What is more, slavery in the Old World was practiced mainly by Arab traders who immensely benefited from it, and only European colonization of Africa put an end to it. In fact, one of goals of the famous Livingstone expedition was to counter-balance the Arab exploitation of East Africa.
That of course, does not mean that European colonialism can be defended by today's moral standards, but that viewing historical events through moral (or ideological) standards can be quite deceptive.
Wissenschaft Ohne Moral :)
Wojtek