Current biogeographic models hypothesize that brown bears (Ursus arctos) migrated from Asia to North America via east Beringia (unglaciated Alaska and Yukon) ca. 100 - 50 ka BP, but did not reach areas south of Beringia until the opening of a mid-continental ice-free corridor (IFC) ca. 12 - 13 ka BP. This model has been problematic because migration to the mid-continent was blocked by glacial ice only during the relatively short period ca. 23 - 12 ka BP—If brown bears entered east Beringia 50–100 ka BP, their path further south should have been relatively unimpeded.

I report the discovery of a well-preserved cranial fragment of a brown bear collected in fluvial gravels near Edmonton, Alberta (Figs. 1, S1; Table S1). Two AMS radiocarbon dates on collagen returned ages of 25,210 ± 560 BP (AA48743) and 27,410 ± 200 BP (OxA-12902). This find indicates that brown bears reached areas south of Beringia well before the postglacial period and prior to the coalescence of Laurentide and Cordilleran glaciers.

Mitochondrial DNA was extracted from the root of the specimen's second molar and the genetic sequence shows the bear belongs to the clade of brown bears (Clade 4) that inhabits the southern portion of brown bear range today, including Edmonton. This clade existed in eastern Beringia prior to 35 ka BP, along with two other clades (Clades 2c and 3c), but has been extinct ever since. In fact, extensive radiocarbon dating and genetic studies of brown bear fossils in eastern Beringia indicates that all brown bear populations became extinct in eastern Beringia between 35–21 ka BP. Brown bears which repopulated eastern Beringia ca. 21 ka BP (presumably from Siberia) belong to new clades (Clades 2b and 3b).

The discovery of a ca. 26 ka BP Clade 4 brown bear near Edmonton challenges the hypothesis that southern brown bears are descended from Beringian populations dispersing through the IFC ca.13 −12 ka BP, since those dispersers would have belonged to clade 2 or 3. A more parsimonious model is that Clade 4 bears penetrated into southern regions well before the last glacial maximum (LGM), became isolated south of the ice during the LGM, and that modern bears there are descended from in situ populations. By implication, northern and southern female brown bears in North America have been genetically isolated from each other for at least 35,000 years. Furthermore, the first appearance of brown bears south of the LGM ice margin should not be used to date the earliest availability of a late glacial IFC for human expansion in the New World.