But that's the point. How did it develop?
The European's first go-to was to enslave Native Americans. But because they brought over diseases and worked the poor bastards to death, that population dropped dramatically. (Perhaps 80% of Native Americans died off by 1600). That's when they got the brilliant idea, "Hey, let's go to Africa and start buying people to be slaves!"
Slavery had largely disappeared in Europe by 1600, and even serfdom was on the way out. The Europeans had to reintroduce the practice on an industrial scale to exploit the new world they found.
And this is what makes what they did so much worse. The influx of goods changed the economy of Africa, where they became more dependent on trading manufactured goods to people than developing their own industries. When the British finally had a pang of conscience and ended the slave trade in 1807, their economies collapsed and left them ripe for colonization.
If you want to truly **** something up, add white people. We excel at that.
The problem was VOLUME. Slavery in sub-Sahara Africa was a thing, but it was more akin to what slavery had been for decades. Slaves were considered part of the household, and their treatment wasn't really that bad. It wasn't until the Europeans came along, and they were enslaving thousands, packing them in like sardines and having half of them die on the way over, that it became a true crime against humanity.
Now you are getting it. Sit down and shut up, you might learn something.