I found a comment claiming that prior to 10.8 (2012, Mountain Lion) it used GNU grep, but nothing I'd feel comfortable citing.
$ type grep
grep is hashed (/usr/bin/grep)
It does seem to be the original grep for this machine (it's a Mac Mini) - it has the same Jan 2006 date as most of the files in /usr/bin, and nothing has an earlier date. There's no other file called grep elsewhere.