Approximately 12,000 years ago, the whole planet was becoming warmer, the polar icecaps were melting, and sea levels were rising. This cut the Japanese islands off from the mainland and made them the shape they are today. The warmer climate caused the sea levels to rise, peaking in the early Jōmon period (approx. 6,000–5,000 years ago), so that the sea cut deep into the inlands and created inlets rich with fish and shellfish. The land also saw the formation of zones with conifer forests, deciduous broad-leaved forests, and evergreen forests, and the deciduous broad-leaved forests that cover much of eastern Japan were filled with not only a variety of animals but also vegetative foods like acorns, sweet chestnuts, and horse chestnuts. Moreover, the invention of the bow and arrow and earthenware improved the technologies for acquiring food as well as for cooking and storing it. As a result, the population grew and settled down, leading to the construction of large-scale stone circles in the late Jōmon period (approx. 4000 years ago).