Genomic data from the extinct California brown bear suggests a source population for reintroduction to California

California brown bears, also known as California grizzlies or golden bears, are an extinct group that once thrived in North America's western coastal habitats. Despite being common in the region as recently as the early 19th century, intense poisoning, trapping, and hunting led to their extinction by 1924. Today, California is emerging as a candidate for brown bear reintroduction as a component of larger ecosystem restoration efforts. Questions remain, however, about whether living brown bears are suitable proxies for the bears that once inhabited California. While recent work suggests that brown bears from California were similar in size and overall diet to brown bears living today in continental North America, the 1) extent to which California bears were genetically differentiated from other populations, and 2) what this means for proposed reintroductions, remain outstanding questions. We generated genomes from two of the last living California brown bears and compared them to genomes from living brown bears. Genomic estimates of divergence time combined with radiocarbon dating points towards brown bears arriving recently in California, having diverged within the last 10,000 years from a common ancestor with brown bears found today in Yellowstone National Park, Wyoming. This timeline, the overall genetic similarity between the California and Yellowstone populations, and the strong pattern of isolation-by-distance we observe all suggest that no closer living relatives are likely to be found. If genetic background is to be a consideration for reintroduction efforts in California, brown bears from Yellowstone might serve as a source population.