North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It can also be considered a northern subcontinent of the Americas. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the west and south by the Pacific Ocean, and to the southeast by South America and the Caribbean Sea. North America covers an area of about 24,709,000 square kilometers (9,540,000 square miles), about 4.8% of the planet's surface or about 16.5% of its land area. As of 2013, its population was estimated at nearly 565 million people across 23 independent states, representing about 7.5% of the human population. Most of the continent's land area is dominated by Canada, the United States, Greenland, and Mexico, while smaller states exist in the Central American and Caribbean regions. North America is the third largest continent by area, following Asia and Africa, and the fourth by population after Asia, Africa, and Europe. The first people to live in North America were Paleoindians who began to arrive during the last glacial period by crossing the Bering land bridge. They differentiated into a number of diverse cultures and communities across the continent. The largest and most advanced Pre-Columbian civilizations in North America were the Aztecs in what is now Mexico and the Maya in Central America. European colonists began to arrive starting in the 16th and 17th centuries, wiping out large numbers of the native populations and beginning an era of European dominance.