As Cryptum established, multiple humans species created a prehistoric, space-fairing civilization which once challenged the Forerunners. They pulled off many amazing feats like create numerous colonies, found an alliance with ancient San 'Shyuum, and constructed a “cure” for the Flood (though what the Primordial has said makes me doubt its legitimacy).
They managed to stave off the Flood, but ultimately lost when fighting the Forerunner. Their civilization was then regressed by the Forerunners. However because their “cure” was lost, Forerunners used them for study after the war.
The Librarian even began to believe that humanity could attain the mantle - and she altered their Geas to promote their survival. She was behind history changing-events such as the SPARTAN program, Cortana, and humanities innate familiarity with Forerunner technology. Humans became Reclaimers.
But what of the other human species living within the Pleistocene? Florians, Neanderthals, Denisovans. Modern humanity consists entirely of -Yoink!- sapiens. They did not pull through to our recent epoch, as they all went extinct. Chackas, in the form of a monitor, even mentioned that modern humans had found fossils of -Yoink!- floresiensis (Florians).
Why did the Forerunners not ensure the survival of other human species through Geas, so that they could pull through to modern times? Forerunners can’t have only cared about -Yoink!- sapiens, because Denisovans were preserved on Installation 07 for study. And even if -Yoink!- sapiens are somehow “special” in the eyes of the Foreunner, what is it that makes it so?
I hope someone has an explanation for this; it’s really confusing for me.