Even during the Pliocene Era (HS Era) there was a not insubstantial amount of Ice Sheet melt that allowed plants to flourish. What there isn't, is any mechanism for continuous evolution to function, let alone Civilisations to arise
Wrong era. The Pliocene runs from c.5.3 to 2.5 million years BP. The Pleistocene (our era - HS) on the other hand runs from c.2.5 million years BP to c.10,000 BP.
Anatomically modern humans (Homo sapiens - Us) development towards civilization is the result of some roughly 195,000 years, which is hardly "a few millenia".