A retired economist in another country wants to know how we know that many ancient slaves were prisoners of war, kidnap victims, or the children of slaves. Ok! Readers who don’t want to hear about slavery and child abandonment might want to skip this one.
So in the Ur III period around 2000 BCE we see massive numbers of people being rounded up and deported into labour camps near Ur. Some were starved to death so their supervisors could sell their rations, and others seem to have been blinded to stop them running away (they could still haul water and do other simple tasks). A bit later we have contracts where parents sell their children to someone willing to feed them during sieges or famines. Moving on to the 8th and 7th century BCE, the archive from Nippur (Oriental Institute Publication 114) and the Iliad describe people being captured by raiders and bandits and either ransomed or enslaved. A little later we see massive numbers of captives being dedicated to the gods in Babylonia, where they would work for the rest of their lives for the temple (although it is worth noting that these širāku had what we would call human rights other than the right to move freely and choose their employer- there were even worse statuses to be placed in). We also see that people with unfree status were tattooed or branded so they could be identified if they ran away. Later stories about Solon around 600 BCE describe how farmers in Attica fell into debt and were forced to sell themselves and their lands, possibly share-cropping for one sixth of the produce (the ἑκτημόριοι “sixth-parters”).