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（在……附近） 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 overlaps¹ in bear habitats. These overlaps then led to hybridization（杂交，配种） , or interbreeding -- an event that caused maternal² DNA³ 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. The results of the study will be published on 7 July 2011 in the journal Current Biology. Polar and brown bears are vastly different species in terms of body size, skin and coat color, fur type, tooth structure, and many other physical features. Behaviorally, they are also quite distinct: Polar bears are expert swimmers that have adapted to a highly specialized⁴, arctic lifestyle, while brown bears -- a species that includes Grizzlies⁵ and Kodiaks -- are climbers that prefer the mountain forests, wilderness⁶ regions, and river valleys of Europe, Asia, and North America. "Despite these differences, we know that the two species have interbred opportunistically and probably on many occasions during the last 100,000 years," Shapiro said. "Most importantly, previous research has indicated that the brown bear contributed genetic⁸ material to the polar bear's mitochondrial（线粒体的） lineage -- the maternal part of the genome, or the DNA that is passed exclusively from mothers to offspring. But, until now, it was unclear just when modern polar bears acquired their mitochondrial genome in its present form."

Although previous researchers had suggested that the ancient female ancestor of modern polar bears lived on the ABC Islands -- the Alaskan islands of Admiralty, Baranof, and Chichagof -- only 14,000 years ago, Shapiro's team found evidence of a much earlier hybridization event. Because of this event, the modern polar bear's mitochondrial DNA probably underwent fixation（固定，定位） -- a drastic reduction in genetic variation and a transition to a state in which the entire gene⁷ pool includes only one form of a particular gene. After performing genetic analyses of 242 brown-bear and polar-bear mitochondrial lineages sampled throughout the last 120,000 years and across multiple geographic⁹ ranges, Shapiro's team found that the fixation of the mitochondrial genome likely occurred during or just before the peak of the last ice age, possibly as early as 50,000 years ago, near present-day Ireland. Shapiro noted¹⁰ that the specific population of brown bears that shared its maternal DNA with polar bears has been extinct for roughly 9,000 years. However, her data offer clear genetic evidence that the two species were in contact long before the brown bear's disappearance¹¹ from the British Isles¹².

Shapiro explained that, although both polar bears and brown bears have experienced long periods of geographic stability, episodes of both warming and cooling during the last 500,000 years or more likely led to environmental conditions favorable to hybridization between the two bear species. "Polar and brown bears likely came into contact intermittently¹³, in particular in coastal¹⁴ regions where the effects of climate change may have been more pronounced," Shapiro said. "Whenever they come into contact, there seems to be little barrier to their mating."