Final answer:
The northern third of the African continent is predominantly made up of the Sahara Desert, the world's largest hot desert covering roughly the size of the United States.
Step-by-step explanation:
The most significant part of the northern third of the African continent is made up of the Sahara Desert. It is the world's largest hot desert, covering an area of approximately 3.6 million square miles, which is about the size of the United States. The Sahara stretches from the Red Sea in the east to the Atlantic Ocean in the west, encompassing much of North Africa and serving as a natural barrier that has historically influenced the movement and interactions of peoples, technologies, and cultures. While the Sahara is known for its arid and inhospitable environment now, it was not always a desert; it used to be a lush region with pastoralists and herds of cattle.
The Sahara's transformation from a green, fertile region to a vast desert happened between 6000 and 2500 BCE. This drastic change to a drier environment led to challenges for the inhabitants who were accustomed to hunting and gathering in a more tropical zone. As the grasslands and lakes gave way to sand, humans retreated to more hospitable regions such as the Nile River valley, a fertile strip made possible by the regular flooding of the Nile, which allowed civilizations to flourish.