North America metropolitan area by population in North America: Greater Mexico City. Canada also breaks into the top ten largest metropolitan areas with the Toronto metropolitan area having six million people. The proximity of cities to each other on the Canada–United States border and Mexico–United States border has led to the rise of international metropolitan areas. These urban agglomerations are observed at their largest and most productive in Detroit–Windsor and San Diego–Tijuana and experience large commercial, economic, and cultural activity. The metropolitan areas are responsible for millions of dollars of trade dependent on international freight. In Detroit-Windsor the Border Transportation Partnership study
North America the third largest continent by area, following Asia and Africa, and the fourth by population after Asia, Africa, and Europe. In 2013, its population was estimated at nearly 579 million people in 23 independent states, or about 7.5% of the world's population, if nearby islands (most notably the Caribbean) are included. North America was reached by its first human populations during the last glacial period, via crossing the Bering land bridge approximately 40,000 to 17,000 years ago. The so-called Paleo-Indian period is taken to have lasted until about 10,000 years ago (the beginning of the Archaic or Meso-Indian period). The