That's a good question.
I guess one way to "prove" that people existed that long ago would be to look at the evolution of languages. We know from our own experience how language changes over time. It is dynamic, but not enough to form into a whole other language within our own lifetime, yeah? So how long would it take for a new language to grow from an old one? Hundreds of years perhaps? And how about to evolve into the many different languages we have? Certainly thousands.
That, and carbon dating.
I think he was talking about individual people. Like Ghengis Khan.
or Phil Collins