aerandir , but there was a comment by /u/speculativereply

Searching at https://redditsearch.io/ points out

• "Did everyone bite their nails before nail clippers were invented?" at https://www.reddit.com/r/AskHistorians/comments/4px0bd/did_everyone_bite_their_nails_before_nail/ . /u/sunagainstgold gave the biggest answer, but yodatsracist also added some information (burying fingernails implies that there's a way to cut them, rather than just file them down).
• "How did people cut their nails before the nail clipper was invented? Did they bite them? If so why is it now considered a bad habit?" at https://www.reddit.com/r/AskHistorians/comments/1uhvu0/how_did_people_cut_their_nails_before_the_nail/ has answers by Toptomcat, with lots of hyperlinks to sources. Also Quietuus talks about Norse.