Polar bears (Ursus maritimus) and brown bears (Ursus arctos) are sister species possessing distinct physiological and behavioural adaptations that evolved over the last 500,000 years. However, comparative and population genomics analyses have revealed that several extant and extinct brown bear populations have relatively recent polar bear ancestry, probably as the result of geographically localized instances of gene flow from polar bears into brown bears. Here, we generate and analyse an approximate 20X paleogenome from an approximately 100,000-year-old polar bear that reveals a massive prehistoric admixture event, which is evident in the genomes of all living brown bears. This ancient admixture event was not visible from genomic data derived from living polar bears. Like more recent events, this massive admixture event mainly involved unidirectional gene flow from polar bears into brown bears and occurred as climate changes caused overlap in the ranges of the two species. These findings highlight the complex reticulate paths that evolution can take within a regime of radically shifting climate.

北极熊 ( Ursus maritimus ) 和棕熊 ( Ursus arctos) 是姐妹物种，在过去 50 万年中进化出独特的生理和行为适应性。然而，比较基因组学和种群基因组学分析表明，一些现存和已灭绝的棕熊种群具有相对较新的北极熊血统，这可能是由于基因从北极熊流入棕熊的地理局部实例的结果。在这里，我们生成并分析了一只大约 100,000 岁的北极熊的大约 20X 古基因组，它揭示了一个大规模的史前混合事件，这在所有现存棕熊的基因组中都很明显。从活北极熊的基因组数据中看不到这一古老的混合事件。就像最近发生的事件一样，这种大规模的混合事件主要涉及从北极熊到棕熊的单向基因流动，并且发生在气候变化导致两个物种的范围重叠时。这些发现突出了进化在气候急剧变化的情况下可能采取的复杂网状路径。