I would like you to cite your "plenty of evidence" for humans 30 thousand years ago--- because we have almost nothing, we have almost nothing 15 thousand years ago; we only have lots of evidence for the past thousand years or so, shoddy evidence for the past 2 thousand, even weaker for the past 5-10 thousand and after that , we really dont have

.
we have samplings of a rare human from this or that ancient time, almost none from the post toba period up until a few thousand years ago
and by reason alone--- stating that humans could have created a civilization before our current one is not at all like claiming aliens visit us, or that magical super empires existed--- its perfectly reasonable and supported by the fact that civilization has risen many times, in our brief period of having the ability to research it , we know about lots of peoples all the way back to when the black lake became the black sea--- because of the conditions which existed in the prolonged ice age on earth, a global civilization would not have been totally irrational to consider --- especially if you take into consideration the similiarity of myth structure among all the peoples of earth.( of course we can attribute this to the biological design of the human as well)
but just dont discount the tenacity of humanity and its ability to socialize in extremely complex ways.