Anthropologists believe that the Japanese archipelago was settled by migrants from the Asian mainland sometime between 140,000 and 500,000 years ago, when falling global temperatures trapped water in glaciers and the polar ice caps, causing sea levels to drop 120 m or more below their present levels, and opening land bridges to Siberia and the Korean peninsula. Permanent village settlements and a cultural complex known as the Jōmon, after the distinctive, cord-marked slab pottery found at most sites, appeared between 14,500 and 10,000 bce. Around 1,000 bce, a new wave of immigrants spread outward from northern Kyushu, intermingling with the Jōmon peoples and displacing their civilization with a new one, which archaeologists have dubbed Yayoi after the location of the first site discovered, in Tokyo in 1884. The newcomers brought with them bronze- and iron-working skills, advanced agricultural techniques, and more sophisticated forms of political organization.