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Evolution

The stoat's direct ancestor was Mustela palerminea, a common carnivore in central and eastern Europe during the Middle Pleistocene, that spread to North America during the late Blancan or early Irvingtonian. The stoat is the product of a process that began 5–7 million years ago, when northern forests were replaced by open grassland, thus prompting an explosive evolution of small, burrowing rodents. The stoat's ancestors were larger than the current form, and underwent a reduction in size as they exploited the new food source. The stoat first arose in Eurasia, shortly after the long-tailed weasel, which is in a different genus (Neogale), arose as its mirror image in North America 2 million years ago. The stoat thrived during the Ice Age, as its small size and long body allowed it to easily operate beneath snow, as well as hunt in burrows. The stoat and the long-tailed weasel remained separated until 500,000 years ago, when falling sea levels exposed the Bering land bridge.

Fossilised stoat remains have been recovered from Denisova Cave. Combined phylogenetic analyses indicate the stoat's closest living relatives are the American ermine (M. richardsonii) and Haida ermine (M. haidarum), the latter of which partially descends from M. erminea. It is basal to most other members of Mustela, with only the yellow-bellied (M. kathia), Malayan (M. katiah), and back-striped (M. strigidorsa) weasels being more basal. The mountain weasel (Mustela altaica) was formerly considered its closest relative although more recent analyses have found it to be significantly more derived. It was also previously thought to be allied with members of the genus Neogale such as the long-tailed weasel, but as those species have since been separated into a new genus, this is likely not the case.