Asia is Earth's largest and most populous continent, located primarily in the Eastern and Northern Hemispheres. It shares the continental landmass of Eurasia with the continent of Europe and the continental landmass of Afro-Eurasia with both Europe and Africa. Asia covers an area of 44,579,000 square kilometres (17,212,000 sq mi), about 30% of Earth's total land area and 8.7% of the Earth's total surface area. In general terms, Asia is bounded on the east by the Pacific Ocean, on the south by the Indian Ocean and on the north by the Arctic Ocean. The western boundary with Europe is a historical and cultural construct, as there is no clear physical and geographical separation between them. The border between Asia and Europe was historically defined by European academics. China and India alternated in being the largest economies in the world from 1 to 1800 CE. The Silk Road became the main East-West trading route in the Asian hinterlands while the Straits of Malacca stood as a major sea route. Asia has exhibited economic dynamism (particularly East Asia) as well as robust population growth during the 20th century, but overall population growth has since fallen. Asia was the birthplace of most of the world's mainstream religions including Christianity, Islam, Judaism, Hinduism, Buddhism, Confucianism, Taoism (or Daoism), Jainism, Sikhism, Zoroastranism, as well as many other religions. Asia has a mix of many different climates ranging from the equatorial south via the hot desert in the Middle East, temperate areas in the east and the continental centre to vast subarctic and polar areas in Siberia.