It seems that when learning about antiquity, most "major" cultures had kept other humans as slaves. I've seen slaves mentioned in information about Ancient Greeks, Romans, Egyptians, Mayans, Aztecs, Incas, Vikings, Celts, Mongols, Kongolese (?), Harappans, and probably more.
Were there any ancient civilizations without slaves? How were slaves distinguished from non-slaves? Were there any anti-slavery movements in the ancient world?
I know this is a very broad subject, but I'm interested in learning if every culture has had people they kept as help, and if anyone knows details about life as a slave in any particular civilization/time period. Thank you!
This is a very broad question, so I can't really provide you an answer. However, my inclination is that it will be very difficult to answer your question for two reasons. One, it will be difficult to find archaeological evidence of slavery in societies without written records. Or rather, the evidence might not be hard to find, but it probably won't look that much different than the evidence you would expect to see for other marginalized groups in society.
Second, not all slavery is equivalent. While you could say that Roman slavery and 18th century plantation slavery are both kinds of slavery, they are pretty significantly different in a lot of important respects including cultural attitudes towards slaves. For both these reasons, determining if slavery is a universal of human societies is difficult.
If nothing else, I can say that there is very little to no good evidence of slavery or similar practices in indigenous societies in the US Southwest pre-Spanish. That said, it is always hard to prove that something didn't happen, as opposed to establishing that something did occur.