[kleer001]

IMHO there can be no direct evidence, only speculation.

My thought is that it's easier to make sounds in a rhythm (on/off) than it is to control the pitch of those sounds (relative to previous sound).

Also thousands, I think, is probably an underestimate by a couple orders of magnitude.

[jacobolus]

It is very weak to say “it is believed that ...” and then make some very large (frankly implausible) claims without describing who believes this or why.

I would maybe give this idea some credibility if it were changed to “millions of years ago before the evolution of vocal anatomy capable of complex speech”.

But even then... completely speculative.