The human population on Earth two million years ago has been estimated at 100,000. At the beginning of the agricultural revolution this number had risen to perhaps five million, thanks to better adaptation, technology, and abundant resources that became available as the ice sheets receded. By 3000 BC, the time of the first Egyptian dynasty, world population had increased to approximately 100 million. By the birth of Christ, world population was well over 200 million.
 
The beginnings of agriculture
Agriculture was a gradual discovery. It is believed that early gatherers first learned the relationship between plants, the foods they produced, and their growing cycle. At some point the gatherers learned how to encourage the plants they depended on and inhibit those of no use. Then came the steps of gathering seeds, planting seeds, and nurturing the plants. By selecting seeds from the strongest and most productive plants for replanting, the early planters interrupted and redirected the process of natural selection to improve the yield of the useful plants. For example, researchers in Mexico have found evidence of the ancient corn plant with only a few kernels that became the much more productive corn plant of ancient America through selection over many thousands of years.
The first domesticated grain is believed to have been a wild wheat that grew in southern Turkey. To domesticate this plant, the early gatherers had to learn how to harvest the grain seeds, extract the wheat kernel, grind it, and bake it, all before they learned how to grow the plant and select it so that it increased in kernel size. This was a complicated learning process that took time. Less is known about how and when the rice plant was domesticated, but it was clearly as important in Asia as wheat was in the Middle East and corn in America.
The agricultural revolution accelerated as innovations increased arable land, crop yields, and farmer productivity. Land was cleared by grazing or fire. Soil was prepared by digging first, and then plowing. Irrigation insured adequate water. Fertilization and crop rotation increased yields. Specific tools like the sickle, scythe, plow, and hoe increased farmer efficiency.

Domestication of animals
Dogs were domesticated from wolves perhaps 15,000 years ago in both America and the Near East, but dogs were useful mainly as companions, guards, and hunting aids. The first domesticated food producing animal was probably the goat, a source of meat, milk, and waterproof hides. This breakthrough occurred in the hills of modern Iraq. Domestication of the goat was followed by sheep (meat and wool), cattle (meat, milk, hides, power), horses (meat, milk, transport), pigs (meat and fat), chickens (meat and eggs), and others.
Cattle are considered the most significant domestication. In addition to providing meat, milk, and hides, they were also valuable as beasts of burden. They pulled wagons, greatly improving land transport. They pulled plows, greatly improving agriculture. The existence of domesticated cattle is thought to have been primarily responsible for the doubling of population in the Near East between 4000 and 5000 BC.
The process of domestication must have been to obtain young animals, raise them in captivity over enough generations so that they grew less wild and more tolerant of being managed. It is unclear why some animals could be domesticated but not others. Why the cow but not the buffalo? Predators would be unlikely choices for domestication, but the wolf was a predator and so was the cat, which was domesticated much later.
Domestication of animals brought many important advantages. They were dependable sources of meat. Cattle and goats converted grass, of little use to humans, into milk and milk products like cheese. Vast grasslands that could previously support only a few hunters could now support much larger populations of herders. Sheep and vicunas produced wool each year. Animals could graze on broken lands of little use for farming.
Horses, oxen, llamas, and other animals provided power for pulling, plowing, and carrying. Military uses were eventually found for onagers, asses, and horses, and to some extent, elephants, although elephants are not considered domesticated. The horse was domesticated first in the north Asian steppes and its use spread from there, probably in the wake of Asian migrations to the south and west. Paleontologists believe the horse evolved in the Americas actually, but went extinct there, perhaps due to hunting pressure.
Mounted Asian barbarians are thought to have overrun the first towns rising in Asia Minor and the Middle East around 6000 BC. The people that were overrun may not have seen a domestic horse previously, much less one on which a warrior could ride.