5-milion years ago instead of 600,000 or 130,000 years ago-
Recent interest in the relationship between polar bears and brown (grizzly) bears from which they emerged has been fueled by the observation of increasing numbers of polar bear – brown bear hybrids. Our recent article on this produced a number of comments, many of them excellent, well informed. Until recently some observers thought that polar bears as a species might have emerged from brown bears as recently as only 130,000 years ago.
However, ice flow living polar bears don’t leave many fossils and the Arctic has been frozen more or less for millions of years, and studies using the new techniques of genetic analysis show polar bears have been around for a long time. A new study “Polar and brown bear genomes reveal ancient admixture and demographic footprints of past climate change” pegs polar bear emergence at about 4.5-million years ago. Very interesting, however, is the concurrent finding that brown bears, black bears and polar bears crossbred intermittently over the entire period, presumably in warmer periods when the ice retreated. Most apparent is the amount of polar bear DNA in the brown bears of Alexander Archipelago in Alaska.
The warmer periods when ice retreated might have reduced the number of polar bears to a degree that genetic bottlenecks occurred from lack of genetic diversity among the polar bears. On the other hand, the trickle of crossbreeding might have resulted in an admixture of genes that would also provide genetic rescue of the polar bears. Currently, and even before the current big melt, polar bear populations were low compared to deep prehistory.
