This is like asking who the first person was to speak Spanish instead of Latin... It's a very organic thing and to suggest that it's black and white like this shows quite a bit of ignorance on the topic of origination.
We'll all admit, I assume, that Spanish and Portuguese and Italian, just to name a few, originated from a Latin vernacular, yes?
Since we know that Latin preceded these languages, would you attempt to delineate who the first person was to speak something other than Latin? By your logic, "There had to be a first..."
Knowing what you know about language from experience, don't you think it's kind of ridiculous to assume that one day a baby was born speaking Spanish instead of his parent's Latin? What good is a new language if there is no one else to speak it with? And how can there be a first if there must be multiples in order for a new language to exist?
That's obviously a silly endeavour, right? Well, so too is attempting to make the claim that there had to be a first human...
Since all things come from previous things, and since all things are constantly in flux, there is no way of nailing down the first person to speak a new language anymore than there is a way to knowing which person was the first human.