Polar bear fossils are confined to the Pleistocene (Kurtén 1964). Both fossil and molecular data indicate that polar bears stemmed from brown bears about the Middle Pleistocene (perhaps 400000–300000 years ago) (Kurtén 1964, Talbot and Shields 1996). Middle Pleistocene climatic cooling probably influenced the evolution of the ancestor of the polar bear (Kahlke 1999). Yu et al. (2004), by combining nuclear and mitochondrial DNA findings, have gained new insight into the evolutionary history of the bears (Ursidae). Their results corroborate other morphologic and genetic evidence indicating that brown and polar bears are most closely related and suggest that polar bears split from brown bears between 1 and 1.5 million years ago (Ma).
The divergence of polar and brown bears probably occurred somewhere along the coasts of Siberia or Alaska, but pre-Holocene fossils are scarce. Heaton et al. (1996) note that differences in mitochondrial DNA sequences suggest that living brown bears of the Alexander Archipelago, southeastern Alaska, comprise a distinct clade and are most closely related to polar bears. Could that be a clue as to their place of origin? I speculate that the drift from an Arctic coastal brown bear with a grayish coat to a polar bear began with the former specializing in scavenging beached marine mammal carcasses. Gradually, the bears might have ranged farther out onto the sea ice (fast ice) where they could actively prey on young seals in their dens and then on adult ringed seals at their breathing holes. In this respect, it is worth noting that grizzly bears (Ursus arctos) are now regular visitors to the Canadian Arctic Islands. On the sea ice of Viscount Melville Sound, remains of seals as well as female polar bears and their cubs have been attributed to kills by grizzly bears. Perhaps one of these bears denned on Melville Island during the winter of 2003–2004 (Doupé et al. 2007). Breakthroughs would come with genetic selection for white coats (camouflage) to aid their hunting (Sterling 1989:26), as well as adaptation to denning in dense snow near sea coasts rather than in earth farther inland, and physical adaptation for more trenchant teeth to support the more carnivorous diet, as well as large, oar-like feet to enhance their swimming ability.
The oldest known polar bear fossils are a relatively large right ulna from presumably Early Weichselian (approximately 70000 BP[?]) gravels of the Thames at Kew Bridge, London, and a left lower jaw from >80000 BP deposits on Svalbard (O. Ingólfsson, personal communication). The Kew Bridge find induced Kurtén (1964) to create a new gigantic Late Pleistocene subspecies Ursus maritimus tyrannus. Stuart (1982) calculated that the Kew locality was at least 140 km from the Devensian (last glaciation) coastline, based on the assumption that sea level was at its lowest then. Further, the presence of a marine mammal fauna, including ringed and bearded seals, suggests that polar bears too were present in the southern North Sea during the Late Pleistocene (at several colder intervals during the Weichselian) (Post 2005). Also, it is possible that outlines of bears in the Paleolithic cave of Ekain, northern Spain, may indicate that polar bears drifted south from the edge of pack ice off southern England (see map on the cover of Preece ) to the northern coast of Spain during heavy ice years toward the close of the Pleistocene. I interpret the bears to be twin subadult polar bears from their teardrop body shapes (Bahn and Vertut 1988: Fig. 90). These images were probably made during the Magdalenian (approximately 17000–12000 years ago).
The latest Weichselian to Early Holocene colonization of the western coastal regions of Norway, Sweden, and Denmark (northern Jutland) are well documented. A large lower jaw of an adult male polar bear dated to 11100 ± 160 BP from near Asdal (Jutland) occurs in the Allerød (Aaris-Sørensen and Petersen 1984). Eight finds (from Scania [two], Halland [one], and Bohuslän [five]) are known from Sweden (Kurtén 1964, Liljegren 1975). The more southerly specimens from Scania are dated at 12710 BP and 12490 BP (Håkansson 1974, 1976). Two of the finds from Bohuslän have been dated to 10620 BP and 10430 BP (Håkansson 1976). A nearly complete skeleton of a large male nearly 30 years old from Judaberg (Finnøy, Norway) has been dated at 10660 BP. It shows that polar bears were present in the Younger Dryas High Arctic marine environment in southwestern Norway (Blystad et al. 1983). Otto Salvigsen (personal communication) has dated (about 8200 BP) polar bear bones from a marine section on Svalbard. Fossil finds from Siberia are rare and not well dated. Vereshchagin (1969) records a right ulna from near the mouth of Mordy-Yahk River on the western coast of the Yamal Peninsula. Fossil remains of walrus and white whale were found there too. Also identified as belonging to the polar bear are: a right first molar stained black (Pleistocene?) found in 1961 on the shore of the Pechora River, as well as abundant remains from the Mesolithic site on Zhokhov Island (approximately 9000–8000 BP) (G. F. Baryshnikov, personal communication). The early inhabitants of Zhokhov Island in the Siberian Arctic evidently relied heavily upon polar bears for subsistence. Many of the bones from the site exhibit cut marks. Skull measurements suggest that female bears with cubs were killed in winter by hunters seeking their dens (Pitulko and Kasparov 1996, Pitulko 2003). In northern North America, the oldest radiocarbon-dated specimens from Vandfeldsnaes, Greenland, and Cape Richard Collinson, Prince of Wales Island, Canada, are only about 2000 years old (Harington 2003a:383).
Fulton and Strobeck (2006: Fig. 3a) provide a succinct view of relationships among bears. Note particularly the closeness and relatively recent evolution of polar and brown bears (Fig. 2).
 Andy Currant of the Natural History Museum–London (personal communication) believes that the Kew Bridge bear ulna represents a huge brown bear rather than a polar bear, based on faunas similar to that at Kew Bridge from many British sites containing dominant steppe bison and reindeer, with wolves and gigantic brown bears moderately represented.
Also, Jonathan Amos (British Broadcasting Corporation science report of 10 December 2007) mentions, regarding the Svalbard polar bear fossil, that Olafur Ingolfsson now considers that the well-preserved jaw from Poolepynten is of last interglacial age (perhaps 130 000–110 000 years old) and represents an adult, possibly a female. The specimen is significant because it indicates that polar bears survived the considerable warmth of the last interglacial and had evolved a good deal earlier.