Complex sentence: The survival of bison into the Holocene and recent times is therefore difficult to explain via overkill, since these animals--separated from human hunters by more than 10,000 years--would almost certainly have been very nearly as naive as native North American large mammals.
Split into simpler sentences: The survival of bison into the Holocene and recent times is therefore inconsistent with the overkill scenario. Given this enormous span of geologic time, bison would almost certainly have been very nearly as naive as native North American large mammals.