Absent Africa there were still slaves, slaves existed long before anyone in Europe met an African.
Quite true. Slavery in all likelihood goes back to Neolithic times.
But what made African slavery in America so intractable was its combination of the ancient institution of slavery with the newly invented ideology of racism. Slaves who could run away and just disappear into the general population would have quickly undermined the institution, just as slavery of whites disappeared in (western) Europe during the Middle Ages.
But the idea that this visually distinct group was somehow not fully human and therefore fair game to be enslaved was somehow more compatible with democracy, freedom, etc. In the minds of confused people, anyway.