The non-controversial that has become controversial

Recently, I came across two people that I will call Mr. Potato and Mr Head that were discussing about God's existence.

Potato: ... How can you continue to say that God exists when science has proved time and time again that men have evolved from monkeys to the present stage as we know ourselves to be today... can't you read books? ...man...

Complained Mr. Potato, a little bit upset in the tone of his voice, impatient to concluded a quite long argument, and confident to have won the discussion.
But a vexed Mr. Head was not ready yet to give up on this discussion; but sensing that Potato was becoming agitated and angry, he decided to cut it short and said:

Head: I tell you what, you have your believes and convictions I have mine; so to be fair why don't we agree to this: My grandparents were humans and yours were monkeys... Ok?

Mr. Potato with a sigh of relief on his face that his friend was finally coming to his senses was quiet and thinking about what Mr. Head just said. To my surprise, a very angry and confused Mr. Potato jumped off the chair screaming, and shouting ..." My grandparents were not monkeys, my grandparents were not monkeys, my grandpa... ... ..."

Where does lie truth? You be the judge. Post your opinion.