DaasYochid- One does not need to observe actual evolution (or any fact, for that matter) in order to come to factual conclusions. Speculation is certainly enough, as long as you “observe” evidence.
Just think about the parsha of two weeks ago which talks about sotah. We don’t require eidim to observe a married woman having an affair, we only need them to speculate after they have observed evidence of her being secluded with a man other than her husband.