what temperature would it be today if man had never walked the face of the planet?
If you've been unable to find an answer, you've never put much effort into looking. Here's the most popular chart of it:
Your answer would be the sum of Greenhouse Gases, Ozone and sulfate. So in 1994, the human contribution since 1900 was around +0.5C. Bringing that up to today, it would be around +0.8C.
Now, there are also scientists exploring how land clearing by early humans may have affected the climate, but there's nothing firm to report there yet.
So, there you go. Sorry that you won't be able to use that talking point any longer. I know it was one of your favorites.