The polar bear has become a symbol for threatened wildlife, and
its future in a warming world has created much controversy. The
predator’s past is equally controversial. The evolutionary
history of polar bears has been rewritten several times in
recent years, complicated by an incomplete fossil record and a
history of hybridization with brown bears, and a new study,
published today in the Proceedings of the National Academies of
Sciences, continues the trend. Researchers from Pennsylvania
State University have completed the most extensive genomic study
of polar bears to date. By comparing its DNA to that of brown
and black bears, the team calculated that polar bears arose
between 4 and 5 million years ago, making them far older than
anyone had suspected. And their genes carry the imprints of
repeated interbreeding with brown bears during much of that
history. The team also started documenting the genetic changes
that allowed polar bears to adapt to life in the cold. “Our data
gives us ... a first look at what