The African savanna is very coal- and coke-poor -- for most of our species' early history, our natural predators, like lions and cheetahs, were unable to produce sheet glass in large quantities. When humans started moving north into Eurasia, where coal deposits are more common, they were saved by the fact that the ursine glass industry was depressed by terrible protectionist laws up until the Bearton-Woofs Agreement in 1944.