You don’t.
Because you don’t know how prehistoric people spoke or how they sounded. Don’t know accents. Don’t know much of anything about their speech.
So you make it up. Figure out how you think they spoke, what their accents were like, whether they talked like professors or peasants.
Which is a way of saying you go with your gut. And if your gut feeling about how they spoke and what words they used is good, then your readers will feel the same and will read their conversation as authentic.
