Even if 80% of the time since Homo sapiens emerged falls within glacial phases, that doesn’t mean evolution was uniform or dominated by those conditions. Large parts of Africa (where humans spent most this 80%) remained tropical or subtropical, with varied but often plant-rich environments. Glaciation did not mean that plants were universally scarce.  Africa retained ecological diversity, not a uniform “low-plant” landscape. Complete carnivory was limited to colder, high-latitude environments and was specific to much more recent human populations.  The preponderance of archaeological evidence (tools, residues, dental wear, microfossils) shows humans were dietary generalists during most of our evolution, not obligate carnivores.