Africa is the world's second-largest and second-most-populous continent. At about 30.2 million km2 (11.7 million sq mi) including adjacent islands, it covers six percent of the Earth's total surface area and 20.4 percent of the total land area. With 1.1 billion people as of 2013, it accounts for about 15% of the world's human population. The continent is surrounded by the Mediterranean Sea to the north, both the Suez Canal and the Red Sea along the Sinai Peninsula to the northeast, the Indian Ocean to the southeast, and the Atlantic Ocean to the west. The continent includes Madagascar and various archipelagos. It has 54 fully recognized sovereign states ("countries"), nine territories and two de facto independent states with limited or no recognition. Africa's population is the youngest among all the continents; 50% of Africans are 19 years old or younger. Algeria is Africa's largest country by area, and Nigeria is the largest by population. Africa, particularly central Eastern Africa, is widely accepted as the place of origin of humans and the Hominidae clade (great apes), as evidenced by the discovery of the earliest hominids and their ancestors, as well as later ones that have been dated to around seven million years ago, including Sahelanthropus tchadensis, Australopithecus africanus, A. afarensis, Homo erectus, H. habilis and H. ergaster – with the earliest Homo sapiens (modern human) found in Ethiopia being dated to circa 200,000 years ago. Africa straddles the equator and encompasses numerous climate areas; it is the only continent to stretch from the northern temperate to southern temperate zones.