Iran‘s complex climate ranges from subtropical to sub polar. It is mild and humid in the Caspian region, but warm and humid in the southern shores. The north-western and north-eastern areas are cold and dry, while the central part has a desert climate. The heights of Zagros and Alborz enjoy mild weather in summer. Because of this complex climate Iran is famed for having four distinct seasons at the same time, so that one can go skiing in the northern mountains and swimming in the southern waters in the same season. Plant and animal life is of great variety. Wildlife includes leopards, bears, mouflon, ibex, wild ass and wild boars. Studies have revealed the presence of a remarkably wide variety of reptiles like crocodiles and turtles. Some 200 varieties of fish live in the Persian Gulf. Sturgeon is one of 30 species found in the Caspian Sea.
Iran is a cradle of human civilization. In the late 4th and early 3rd millennia BC Elamite civilization rose on the lowland Khuzestan, in the South-west of Iran. In the late 2nd millennia BC the Iranians migrating southward from the Central Asian Steppes, began to settle in the Iranian plateau. By the mid-9th century BC two major groups of Iranians rose to be the dominant force on the plateau: the Medes and the Persians. In 550 BC the Persians defeated the Medes, and the Achaemenid kings appeared on the international scene. The Parthian and Sassanian dynasties ruled Iran successively. In 625 the Sassanians were defeated by the Muslim armies and the vast majority of Iranians converted to Islam, attracted by the divine teachings of this religion.