GenZedong

The advent of class society brought slavery. Class society appeared when humans were able to produce more than they consumed. This happened through several advances such as agriculture, clay pots (which allowed for storing grain and water for later consumption), and of course tools. From the moment we shaped a stone into an arrow tip we have been performing socially necessary labor.

As people produced items of consumption, this freed up time for other people in the clan/tribe/village to do other things. One very important task that appeared and has been with us since then is accounting. Of course back in these days it wasn't done with tables and accounts, but the principle was the same. There is a stockpile of goods, and it was someone's job to know what was in that stockpile. Later it also became their job to decide how that stockpile would be assigned.

This is likely how class society arose; over time, this accounting strata became dominant and exploitative. They controlled the output of production and could easily control the means of productions themselves after that. You need some grain? Well, make a spear first. You need a spear? Get me some grain. Regardless of how it actually played out, they could easily appropriate the surplus for themselves.

Slavery was the first dominant mode of production because it's nakedly exploitative. It can't get any more exploitative than that: you literally own someone as property, and put them to work to produce more than they consume, but everything they produce belongs to you. They don't receive any of their surplus value back, they don't even receive any value beyond the bare minimum to survive!

^ There is a wrench in this theory because not all populations went from nomadic hunter-gatherer lifestyle to settled agriculture. Some (and this is attested to in China, though they were not the Chinese but were incorporated into it) moved back and forth as climate changed and settled over millennia. I don't know their mode of production though. Note also that nomadic doesn't mean they were hunter-gatherers-- they herded horses, for example, and you can have slaves for this.

While people did produce a surplus, this amounted to almost nothing. Yields were very low. Therefore it was difficult to provide food and other items to slaves, while still creating a surplus with their labor. And if they didn't create a surplus, then there was no need for slaves. Does that make sense? If output < 1, then you wouldn't keep slaves around. If output ~= 1.2, then slaves became interesting so you could appropriate that 0.2 for yourself. But with a positive factor of barely 0.2, you couldn't really give them 0.1 for themselves and keep 0.1 for yourself. They also didn't produce the same output all days of the year. In winter, a slave's production might equal 0.9. So what do you do then? You reduce their rations so that they still produce an even 1 in winter, at the detriment of their well-being. That's barely anything and again slaves become less interesting. When output becomes > 4, 5 or 6, then a proletariat is materially possible (possible but not guaranteed, but that's not the topic here).

Why did slavery last so long? Because for most of human history the value output of labor was barely above 1. And history moves dialectically, not prescriptively. People didn't say "well it's the year 1200 now so I guess it's time to move into serfdom". Class struggles happened, revolutions happened. Though in Europe it seems to me this change was very gradual; the earliest form of serfdom was slavery all but in name. Lords were in fact previously slave owners as I understand it, sometimes both at once. The very first form of tax payment from the serf was the corvée, in which the serf worked 4 days of the week on his own land, and the other 2 days on his lord's land, using his own tools. The textbook of political economy of the USSR talks about this but would you believe that even though it's only chapter 3 I'm not there yet. I can think of some advantages for nobles having serfs instead of slaves though; as they are tied to the land they are less likely to run away, since they own some meager possessions. They can also be returned to their lord more easily if trying to escape. There is also a problem with slavery in that the more slaves you have, the more police you need. At some point you're gonna have more slaves than you have police and this is a problem the Roman Empire ran into.

At least this is how I believe it played out.