$ uname -v
Darwin Kernel Version 8.11.1: Wed Oct 10 18:23:28 PDT 2007; root:xnu-792.25.20~1/RELEASE_I386
$ grep --version
grep (GNU grep) 2.5.1
Other man pages: ed says 1993, sed says BSD 2004, cat says 3rd Berkeley Distribution 1995.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.