Once slavery became a major part of the workforce, slave trading became a business. Ancient Greece was the first civilization where slaves made up a large part of the population. Between the 4th and 6th centuries (BC) it is estimated that from one-third to one-half of the population were slaves. There were many sources of slaves. Prisoners captured in war, kidnapping, people selling their children and criminals were some of the more common sources. The market price for slaves would go down after a battle in which many slaves were taken. Slavery was a large part of the Greek economy. A slave could earn money and could earn enough to purchase their freedom. Ancient Rome was even more dependent on slaves.