It happen further back in time:
Brown bears—some of which are called grizzlies—and polar bears are closely related and are even able to interbreed. In the past few years, researchers have used genetic information to sort out this relationship and to understand how polar bears thrive in the frigid Arctic, feeding primarily on seals and other marine life captured from holes in the ice. This work has included sequencing the animals’ genomes, which has indicated that polar bears are truly a distinct species that at times lived apart from brown bears and at times intermingled and interbred with them.
In the latest sequencing effort, Willerslev and researchers from Denmark, China, and the United States analyzed the genomes of 80 polar bears from Greenland and 10 brown bears from North America and Europe. “[It’s] the most comprehensive genomic data set to date, as far as bears are concerned,” says Frank Hailer, an evolutionary biologist from Goethe University Frankfurt in Germany.
Drawing on that data, Willerslev and his colleagues conclude that
polar bears split off from brown bears between 343,000 and 479,000 years ago. Although little more than a blink in time from an evolutionary perspective, that was long enough for key genetic differences to evolve, they note in a report today in Cell.
SOURCE:
http://news.sciencemag.org/biology/2014/05/polar-bear-evolution-was-fast-and-furious