A quick search of this subreddit for "fingernails" turned up quite a few posts asking the same question. Here is a response including links to several answers.

What do you mean by "early mankind?" Do you mean before the invention of nail clippers? You have to have either bitten or picked your nails sometime in your life, right?

I'm not being snarky--I'm just trying to figure out what you're asking. There are some weird assumptions embedded in your question.