Some of the world's major rivers, the Ganges, Indus, Brahmaputra, Yangtze, Mekong, Salween, Red River (Asia), Xunjiang, Chao Phraya, Irrawaddy River, Amu Darya, Syr Darya, Tarim River and Yellow River, arise in the Himalayas, and their combined drainage basin is home to some 3 billion people (almost half of Earth's population) in Afghanistan, Bangladesh, Bhutan, People's Republic of China, India (almost half of the population of India live within 500 km of the Himalayan range), Nepal, Burma, Cambodia, Tajikistan, Uzbekistan, Turkmenistan, Kazakhstan, Kyrgyzstan, Thailand, Laos, Vietnam, Malaysia and Pakistan.
The Himalayas, due to their large size and expanse, have been a natural barrier to the movement of people for tens of thousands of years. In particular, this has prevented intermingling of people from the Indian subcontinent with people from China and Mongolia, causing significantly different languages and customs between these regions. The Himalayas have also hindered trade routes and prevented military expeditions across its expanse. For instance, Genghis Khan could not expand his empire south of the Himalayas into the subcontinent.