In the years just before human beings first evolved, the world was a wetter place than it is now, and a lot of Africa was covered in tropical jungle, full of fruit trees and small animals. But around six million years ago these jungles began to dry up and grasslands - wide, sunny meadows - grew where the cool jungles had been. We don't know why, except that the world seems to always be either warming up or cooling down. Early humans evolved from their monkey ancestors to take advantage of the new grasslands. Instead of climbing trees, they evolved to run and throw things. They started to eat the grain that grew in the grasslands as well as meat and fruit.
But there were many more climate changes ahead. About 2.5 million years ago, the current Ice Age began, and the whole Earth went through a series of colder periods where a lot of the land in northern Europe, Asia, and North America was covered in ice. Further south, the Sahara Desert was not a desert at all, but nice meadows and little groups of trees, with small rivers running through it. People crossed it to spread out over Asia and Europe around 60,000 years ago, just before the big Toba volcanic eruption around may have killed most of the humans alive at that time.
About 10,000 years ago (8000 BC), the world began to warm up, very very slowly. 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. The first farmers began to grow wheat and barley and figs in the hills of West Asia. They tamed donkeys to help them farm the land, and sheep to eat. This good time may be the origin of the Garden of Eden story from the Bible.
But by about 5000 BC, some places in West Asia, India, and Africa began to get too hot. The Sahara desert formed, and people couldn't live there anymore, or even cross it easily. In West Asia, deserts formed in Iraq and Iran, and in the Arabian peninsula. There wasn't enough rain anymore to farm in the hills, and people moved into the river valleys, where they could use the river water to irrigate their fields. They moved into the Nile river valley in Egypt, and the Indus river valley in Pakistan, and the Tigris and Euphrates valleys in Iraq, other river valleys in Iran, and the Yangtze and Yellow River valleys in China.
Massive irrigation canals have to be organized though, 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. These were the earliest civilizations - the Egyptians, the Sumerians, the Chinese, 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. People began to catch diseases from their farm animals, too - they caught measles, smallpox, mumps, and other diseases from animals. They may have given tuberculosis to their cows.