Eurasia ended up with the most domesticated animal species in part because it's the world's largest land mass and offered the most wild species to begin with.

Although native Africans domesticated some plants in the Sahel and in Ethiopia and in tropical West Africa, they acquired valuable domestic animals only later, from the north.

The southward advance of native African farmers with Central African crops halted in Natal, beyond which Central African crops couldn't grow - with enormous consequences for the recent history of South Africa.

Jared Diamond Quotes