The second-largest continent, the second most populous, and the one every human being alive is descended from. Africa carries about 1.5 billion people on roughly 30.3 million square kilometres — a fifth of the land on Earth, big enough to swallow China, India, the United States and most of Europe at once, and consistently drawn far smaller than that on the flat maps everyone grew up with. It is the only continent that sits in all four hemispheres, cut almost in half by the equator, which is why its climate belts mirror each other going north and going south: rainforest at the middle, savanna either side, then the Sahara above and the Kalahari and Namib below. The course starts with that physical frame. The Sahara covers about 9.2 million square kilometres and is the largest hot desert in the world. The Nile runs some 6,650 kilometres, the Congo carries more water than any river except the Amazon and is the deepest on Earth, and the Great Rift Valley is quite literally tearing the continent apart along 6,000 kilometres from the Red Sea to Mozambique. Kilimanjaro stands at 5,895 metres with glaciers three degrees from the equator; Lake Assal in Djibouti lies 155 metres below sea level. Then the regions, four episodes across them: North Africa and the Sahara, where the Nile valley, the Atlas mountains and five thousand years of written history sit on the Mediterranean edge; West Africa and the Sahel, with a Nigeria of more than 230 million people and the empires of Ghana, Mali and Songhai that grew rich on Saharan trade; East Africa and the Horn, where humanity began, where Ethiopia kept its independence and its own alphabet and calendar, and where the Serengeti migration still moves nearly two million animals a year; and Central and Southern Africa, from the Congo rainforest to the Okavango Delta, Victoria Falls, the Cape and a South Africa with twelve official languages. It closes on the layer built over the map: 54 states in the United Nations and 55 in the African Union, around 2,000 languages from four great families, the youngest population on the planet at a median age near 19, and the largest free-trade area in the world by number of countries.