What are some reasons why North American and African tribes were always hunter-gatherers at a time where most other countries had long since stopped?

Animals

One reason was that unlike Eurasian animals such as ox, cow, hen, horse, pig, buffalo and asian elephant, African & American animals are not easy to tame. Even with 1000s of years of animal husbandry knowledge you cannot tame a Zebra,
Hippo, Rhino, African Elephant in the same you do their Asian counterparts. Without the animals to help, many African tribes were at a huge disadvantage. 

Crops

Eurasia took to key crops such as wheat and rice very quickly. Most parts of Africa didn't have an indigenous crop to rival wheat or rice. 

Connections

Eurasia & North Africa behaved like a single cultural unit. Inventions from one part quickly spread to the other. Wheel, writing systems and other key ideas from different parts made everyone collectively smart. Geography disconnected a big chunk of Sub-saharan Africa, Australia and North America from the rest of the world.
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