Polar bear evolution over millions of years has been influenced by climate change and interbreeding with brown bears, a DNA study by U.S. researchers found. Researchers at Pennsylvania State University and the University at Buffalo, as part of an international study, found evidence in newly sequenced polar bear genomes that the size of the polar bear population fluctuated with key climatic events in the past million years, growing during periods of cooling and shrinking in warmer times. The research also suggests while polar bears evolved into a distinct species as long as 4 million to 5 million years ago, the animals may have interbred with brown bears until much more recently as changes in the Earth\'s climate and the retreat of glaciers brought the two species into greater contact as their ranges overlapped, a university release said Monday. \"Maybe we\'re seeing a hint that in really warm times, polar bears changed their lifestyle and came into contact, and indeed interbred, with brown bears,\" Penn State researcher Stephan Schuster said. Researchers said they sequenced and analyzed the nuclear genomes of 28 bears, with many DNA samples provided by the U.S. Geological Survey and the Norwegian Polar Institute. A continuous decline in the size of the population during warmer times has meant polar bears occur in much smaller numbers today than during prehistory, researchers said. \"They have indeed lost a lot of their past genetic diversity, and because of this, they are very likely more sensitive to climate change threats today,\" study senior author Charlotte Lindqvist of the University of Buffalo said.