What major barrier divides Africa from Eurasia?In Jared Diamond's Guns Germs and Steel, chapter 19-How Africa Became Black

To me, Diamond is not really saying that there is a major
barrier that divides all of Africa from Eurasia.  Instead,
there is a major barrier that separates central and southern Africa from northern Africa
and Eurasia.  That barrier is the Sahara Desert.

Diamond
talks in this chapter about how that barrier prevented the spread of crops and
domesticated animals (the tsetse fly helped with the animals) from Eurasia and northern
Africa down into the rest of the continent.  This is the real split -- not the split
between Africa as a whole and Eurasia.