What is the primary reason that the Americas and sub-Saharan Africa only shared in axial age thinking much later?

Question

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Both the Americas and sub-Saharan Africa were geographically isolated from the Eurasian landmass where many of the early axial age civilizations emerged. This limited their exposure to the ideas and innovations of these civilizations and slowed the development of their own distinct systems of thought.

What Is the Primary Reason That the Americas and Sub-Saharan