 The stoat is the product of a process begun 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 arose as its mirror image in North America 2 million years ago.