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”).
Starting with the first surviving Greek historians Thucydides and Herodotus at the end of the 5th century BCE, we have account after account of prisoners of war and the populations of captured cities being sold into captivity or executed by Greek cities or Persian kings. We also know that one of the biggest employers of slaves near Athens was the silver mines, and mine slaves seem to have had a very short life expectancy. Early Greek and Latin had words for “home-born slave” and “bought slave” and slave-holders told each other that the first kind was best. In Egypt under the Ptolemies, letters and slave names indicate that many slaves were abandoned children left out with the trash: whoever took them in and fed them owned them. There is an Akkadian phrase which may mean something similar: where Greeks said that these foundlings came “from the dungheap” (ἀπὸ κοπρίας), Babylonians seem to say they came “from the dog’s mouth” (ie. they were about to be eaten by stray dogs: ša pī kalbi). The idea that if someone was going to die and you save them, you can do what you like with them is very common in different cultures because it let masters tell themselves that whatever they did was not as bad as killing.