New research focused on the full genomes of polar, brown and black bears has pushed back by almost tenfold the date of the development of the polar bear as a distinct species. The New York Times reports that besides dating back about 5 million years, the polar bear genome shows evidence of interbreeding between polar and brown bears ever since.
The last estimate, published in April in Science, put it at 600,000 years ago, and the new study has it at five million. It reinforces the previous conclusion that polar bears are not a recent spinoff of brown bears, and it also paints a more detailed picture of what the bears have been doing all this time as they evolved into the charismatic megafauna we now use to sell soft drinks and promote environmental causes. ...
The progress of species formation, at least in this case, is a bit like a long, ambivalent divorce in which the two parties separate but occasionally fall into bed together even after the official decree.
Read more at The New York Times: DNA Shows Split Between Brown and Polar Bears and Later Coupling