Strait of Tartary
The Strait of Tartary is currently only wide and is shallow enough that the divide is covered by an ice bridge during the winter that can be traversed by foot or dog-sledge. At the glacial maximum of the Ice Age, sea levels were lower than they are today. The Eurasia continent was connected to Sakhalin via the Strait of Tatar and Hokkaidō via the Soya Strait of which humans migrated. This connection explains the similarities of trees, plants, and animals including now extinct mammoths. The receding ice age warmed the area allowing greater tree cover and wildlife, thus new resources for the Nivkhs to exploit. The opening of the Soya and then the shallower Strait of Tartary allowed warm pacific currents to bathe the island and the lower Amur River.