In a really interesting discovery, scientists have uncovered evidence that suggests all living polar bears shared a single ancestor. This ancestor was a female brown bear who lived in Ireland.
The scientists were able to discover this info by analyzing maternally inherited DNA from various bear lineages. This DNA proved that polar bears interbred with brown bears from 20,000 — 50,000 years ago. From the source article:
When some ancient polar bear interbred with an Irish bear, the offspring inherited the brown bear momís mitochondria and with it the brown bear mitochondrial DNA. That mitochondrial DNA apparently spread through polar bear descendants until it replaced original polar bear versions. Today, all living polar bears have this brown bear mitochondrion.
Despite this evidence, polar bears and brown bears will still be classified as two separate species. Today, polar bears are listed as “threatened”, since climate change is rapidly deteriorating their natural habitat.