Eurasia covers around 55,000,000 square kilometres (21,000,000 sq mi), or around 36.2% of the Earth's total land area. The landmass contains around 5.0 billion people, equating to approximately 70% of the human population. Humans first settled in Eurasia between 60,000 and 125,000 years ago. Some major islands, including Great Britain, Iceland, and Ireland, and those of Japan, the Philippines and Indonesia, are often included under the popular definition of Eurasia, in spite of being separate from the contiguous landmass.
Physiographically, Eurasia is a single continent. The concepts of Europe and Asia as distinct continents date back to antiquity and their borders are geologically arbitrary. In ancient times the Black Sea and the Sea of Marmara, along with their associated straits, were seen as separating the continents, but today the Ural and Caucasus ranges are more seen as the main delimiters between the two. Eurasia is connected to Africa at the Suez Canal, and Eurasia is sometimes combined with Africa to make the largest contiguous landmass on Earth called Afro-Eurasia. Due to the vast landmass and differences in latitude, Eurasia exhibits all types of climate under the Köppen classification, including the harshest types of hot and cold temperatures, high and low precipitation and various types of ecosystems.
Eurasia formed 375 to 325 million years ago with the merging of Siberia, Kazakhstania, and Baltica, which was joined to Laurentia, now North America, to form Euramerica. Chinese cratons collided with Siberia's southern coast.
Eurasia has been the host of many ancient civilizations, including those based in Mesopotamia, the Indus Valley and China. In the Axial Age (mid-first millennium BC), a continuous belt of civilizations stretched through the Eurasian subtropical zone from the Atlantic to the Pacific. This belt became the mainstream of world history for two millennia.
Originally, “Eurasia” is a geographical notion: in this sense, it is simply the biggest continent; the combined landmass of Europe and Asia. However, geopolitically, the word has several different meanings, reflecting the specific geopolitical interests of each nation. “Eurasia” is one of the most important geopolitical concepts; as Zbigniew Brzezinski observed: