Slavery, eh? Allow me to say this about that.
Nowadays the first thing that pops into your head when you think of "slavery" is, of course, slavery in America and the American Civil War. But slavery still exists all around us - indentured labour, usually migrants exploited by criminals, exists in most western societies and a lot of prostitution amounts to sex slavery. In many cases the individuals concerned have tragically sold themselves into slavery, thinking they were just getting passage to another country. Then when they get there the people traffickers demand years of service in return. And slavery still very much exists in Africa, particularly in Sudan, where the natural resources to be extracted exceeds the free labour power available. It might make you feel uneasy to realize that slavery is involved in some of the products we all enjoy from Africa.
The origin of slavery lies essentially in superior physical strength of individuals or groups over others, and the outcome of conflict. Many ancient societies had archaic laws about slavery, and in some you could only imprison foreigners who were defeated in battle.
The origin of this practice in Ancient Greece and the ideas behind it were not racial, as we tend to think of slavery as being. Instead, this sort of slavery was based on a disdain of the qualities of a particular individual. Slavery has its origin in the fact that some people can overpower others and force them to do their will, but clearly an individual in this situation faces a choice between slavery and death; the putative slave could fight the more powerful individual and die, thus robbing him of his prize because no-one can command dead men (this is the genius and special dignity behind passive resistance).
Any man who submitted to slavery rather than die was considered to be little better than an animal because he was allowing his brute desire to survive to win out over his human dignity, which wants to be free. In fact, it was believed that it was because he was himself a slave to this base side of himself that he had allowed himself to become the slave of another man rather than dying like a noble person would have done. And the tasks to which slaves were set hence also had the lowest dignity - they were to provide for all the basic menial tasks that higher, more dignified men would not deign to do themselves, such as the unending process of food preparation and cleaning which consumes so much energy but leaves no lasting human monument. With all these slaves to take care of that, the Ancient Greeks could devote themselves to politics, the arts and philosophy; and not without them.
Nowadays the process of providing for comfortable human life has actually become the central goal of our politics rather than something that goes on behind closed doors in kitchens and laundry rooms because it is too undignified to be done in the open. This used to be the province of slaves but is now the province of vast government departments dedicated to maximizing economic growth and making everyone as comfortable as possible - without the need for slavery, except over the microchips and clockwork which run the labour-saving devices that make all this possible. So this kind of slavery and the idea that anyone concerned just with the mere process of staying alive is a "slave" because they can't engage in some form of activity considered higher is quite alien to us.
Easier to understand is slavery as it exists in Africa and has existed in Africa for the entirety of recorded history. In Africa, land tends to be poor but is abundant; hence it was much more fruitful to control people than land, because you can never possibly use all the land available. Hence, slavery in Africa was a way of getting more crucial labour to allow for the extraction of natural resources and food from the land. Slavery also addressed the fact that people had the tendency to wander away from African statelets rather than submit to their control and taxation because there was just so much land to run away into. A similar situation obtained once in South East Asia, where some rulers used to actually go to the length of sticking a tattoo on people to mark them as citizens and make it more difficult for them to escape.
African tribes would raid one another and the peoples who lived around their areas of settlement and bring them back to increase their own productivity - and women to impregnate to increase their numbers. And if you can guess why the slave trade developed as an outcome of this tendency before the end of this sentence, you get ten points.
No? Okay, well it goes like this. If an African tribe raided its neighbour and brought some slaves back, then these slaves could just leg it back to their home at the earliest opportunity - they know the way and it is not far to go. But if the raiders sell the slaves to a distant tribe - probably in exchange for other slaves - then the slaves wouldn't have a clue where they were, and wouldn't be able to get back. Hence, they'd find it much harder to run away and would be stuck as slaves to the tribe that purchased them. And that's why there was such a vibrant slave trade - as opposed to just a custom of raiding - in Africa before the Europeans arrived and made things even worse.