The new dates for pre-Clovis sites posed new problems. We know that the ice-free corridor between the Laurentide and Cordilleran ice sheets was open at least 13,000 years ago—there’s evidence for gene flow between bison populations north and south of the ice sheets occurring at this time, osteological evidence for elk migration through the corridor by about 12,800 years ago, and direct evidence of humans in the center of the corridor by 12,350 years ago (21). But was the corridor open long enough to allow people to walk through it in time to populate the pre-Clovis sites? This is a
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