The Pleistocene (1.64-0.01
epoch was marked by dramatic climate fluctuations (MacFadden 1994), most
notably the periodic advance and retreat of glaciers from the Arctic over
the northern hemisphere (WGBH 2001). In the northern latitudes, some areas
had ice over a kilometer thick, which had a severe cooling effect on the
surrounding areas (WGBH 2001). The Pleistocene also saw the migration
of humans to the Americas (MacFadden 1994).