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Current biogeographic models hypothesize that brown bears (Ursus arctos) migrated from Asia to North America via east Beringia (unglaciated Alaska and Yukon) ~100 to 50 thousand radiocarbon years ago (ka) but did not reach areas south of Beringia until the opening of a mid-continental ice-free corridor (IFC) -13 to 12 ka (1, 2). This model has been problematic because migration to the mid-continent was blocked by glacial ice only during the relatively short period ~23 to 12 ka, meaning the midcontinent was largely icefree before the Last Glacial Maximum (LGM), ~24 to 18 ka (3-5).
Thirty-one brown bear fossils have been radiocarbon-dated in east Beringia (2). Three have infinite ages (>53 ka), the rest are <48 ka, and there is a hiatus in their record from ~35 to 21 ka, suggesting brown bears were absent during that interval (2). Ancient mitochondria! DNA studies have shown that three clades of brown bears inhabited east Beringia before 35 ka: clades 2c, 3c, and 4 (2) (Fig. 1). Populations that recolonized east Beringia after 21 ka all belonged to subclades that were not present before 35 ka (2). Clade 4 never reappeared in east Beringia. After 10 ka, the modern phylogeographic structure of North American brown bears became...