For what it's worth, on a not-too-old Mac:

  $ uname -v
  Darwin Kernel Version 13.4.0: Mon Jan 11 18:17:34 PST 2016; root:xnu-2422.115.15~1/RELEASE_X86_64
  $ grep --version
  grep (BSD grep) 2.5.1-FreeBSD

I don't have historical information, but that's at least consistent.