Answer: different ethnic groups
Step-by-step explanation:
Modern notions of race emerged only after Africans of different ethnic groups were mixed together in the slave trade and as Europeans began enslaving Africans and Native Americans exclusively.
Africa comprises of more than 3,000 different tribes with a lot of them speaking different languages and practicing a unique culture. When the Europeans reached sub-Saharan Africa and began to enslave Africans, once they transported them to the Americas, they considered them as one people.
In grouping them with Native Americans and enslaving both groups exclusively, they began to make slavery a thing of race where the "lower" races were the ones to be enslaved even though slavery had traditionally been practiced on any race.