Did Slavery had existed since the ancient Greek and Roman times?

Yes it did. The Arabs and the Africans had slavery. There was some degree of slavery in Europe as well. The Europeans also became the biggest slavers in History. They bought slaves from Africans by selling cheap gods. They shipped millions of them to the US, the West Indies, Central America, Colombia, Venezuela and Brazil. Britain abolished the transatlantic slave trade in 1807 and slavery in 1833. Slavery was abolished in the US in 1865. Slavery still exists in some parts of the world.