An international team of scientists has discovered that the female ancestor of all living polar bears was a brown bear that lived in the vicinity of present-day Britain and Ireland just prior to the peak of the last ice age — 20,000 to 50,000 years ago. Beth Shapiro, the Shaffer Associate Professor of Biology at Penn State University and one of the team’s leaders, explained that climate changes affecting the North Atlantic ice sheet probably gave rise to periodic overlaps in bear habitats. These overlaps then led to hybridization, or interbreeding — an event that caused maternal DNA from brown bears to be introduced into polar bears. The research, which is led by Shapiro and Daniel Bradley of Trinity College Dublin, is expected to help guide future conservation efforts for polar bears, which are listed as threatened under the Endangered Species Act.