I don't think you can make the historical case that the past was uniformly more brutal or unjust than the present. I think it would be more accurate to say that

I don't think you can make the historical case that the past was uniformly more brutal or unjust than the present. I think it would be more accurate to say that in our current place and time, our current evolution of moral standards and expectations looks unfavorably (and rightfully so!) on human slavery, but there's no guarantee that we or others aren't regressing on other issues. That's probably a cause for humility as future generations will almost certainly look back on us and wonder how we could have tolerated those things that they will consider injustices.