Environmental Change
At the end of the last Ice Age, about 10,000 years ago (8000 BC), the world began to warm up, very very slowly. We don't know why, except that the world seems to always be either warming up or cooling down.
At first, this warming made life a lot easier for early people. They didn't have to worry so much about freezing, there was more food available, and more places where people could live easily. Most people were still hunting and gathering, but the climate was so good that they began to settle down instead of travelling around. Some of these settled people began to farm a little in the hills of West Asia.
But by about 5000 BC, some places in West Asia, India, and Africa began to get too hot. The Sahara desert began to form, and people couldn't live there anymore. In West Asia, deserts formed in Iraq and Iran, and in the Arabian peninsula. People had to move into the river valleys, where there was still enough water and food. They moved into the Nile river valley in Egypt, and the Indus river valley in India, and the Tigris and Euphrates valleys in Iraq, and other river valleys in Iran as well.
With all these people moving into river valleys, though, the valleys got crowded, and by 3000 BC people were beginning to organize themselves into cities and governments, to try to deal with all these people living so close together. And to feed all these people, they began to farm, and to irrigate more land, and they tamed animals to help them farm the land, and to eat. These were the earliest civilizations - the Egyptians, the Sumerians, and the Harappans. A lot of serious diseases also got started around this time, because people living together in crowded and dirty cities, who weren't really getting enough good food to eat, and who were working too hard, made it easy for germs to spread from one person to another.
